Submillimeter-scale Innovation and Unlock New Design Possibilities
ALMA (Atacama Large Millimeter/submillimeter Array)
ALMA Facilities ALMA links with other Observatories Selected Imagery References
The ALMA Observatory is an international astronomy facility, a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and NINS (National Institutes of Natural Sciences) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the NRC (National Research Council) of Canada and the NSC (National Science Council) of Taiwan and by NINS of Japan in cooperation with the Academia Sinica (AS) in Taiwan, and KASI (Korea Astronomy and Space Science Institute) Korea. 1) 2)
ALMA construction and operations are led on behalf of Europe by ESO on behalf of its Member States; by NRAO (National Radio Astronomy Observatory), managed by AUI (Associated Universities, Inc.), on behalf of North America; and by NAOJ (National Astronomical Observatory of Japan) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Figure 1: Global partnerships of the ALMA Program (image credit: ALMA partnership) 3)
ALMA isthe largest astronomical project in existence, it is a single telescope of revolutionary design, composed of 66 high precision antennas (forming a sparse array of antennas) of 12 m and 7 m in diameter. ALMA is located at a truly unique and unusual place: the Chilean Atacama desert. While the astronomers will operate the telescope from the OSF (Operations Support Facility) Technical Building, at 2,900 m above sea level, the array of antennas will be located at the Altiplano de Chajnantor, a plateau at an altitude of 5,000 m altitude. This location was selected because of many well justified scientific reasons, particularly dryness and altitude. The ALMA site with the average annual rainfall below 100 mm is the perfect place for a new telescope capable of detecting radio waves just millimeters in wavelength. Indeed, radio waves penetrate a lot of the gas and dust in space, and can pass through the Earth's atmosphere with little distortion. However, if the atmosphere above ALMA contained water, the radio signals would be heavily absorbed – the tiny droplets of water scatter the radio waves in all directions before they reach the telescope, and would degrade the quality of the observations.
Furthermore, the flat and wide land at the ALMA site is suitable for the construction of a large-scale array. Considering these aspects, the ALMA Observatory will not only be unique because of its ambitious scientific goals, and the unprecedented technical requirements, it will also be unique because of the very specific, harsh environment and living conditions in which the most challenging radio telescope array will operate with high efficiency and accuracy.
ALMA is an international astronomy facility, a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led on behalf of Europe (ESO), North America (NRAO/AUI), and East Asia (NAOJ). The JAO (Joint ALMA Observatory) provides the unified leadership and management of the construction, commissioning and operation of ALMA. The JAO coordinates the ALMA Development Program in order to effectively manage the technological evolution of the ALMA facility. Periodically, solicitations ("calls") are issued by each of the international partners to identify and fund development initiatives ("upgrades") which will enhance the performance of the ALMA facility. The implementation of ALMA upgrades are assigned on a competitive basis.
| On 6 November 1963, the initial agreement between the European Southern Observatory (ESO) and the Government of Chile, the Convenio, was signed, enabling ESO to place its telescopes beneath the exceptionally clear Chilean skies. The birth of ALMA dates back to the end of the 20th century. Large millimeter/submillimeter array radio telescopes were studied by astronomers in Europe, North America and Japan and different possible observatories had been discussed. After thorough investigations, it became obvious that the ambitious projects of all of these studies could hardly be realized by a single community. Consequently, a first memorandum was signed in 1999 by the North American community, represented through the NSF (National Science Foundation), and the European community, represented through ESO (European Organization for Astronomical Research in the Southern Hemisphere), followed in 2002 by an agreement to construct ALMA on a plateau in Chile. Thereafter, Japan, through the NAOJ (National Astronomical Observatory of Japan), worked with the other partners to define and formulate its participation in the ALMA project. An official, trilateral agreement between ESO, the NSF, and the National Institutes for Natural Sciences (NINS, Japan) concerning the construction of the enhanced Atacama Large Millimeter / submillimeter Array was signed in September 2004. This agreement was subsequently amended in July 2006. NAOJ will provide four 12-meter diameter antennas and twelve 7-meter diameter antennas for a compact array (ACA), the ACA correlator and three receiver bands. With the inclusion of the Asian partners, ALMA has become a truly global astronomical facility, involving scientists from four different continents. • On Nov. 17, 2009, ALMA made its first measurements using just two of the 66 antennas that will comprise the array. As of January 4, 2010, three antennas are working in unison. In October 2011, ALMA has officially opened for astronomers. About a third of ALMA's 66 radio antennas are installed. 6)- ALMA is the largest and most ambitious ground-based observatory ever created with full service provision expected in 2013. 6) • On 3 October 2011, ALMA opened officially for astronomers - using the partially constructed antenna array. | |
| • ALMA was inaugurated in an official ceremony on March 13, 2013. This event marks the completion of all the major systems of the giant telescope and the formal transition from a construction project to a fully fledged observatory. The telescope has already provided unprecedented views of the cosmos with only a portion of its full array. 7) • The 66th ALMA antenna was transported to the AOC (Array Operations Site) on 13 June 2014. This is an important milestone for the ALMA project. The 12 m diameter dish is the 25th and final European antenna to be transported up to the Chajnantor Plateau. It will work alongside its European predecessors, as well as 25 North American 12 m antennas and 16 East Asian (four 12 m and twelve 7 m) antennas. 8) 9) • In March 2015, ALMA combined its immense collecting area and sensitivity with that of the APEX (Atacama Pathfinder Experiment) Telescope to create a new, single instrument through a process known as VLBI (Very Long Baseline Interferometry). In VLBI, data from two independent telescopes are combined to form a virtual telescope that spans the geographic distance between them, yielding extraordinary magnifying power. 10) • In July 2015, ALMA successfully opened its eyes on another frequency range after obtaining the first fringes with a Band 5 receiver, specifically designed to detect water in the local Universe. Band 5 will also open up the possibility of studying complex molecules in star-forming regions and protoplanetary discs, and detecting molecules and atoms in galaxies in the early Universe, looking back about 13 billion years. 11) • Nov. 4, 2015: A new instrument attached to the 12 m APEX (Atacama Pathfinder Experiment) telescope at 5000 m above sea level in the Chilean Andes, is opening up a previously unexplored window on the Universe. The SEPIA (Swedish–ESO PI receiver for APEX) will detect the faint signals from water and other molecules within the Milky Way, other nearby galaxies and the early Universe. 13) - The SEPIA wavelength region of 1.4–1.9 mm is of great interest to astronomers as signals from water in space are found here. Water is an important indicator of many astrophysical processes, including the formation of stars, and is believed to play an important role in the origin of life. Studying water in space — in molecular clouds, in star-forming regions and even in comets within the Solar System — is expected to provide critical clues to the role of water in the Milky Way and in the history of the Earth. In addition, SEPIA's sensitivity makes it a powerful tool for also detecting carbon monoxide and ionised carbon in galaxies in the early Universe. | |
| • July 12, 2018: After half a decade of ALMA operations, the original science goals of the observatory have been essentially met. To maintain the leading-edge capabilities of the observatory, the ALMA Board designated a Working Group to prioritize recommendations from the ALMA Science Advisory Committee (ASAC) on new developments for the observatory between now and 2030. 14) The Working Group concluded, based on the ASAC recommendations, that the science drivers that will support further developments shall be: - to trace the cosmic evolution of key elements from the first galaxies through the peak of star formation in the Universe; - to trace the evolution from simple to complex organic molecules through the process of star and planet formation down to solar system scales; - to image protoplanetary disks in nearby star formation regions to resolve their Earth-forming zones, enabling detection of the tidal gaps and inner holes created by forming planets. Even with the outstanding capabilities of the current ALMA array, achieving these ambitious goals is currently impossible. The ALMA observatory needs to become more powerful to address these new challenges and stay at the forefront of astronomy by continuing to produce transformational science and enabling fundamental understanding of the Universe for the decades to come. The top priority upgrades for ALMA will be focused on the receivers (the signal detectors), the digital systems (data transmission), and the correlator (the supercomputer data processor at the heart of the telescope).In addition, to keep up with the new powerful capabilities of the observatory, the ALMA Archive will be further developed, becoming the primary source for the ever-increasing number of publications using advanced data mining tools. The recently appointed ALMA Director, Sean Dougherty, is very enthusiastic about these new developments being implemented in the coming years, as "they will ensure a front-row seat for ALMA over the next decade, through the development of state of the art technology that will advance our understanding of the Universe. This is an important step in continuing the quest for our cosmic origins". The proposed developments will advance a wide range of scientific studies by significantly reducing the time required for the complex observations required by the astronomical community to achieve its ambitious science goals. "This is an exciting moment in the history of ALMA – says Toshikazu Onishi, Chair of the ALMA Board – as we are advancing the future capabilities of this extraordinary facility we built to explore the Universe.". |
Table 1: Some development stages of ALMA 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
Figure 2: A state-of-the-art telescope to study light with wavelengths of about one millimeter, shining from some of the coldest objects in the Universe, ALMA is a cooperation of the European Southern Observatory (ESO), together with its international partners. The site of ALMA is the 5000-m altitude Chajnantor plateau in northern Chile, one of the driest places on Earth (video credit: ESO, ALMA (ESO/NAOJ/NRAO), C. Malin , P. Horálek, Liam Young, B. Tafreshi, J. J. Tobin (University of Oklahoma/Leiden University), M. Kaufman, Theofanis N. Matsopoulos, H. H. Heyer, S. Argandoña and H. Zodet. Music by Movetwo, Published on Dec 7, 2016)
Figure 3: The Atacama Large Millimeter/submillimeter Array is an astronomical interferometer of radio telescopes in the Atacama desert of northern Chile. Since a high and dry site is crucial to millimeter and submillimeter wavelength operations, the array has been constructed on the Chajnantor plateau at 5,000 metres altitude, near Llano de Chajnantor Observatory and Atacama Pathfinder Experiment (video credit: ESO, Published on Oct 7, 2017)
Major ALMA Facilities
Antennas
ALMA will be the world's most powerful telescope for studying the Universe at submillimeter and millimeter wavelengths, on the boundary between infrared light and the longer radio waves. However, ALMA does not resemble many people's image of a giant telescope. It does not use the shiny, reflective mirrors of visible- and infrared-light telescopes; it is instead comprised of many "antennas" that look like large metallic satellite dishes. 15)
Several antennas have already been installed in the harsh conditions of the 5000 m altitude Chajnantor plateau, and more are under construction at the 2900 m altitude OSF (Operations Support Facility). When ALMA is fully operational, visitors to Chajnantor will encounter 66 antennas, 54 of them with 12 m diameter dishes, and 12 smaller ones, with a diameter of 7 m each.
Figure 4: Photo of the first ALMA 12 m antenna, manufactured by Mitsubishi Electric Corporation (image credit: ALMA ,ESO/NAOJ/NRAO)
The most visible part of each antenna is the dish, a large reflecting surface. Most of ALMA's dishes have a diameter of 12 m. Each dish plays the same role as the mirror of an optical telescope: it collects radiation coming from distant astronomical objects, and focuses it into a detector that measures the radiation. The difference between the two types of telescopes is the wavelength of the radiation detected. Visible light, captured by optical telescopes, is just a small part of the spectrum of electromagnetic radiation, with wavelengths between roughly 380 and 750 nm. ALMA, in contrast, will probe the sky for radiation at longer wavelengths from a few hundred µm to about 1 mm (about one thousand times longer than visible light). This is known, perhaps unsurprisingly, as mm and sub-mm radiation, and lies at the very short-wavelength end of radio waves.
This longer wavelength range is the reason, why ALMA's dishes are not mirrors, but have a surface of metallic panels. The reflecting surfaces of any telescope must be virtually perfect: if they have any defects that are larger than a few percent of the wavelength to be detected, the telescope won't produce accurate measurements. The longer wavelengths that ALMA's antennas detect mean that although the surfaces are accurate to within 25 µm — much less than the thickness of a single sheet of paper, the dishes do not need the mirror finish used for visible-light telescopes. So although ALMA's dishes look like giant metallic satellite dishes, to a submillimeter-wavelength photon (light-particle), they are almost perfectly smooth reflecting surfaces, focusing the photons with great precision.
Not only are the dish surfaces carefully controlled, but the antennas can be steered very precisely and pointed to an angular accuracy of 0.6 arcseconds (one arcsecond is 1/3600 of a degree). This is accurate enough to pick out a golf ball at a distance of 15 km.
ALMA will combine the signals from its array of antennas as an interferometer — acting like a single giant telescope as large as the whole array. Thanks to the two antenna transporter vehicles, astronomers will be able to reposition the antennas according to the kind of observations needed. So, unlike a telescope that is constructed and remains in one place, the antennas are robust enough to be picked up and moved between concrete foundation pads without this affecting their precision engineering.
In addition, the antennas achieve all this without the protection of a telescope dome or enclosure. The dishes are exposed to the harsh environmental conditions of the high altitude Chajnantor plateau, with strong winds, intense sunlight, and temperatures between ±20 ºC. Despite Chajnantor being in one of the driest regions on the planet, there is even sometimes snow here, but ALMA's antennas are designed to survive all these hardships.
The production of the antennas is being shared between the ALMA partners. ESO has ordered twentyfive 12 m antennas, with an option for an additional seven, from the AEM Consortium (Alcatel Alenia Space France, Alcatel Alenia Space Italy, European Industrial Engineering S.r.L., MT Aerospace). The North American partners have placed an order of the same size with Vertex RSI, while the four 12 m and twelve 7 m antennas comprising ALMA's ACA (Atacama Compact Array) have been ordered by NAOJ from MELCO (Mitsubishi Electric Corporation).
Apart from the obvious difference in size between the 12-meter and 7-meter antennas, careful observers will spot subtle differences in the antenna design from each partner. However, all the antennas are designed to meet the stringent technical specifications, and work together smoothly as parts of the whole. These state-of-the-art dishes, combined in a single revolutionary telescope, reflect the cooperative nature of the global ALMA project.
Figure 5: Photo of the ALMA antenna array (image credit: ALMA partnership, Ref. 3)
Figure 6: The 12th 7 m antenna developed by Japan was delivered to the high site in Chajnantor on April 29, 2013. Now all the 16 antennas of the ACA (Atacama Compact Array) are installed at the Array Operations Site at an altitude of 5,000 m, waiting to unveil secrets of the universe (image credit: ALMA partnership) 16)
Figure 7: The final antenna of the ALMA project is here seen arriving to the high site at the ALMA Observatory, 5000 m above sea level. Its arrival completes the complement of 66 12 m ALMA antennas on the Chajnantor Plateau in the Atacama Desert of northern Chile (image credit: ALMA, ESO/NAOJ/NRAO, A. Marinkovic) 17)
ALMA Front End Integration Centers: A construction project like ALMA, involving several partners in four different continents, requires consensus on several organizational and managerial decisions concerning the actual execution of certain construction activities. Several different scenarios for assembling and integrating the Front End components were extensively studied. This study revealed that the best solution was a "parallel approach", installing half of the Front End in Europe and the other half in North America with identical and parallel procedures. This scenario was preferred in view of logistics, organization and program risks. Mainly based on considerations of risk mitigation, the parallel FEIC (Front End Integration Centers) was selected. The European FEIC is located at Rutherford Appleton Laboratory (UK) and the North American FEIC at NRAO. A third FEIC is installed in Taiwan to carry out the integration of Front End assemblies required for the antennas supplied by NAOJ.
Figure 8: Aerial view of the ALMA OSF (Operation Support Facility) at 2,900 m altitude (image credit: ALMA partnership)
A world-class observatory site in the desert: 18)
The ALMA Observatory is operated at two distinct sites, far away from comfortable living conditions of modern civilization. The ALMA OSF is the base camp for the every-day, routine operation of the observatory. It is located at an altitude of about 2900 m, quite high compared to standard living conditions, but still quite acceptable for scientific projects in astronomy of similar scope. However, the OSF will not only serve as the location for operating the Joint ALMA Observatory, it is also the AIV (Assembly, Integration and Verification) station for all the high technology equipment before being moved to the AOS (Array Operations Site), located at 5000 m altitude. Antenna assembly is done at the OSF site at three separate areas, one each for the antennas provided by North America (VERTEX), Japan (MELCO), and Europe (AEM Consortium).
The OSF is also the center for activities associated with commissioning and science verification as well as Early Science operation. During the operations phase of the observatory it is the workplace of the astronomers and of the teams responsible for maintaining proper functioning of all the telescopes.
The construction of the OSF and AOS sites and their access required substantial efforts of the ALMA project. Obviously, there was no access to these two remote locations (Figure 9). The OSF site, located at 2900 m altitude, is about 15 km away from the closest public road, the Chilean highway No. 23. The AOS is another 28 km away from the OSF site. Thus, one of the first projects to be accomplished by ALMA was to construct an access road not only to the OSF but also to the AOS road, 43 km in length, not only at high altitudes, but also with sufficient width to regularly transport a large number of large radio telescopes with a diameter of 12 m.
The geographical location of ALMA (at Altiplano de Chajnantor) is latitude: -23.029° ; longitude: -67.755°
Figure 9: Access to the AOS and OSF facilities (image credit: ALMA partnership)
ALMA Front End System
The ALMA Front End system is the first element in a complex chain of signal receiving, conversion, processing and recording. The Front End is designed to receive signals of ten different frequency bands. 19)
The ALMA Front End is far superior to any existing systems. Indeed, spin offs of the ALMA prototypes are leading to improved sensitivities in existing millimeter and submillimeter observatories around the world. The Front End units are comprised of numerous elements, produced at different locations in Europe, North America, East Asia and Chile.
ALMA Cryostats: The largest single element of the Front End system is the cryostat (vacuum vessel) with the cryo-cooler attached. The cryostats will house the receivers, which are assembled in cartridges and can relatively easily be installed or replaced. The corresponding warm optics, windows and infrared filters were delivered by the IRAM (Institut de Radio Astronomie Millimétrique) of France. The operating temperature of the cryostats will be as low as 4 K (equivalent to -269ºC).
ALMA Receiver Bands: In the initial phase of operations, the antennas will be equipped with at least four receiver bands: Band 3 (3 mm), Band 6 (1 mm), Band 7 (0.85 mm), Band 9 (0.45 mm). It is planned to equip the antennas with the missing bands at a later stage of ALMA operations. The development programs were successful, as the requirements could be met – and sometimes the performance is even better than defined in the specifications.
| ALMA band | Frequency range (GHz) | Receiver noise (K) over 80% of the RF band | Temperature (K) at any RF frequency | To be produced by | Receiver technology |
| 1 | 31 - 45 | 17 | 26 | TBD (To be decided) | HEMT |
| 2 | 67 - 90 | 30 | 47 | TBD | HEMT |
| 3 | 84 - 116 | 37 | 60 | HIA | SIS |
| 4 | 125 - 163 | 51 | 82 | NAOJ | SIS |
| 5 | 162 - 211 | 65 | 105 | OSO | SIS |
| 6 | 211 - 275 | 83 | 136 | NRAO | SIS |
| 7 | 275 - 373 | 147 | 219 | IRAM | SIS |
| 8 | 385 -500 | 196 | 292 | NAOJ | SIS |
| 9 | 602 - 720 | 175 | 261 | NOVA | SIS |
| 10 | 787 - 900 | 230 | 344 | NAOJ | SIS |
| IRAM (Institut de Radio Astronomie Millimétrique), Grenoble, France | |||||
Table 2: The 10 frequency bands of the ALMA antennas
Modular Cryogenic Receiver Concept. The complete front end unit will have a diameter of 1 m, be about 1m high and have a mass of about 750 kg. The cryostat will be cooled down to ~4 K by a 3-stage commercial closed-cycle cryocooler based on the Gifford – McMahon cooling cycle. The individual frequency bands are implemented in the form of modular cartridges that will be inserted in a large common cryostat. This cartridge concept allows for a great flexibility in construction and operation of the array. Figure 10 shows an example of such a receiver cartridge. Another advantage of the cartridge layout with well-defined interfaces is the fact that different cartridges can be developed and built by different groups within the ALMA Project with a large degree of independence but without the risk of incompatibility between them. 20)
Figure 10: Example of a, Band 6, receiver cartridge. The larger diameter metal plate in the middle is the boundary between cooled receiver electronics inside the cryostat (right hand side) and the room temperature electronics (left hand side), image credit: ALMA partnership)
Figure 11: Photo of one typical receiver cartridge built for ALMA ((image credit: ALMA partnership)
Band 5 — July 17, 2015: After more than five years of development and construction, ALMA successfully opened its eyes on another frequency range after obtaining the first fringes with a Band 5 receiver, specifically designed to detect water in the local Universe. Band 5 will also open up the possibility of studying complex molecules in star-forming regions and protoplanetary discs, and detecting molecules and atoms in galaxies in the early Universe, looking back about 13 billion years (Ref. 11).
"Band 5 will open up new possibilities to explore the Universe and bring new discoveries," explains ESO's Gianni Marconi, who is responsible for the integration of Band 5. "The frequency range of this receiver includes an emission line of water that ALMA will be able to study in nearby regions of star formation. The study of water is, of course, of intense interest because of its role in the origin of life." With Band 5, ALMA will also be able to probe the emission from ionized carbon from objects seen soon after the Big Bang, opening up the possibility of probing the earliest epoch of galaxy formation. "This band will also enable astronomers to study young galaxies in the early Universe about 500 million years after the Big Bang," added Gianni Marconi.
The Band 5 receivers were originally designed and prototyped by Onsala Space Observatory's Group for Advanced Receiver Development (GARD) at Chalmers University of Technology in Sweden, in collaboration with the Rutherford Appleton Laboratory, UK, and ESO, under the European Commission supported Framework Program FP6 (ALMA Enhancement). After having successfully tested the prototypes, the first production-type receivers were built and delivered to ALMA by a consortium of NOVA and GARD in the first half of 2015. Two receivers were used for the first light. The remainder of the 73 receivers ordered, including spares, will be delivered between now and 2017.
Figure 12: Photo of one of the Band 5 receiver cartridges built for ALMA. Extremely weak signals from space are collected by the ALMA antennas and focussed onto the receivers, which transform the faint radiation into an electrical signal (image credit: ALMA partnership)
ALMA Back End and Correlator
The ALMA Back End systems deliver signals generated by Front End units installed in each antenna to the Correlator installed in the AOS (Array Operations Site) Technical Building, located at an altitude of 5,000 m. Signal processing and data transfer is schematically shown in Figure 13. Analog data, produced by the Front End electronics, is processed and digitized before entering into the data encoder, followed by the optical transmitter units and multiplexers. All these elements are installed in the receiver cabins of each antenna. Optical signals are then transmitted by fibers to the AOS Technical Building. The total distance is, in one antenna configuration, about 15 km. At the Technical Building the incoming optical signals are de-multiplexed and de-formatted before entering the Correlator. 21) 22) 23)
ALMA main array Correlator: The ALMA main array Correlator, to be installed in the AOS Technical Building, is the last component in the receiving end of the data transmission. It is a very large data processing system, composed of four quadrants, each of which can process data coming from up to 504 pairs of antennas. The complete correlator will have 2912 printed circuit boards, 5200 interface cables, and more than 20 million solder points. Integral parts of the Correlator are TFB (Tunable Filter Bank) cards. The layout is such that four TFB cards are needed for the data coming from a single antenna. The TFB cards have been developed and optimized by the University of Bordeaux over the last few years.
ACA (Atacama Compact Array) Correlator: The ACA Correlator is designed to process the signals detected by the Atacama Compact Array (ACA). This correlator consists of 52 modules connected with each other through optical-fiber cables. All the modules are installed in 8 racks in the AOS Technical Building. The power spectra issued from the correlation are transferred to the ACA data processing computers.
Figure 13: Schematic of the ALMA signal processing and data transfer from the Front End to the Correlator (image credit: ALMA partnership)
• May 15, 2020: The contract has been signed for the production of the final set of receivers to be installed on the Atacama Large Millimeter/submillimeter Array (ALMA). Of the originally foreseen ten receiver bands, eight have already been installed, and the ninth, Band 1, is currently in production in East-Asia. Now, contracts have been signed to start the production of the final band in the original ALMA definition — Band 2, led by ESO. Exceeding the originally defined frequency range for this Band (69-90 GHz), the proposed receiver will operate at the full 67-116 GHz frequency window. The hugely successful Band 3 receiver has already opened up the 84-116 GHz frequency range years ago, but the new Band 2 will allow for observations across the entire 67-116 GHz atmospheric window using a single receiver. The project will involve multiple international partners as detailed below. 24)
Figure 14: The figure shows the typical contributions of continuum emission sources with respect to the ALMA Band 2 frequency coverage (purple shaded region), and the Band 1 frequency coverage (pink shaded region). ALMA Band 2 will on average span one of the cleanest mm or submm-wave windows available. The grey bands depict the frequency bands of the Planck satellite, which covered similar frequencies as ALMA. Figure adapted from Planck Collaboration et al 2016 (A&A 594, A10 (2016)), image credit: ESO
Following successful tests of a prototype Band 2 receiver (Yagoubov et al. 2020, A&A 634, A46), the ALMA board has approved the pre-production of a series of six cartridges, with the goal of eventually moving into the production of the full set, one for each of the ALMA antennas. This will depend on the verification of the performance and series production readiness based on the pre-production receiver cartridges.
The 67-116 GHz atmospheric window is rich in strong molecular lines with less crowding than other atmospheric windows. Due to the relatively low excitation energies, these lines are exceptionally suited for studying the dense molecular gas and early phases of star formation. Furthermore, the window is rich in complex organic molecules (COMs), tying in directly with the new fundamental science drivers defined in the ALMA 2030 Development Roadmap. Band 2 will also fill a gap in ALMA's ability to observe the molecular reservoir in redshifted galaxies. Finally, Band 2 continuum measurements have the lowest intrinsic background, as thermal dust emission, CMB emission, free-free emission and synchrotron emission conspire to form a local emission minimum in frequency space.
The production of the Band 2 receiver cartridges will be undertaken by a consortium comprising the Netherlands Research School for Astronomy (NOVA), Chalmers University, Gothenburg, Sweden, and the Italian National Institute for Astrophysics (INAF). The National Astronomical Observatory of Japan (NAOJ) will contribute to the production and testing of receiver optics as an East Asia contribution to the ALMA Development Program. The National Radio Astronomy Observatory (NRAO) and the University of Chile have been involved in the development and production of some components of the receivers, which will be sent to ESO for testing and integration.
ALMA links with other observatories to create an Earth-size telescope
November 2015: ALMA continues to expand its power and capabilities by linking with other millimeter-wavelength telescopes in Europe and North American in a series of VLBI (Very Long Baseline Interferometry) observations. In VLBI, data from two or more telescopes are combined to form a single virtual telescope that spans the geographic distance between them. The most recent of these experiments with ALMA formed an Earth-size telescope with extraordinarily fine resolution. 25)
Figure 15: ALMA combined its power with IRAM and VLBA in VLBI separated observations (image credit: A. Angelich, NRAO/AUI/NSF)
These experiments are an essential step in including ALMA in the EHT (Event Horizon Telescope), a global network of millimeter-wavelength telescopes that will have the power to study the supermassive black hole at the center of the Milky Way in unprecedented detail.
Before ALMA could participate in VLBI observations, it first had to be upgraded adding a new capability known as a phased array. This new version of ALMA allows its 66 antennas to function as a single radio dish 85 m in diameter, which then becomes one element in a much larger VLBI telescope.
• The first test of ALMA's VLBI capabilities occurred on 13 January 2015, when ALMA successfully linked with the APEX (Atacama Pathfinder Experiment Telescope), which is about two kilometers from the center of the ALMA array.
• On 30 March 2015, ALMA reached out much further by linking with IRAM (Institut de Radioastronomie Millimetrique), the 30 m radio telescope in the Sierra Nevada of southern Spain. Together they simultaneously observed the bright quasar 3C 273. Data from this observation were combined into a single observation with a resolution of 34 µarcsec (1 microarcsecond = 2.8º x 10-10). This is equivalent to distinguish an object of less than 10 cm on the Moon, seen from Earth. - The March observations were made during an observing campaign of the EHT at a wavelength of 1.3 mm.
• The most recent VLBI observing run was performed on 1–3 August 2015 with six of the VLBA (Very Long Baseline Array) antennas of NRAO (National Radio Astronomy Observatory). This combined instrument formed a virtual Earth-size telescope and observed the quasar 3C 454.3, which is one of the brightest radio beacons on the sky, despite lying at a distance of 7.8 billion light-years. These data were first processed at NRAO and MIT-Haystack in the United States and further post-processing analysis is being performed at the MPIfR (Max Planck Institute for Radio Astronomy) in Bonn, Germany.
- The VLBA is an array of 10 antennas spread across the United States from Hawaii to St. Croix. For this observation, six antennas were used: North Liberty, IA; Fort Davis, TX; Los Alamos, NM; Owens Valley, CA; Brewster, WA; and Mauna Kea, HI. The observing wavelength was 3 mm.
• The new observations are a further step towards global interferometric observations with ALMA in the framework of the Global mm-VLBI Array and the EHT (Event Horizon Telescope), with ALMA as the largest and the most sensitive element. The addition of ALMA to millimeter VLBI will boost the imaging sensitivity and capabilities of the existing VLBI arrays by an order of magnitude.
Enhancing ALMA's Future Observing Capabilities
• June 2021: With each observing cycle at the ALMA (Atacama Large Millimeter/submillimeter Array) new features and observing modes are offered. Here we provide some background about how these new capabilities are tested and then made available to ALMA users. These activities help to drive the cutting-edge science conducted with ALMA and to maintain ALMA's position as the foremost interferometric array operating at millimeter and submillimeter wavelengths. We focus in particular on opening up high-frequency observing using ALMA's longest baselines, which offers the highest possible angular resolution. 26)
Extension and optimization of new capabilities
- The global effort of adding new capabilities to ALMA is referred to as Extension and Optimization of Capabilities (EOC). EOC was the natural progression after moving away from initial tests when ALMA was commissioned. During the final years of construction and during Cycle 0 operations, almost ten years ago, the development of new modes was called Commissioning and Scientific Verification (CSV). CSV was conducted to ensure that the capabilities offered were fully operational and valid. Following this, with ALMA as a fully operational telescope, testing as part of EOC activities has continued as an ALMA-wide effort encompassing all partners1: the Joint ALMA Observatory (JAO) in Chile and the ALMA Regional Centres (ARCs) in East Asia, North America and Europe. In Europe there are also contributions from the ARC network (see Hatziminaoglou et al., 2015). The entire EOC effort, including all coordination, planning and the intricate steps involved, is led by the JAO (see Takahashi et al., 2021).
- In this article we provide an overview of EOC, and what features might be expected in the coming cycles, with a specific focus on pushing ALMA to achieve the highest angular resolutions possible (a study involving significant input from the European ARC). We also highlight how the ALMA community benefits from each capability potentially offered.
ALMA's process for offering new capabilities
- Behind the scenes, the process that makes new capabilities possible is the ObsMode process (Takahashi et al., 2021), which is led and coordinated by the JAO. The intention of the ObsMode process is to enable all observing modes that ALMA was designed to support, as well as any additional ones identified since construction began.
- Unfortunately, ALMA cannot simply test a new observing mode on the telescope and thereafter open it directly to the community. This is because all parts of the observing chain involving the so-called subsystems (Control software, Observing Tool [OT], Scheduling, Quality Assurance [QA], Pipeline, and Archive, to name just a few) must be up to the task. Before opening a new capability to the community, ALMA must be able to demonstrate the entire workflow: the correct creation of the observation files; successful, error-free observations; data reduction — first using manual scripts and thereafter with the ALMA Pipeline; and finally data and product ingestion into the ALMA Archive such that it can be delivered to any Principal Investigator (PI) and used in any future Archive mining exercises.
- The ObsMode process therefore follows a yearly structure and is aligned with ALMA observing cycles. For example, the majority of work in 2021 began in October 2020 and will finish in October 2021 (Figure 16). This system includes a two-year lead time, such that any capability planned for release in Cycle 9 (due to start in October 2022) must be fully tested and verified in Cycle 7a. Final tests during the first half of Cycle 8, before the Cycle 9 Call-for-Proposals (CfP) pre-announcement is made, mark the final date to confirm the readiness of a capability for scientific operations. The main considerations throughout the year include:
Figure 16: Simplified schematic of the general ObsMode timeline focused around the year 2021 (Cycle 7 — restarted because of the pandemic), starting from the identification of new capabilities at the end of the previous year and leading up to the point where they are planned for use in Cycle 9, two years later. In reality the EOC is a continual process as there is an intrinsic overlap of testing and development between the years (image credit: ALMA Team)
- Proposed capabilities and priorities: A list is drawn up of capabilities aimed at science operations two years later. Given the ten years of ALMA operations, there is a natural continuation from previous years. ALMA management, together with the science and operations teams, arrange and discuss the priorities with the ALMA Science Advisory Committee (ASAC), which confirms that these align with the community input.
Initial capability plan: Plans are made by the expert teams leading each capability. These must provide a technical summary, identify the on-sky time requirements, and detail each team member's role. Most importantly, the plans set the criteria for declaring a particular capability as ready.
- Test Observations: EOC observations are scheduled to have a minimal impact on standard science observations, being conducted in small time windows or when science observations cannot take place. Where possible, observations use Scheduling Blocks (SBs) constructed with the OT, however some tests require custom command-line scripts to operate ALMA in a manual mode.
- Data reduction and problem reporting: Custom scripts are employed, using the Common Astronomy Software Applications package (CASA; McMullin, 2007) reduction software with extra analysis and heuristics. Extra system-level stability and data-validity checks are also made. EOC teams aim to provide QA-like reduction workflows to enable an easier transition to science operations.
- Technical readiness: In September and October the EOC teams report their findings and provide a technical report to specific expert reviewers. These reports are used for a readiness assessment to confirm whether the capability meets the initial readiness criteria.
- Subsystem impact: Requirements are created continually throughout the year for the subsystems involved. Although developments are continual, a capability can only be declared operational when all subsystems integrate the required modifications. Examples of subsystem changes are: (1) the addition of new OT features that allow SBs to be generated, and (2) modification of the QA2 process to provide the correct reduction path (see, for example, Petry et al., 2020).
- Documentation: Before the CfP is issued, ALMA provides users with a Proposer's Guide2 and a Technical Handbook3. These documents must fully detail and explain any newly offered capabilities.
Focusing on high frequencies and long baselines with band-to-band (B2B)
- The European ARC (ALMA Regional Centre) is particularly involved with EOC activities to offer high-frequency observations (Bands 8, 9, and 10, > 385 GHz) using the most extended array configurations (C-8, C-9 and C-10, with maximal baselines of ~ 8.5, ~ 13.9 and ~ 16.2 km, respectively). Theoretically, the highest frequencies coupled with the longest-baseline array would achieve an angular resolution of 5 milliarcseconds. This translates to sub-au scales for sources within 200 parsecs and would provide the most detailed sub-mm picture of protoplanetary discs. For extragalactic targets, parsec scales could be resolved for sources within 40 Mpc, offering unprecedented details of galactic structures.
Note: As of 19 March 2020, the previously single large ALMA file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the ALMA project mission and its imagery in the period 2020 and 2019, in addition to some of the mission milestones.
•ALMA imagery in the period 2018-2011
Some status and selected observation imagery provided by ALMA in 2021-2019
Only a selected few images can be shown here. The interested reader is referred to the ALMA Press Release site for more details. 27) 28)
• November 4, 2021: A new discovery is shedding light on how fluorine — an element found in our bones and teeth as fluoride — is forged in the Universe. Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, a team of astronomers have detected this element in a galaxy that is so far away its light has taken over 12 billion years to reach us. This is the first time fluorine has been spotted in such a distant star-forming galaxy. 29)
- "We all know about fluorine because the toothpaste we use every day contains it in the form of fluoride," says Maximilien Franco from the University of Hertfordshire in the UK, who led the new study, published today in Nature Astronomy. Like most elements around us, fluorine is created inside stars but, until now, we did not know exactly how this element was produced. "We did not even know which type of stars produced the majority of fluorine in the Universe!"
- Franco and his collaborators spotted fluorine (in the form of hydrogen fluoride) in the large clouds of gas of the distant galaxy NGP–190387, which we see as it was when the Universe was only 1.4 billion years old, about 10% of its current age. Since stars expel the elements they form in their cores as they reach the end of their lives, this detection implies that the stars that created fluorine must have lived and died quickly.
- The team believes that Wolf–Rayet stars, very massive stars that live only a few million years, a blink of the eye in the Universe's history, are the most likely production sites of fluorine. They are needed to explain the amounts of hydrogen fluoride the team spotted, they say. Wolf–Rayet stars had been suggested as possible sources of cosmic fluorine before, but astronomers did not know until now how important they were in producing this element in the early Universe.
- "We have shown that Wolf–Rayet stars, which are among the most massive stars known and can explode violently as they reach the end of their lives, help us, in a way, to maintain good dental health!" jokes Franco.
- Besides these stars, other scenarios for how fluorine is produced and expelled have been put forward in the past. An example includes pulsations of giant, evolved stars with masses up to few times that of our Sun, called asymptotic giant branch stars. But the team believes these scenarios, some of which take billions of years to occur, might not fully explain the amount of fluorine in NGP–190387.
Figure 17: This artist's impression shows NGP–190387, a star-forming, dusty galaxy that is so far away its light has taken over 12 billion years to reach us. ALMA observations have revealed the presence of fluorine in the gas clouds of NGP–190387. To date, this is the most distant detection of the element in a star-forming galaxy, one that we see as it was only 1.4 billion years after the Big Bang — about 10% of the current age of the Universe. The discovery sheds a new light on how stars forge fluorine, suggesting short-lived stars known as Wolf–Rayet are its most likely birthplace (image credit: ESO/M. Kornmesser)
- "For this galaxy, it took just tens or hundreds of millions of years to have fluorine levels comparable to those found in stars in the Milky Way, which is 13.5 billion years old. This was a totally unexpected result," says Chiaki Kobayashi, a professor at the University of Hertfordshire. "Our measurement adds a completely new constraint on the origin of fluorine, which has been studied for two decades."
- The discovery in NGP–190387 marks one of the first detections of fluorine beyond the Milky Way and its neighbouring galaxies. Astronomers have previously spotted this element in distant quasars, bright objects powered by supermassive black holes at the center of some galaxies. But never before had this element been observed in a star-forming galaxy so early in the history of the Universe.
- The team's detection of fluorine was a chance discovery made possible thanks to the use of space and ground-based observatories. NGP–190387, originally discovered with the European Space Agency's Herschel Space Observatory and later observed with the Chile-based ALMA, is extraordinarily bright for its distance. The ALMA data confirmed that the exceptional luminosity of NGP–190387 was partly caused by another known massive galaxy, located between NGP–190387 and the Earth, very close to the line of sight. This massive galaxy amplified the light observed by Franco and his collaborators, enabling them to spot the faint radiation emitted billions of years ago by the fluorine in NGP–190387.
- Future studies of NGP–190387 with the Extremely Large Telescope (ELT) — ESO's new flagship project, under construction in Chile and set to start operations later this decade — could reveal further secrets about this galaxy. "ALMA is sensitive to radiation emitted by cold interstellar gas and dust," says Chentao Yang, an ESO Fellow in Chile. "With the ELT, we will be able to observe NGP–190387 through the direct light of stars, gaining crucial information on the stellar content of this galaxy."
- This research was presented in the paper "The ramp-up of interstellar medium enrichment at z > 4" to appear in Nature Astronomy. 30)
• September 8 2021: New receivers that will allow ALMA to study the early Universe in longest wavelengths yet successfully tested. 31)
- First light has been achieved for a new set of receivers installed on ALMA, in which ESO is a partner. The band 1 receivers pick up radio waves between 6 and 8.5 mm in length, setting a new record for the longest wavelengths that ALMA can observe. They will allow astronomers to view the early, distant Universe and to explore how planets form as never before.
- To capture radiation from cosmic sources over a broad range of wavelengths, ALMA's 66 antennas, located on the Chajnantor plateau in Chile, are equipped with precisely tuned receivers. Each receiver type is sensitive to a particular "band" or range of wavelengths of the electromagnetic spectrum. Until recently, eight out of a planned ten receivers had been mounted on the antennas, covering altogether the window between 0.3 and 3.6 mm (bands 10 to 3).
- Now, thanks to band 1 receivers, ALMA will be equipped to observe at even longer wavelengths of light, opening a new window to explore the Universe. Scientists hope to be able to study gas in the reionization epoch of the Universe, the time when the first stars and galaxies formed. In addition, as it is able to detect larger dust grains than the other ALMA bands, band 1 is ideal to look at the growth of such grains in discs around stars, allowing astronomers to gain more insight into how planets form.
- The development of the ALMA band 1 receivers has been led by Taiwan's Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), accompanied by an international team comprising the National Astronomical Observatory of Japan (NAOJ), the University of Chile, the National Radio Astronomy Observatory (NRAO), the Herzberg Institute of Astrophysics in Canada and the National Chung-Shan Institute of Science and Technology in Taiwan. The University of Chile has been involved in the project since the beginning, helping to develop and produce optical elements such as the lenses and horn antennas for the band 1 receivers.
- Recently, the ALMA board signed a contract for the development of the final, band 2, set of ALMA receivers, which will be led by a consortium of European institutions.
Figure 18: In this image, twenty of the cold cartridges of the band 1 receivers are lined up at the Front-end Testing Lab in Taichung, Taiwan, ready to be taken to Chile and installed on ALMA (Atacama Large Millimeter/submillimeter Array). The band 1 receivers pick up radio waves between 6 and 8.5 mm in length, the longest wavelength that ALMA is able to measure (image credit: ASIAA)
Figure 19: Assembly of ALMA band 1 cartridges. Electronics Technician Nelson Tabilo assembles one of the cold and warm cartridges which form part of the band 1 receivers installed on ALMA. The band 1 receivers pick up radio waves between 6 and 8.5 mm in length, the longest wavelength that ALMA is able to measure [image credit: G. Siringo, ALMA (ESO/NAOJ/NRAO)]
• July 29, 2021: A new study from scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) suggests that previously displaced gases can re-accrete onto galaxies, potentially slowing down the process of galaxy death caused by ram pressure stripping, and creating unique structures more resistant to its effects. 32)
- "Much of the previous work on ram pressure stripped galaxies is focused on the material that gets stripped out of galaxies. In this new work we see some gas that rather than being thrown out of the galaxy never to return is instead moving like a boomerang, being ejected out but then circling around and falling back to its source," said William Cramer, an astronomer at Arizona State University and the lead author on the new study. "By combining Hubble and ALMA data at very high resolution, we are able to prove that this process is happening."
- Ram pressure stripping refers to the process that displaces gas from galaxies, leaving them without the material needed to form new stars. As galaxies move through their galaxy clusters, hot gas known as the intra-cluster medium—or, the space between—acts like a forceful wind, pushing gases out of the traveling galaxies. Over time, this leads to the starvation and "death" of once-active star-forming galaxies. Because ram pressure stripping can speed up the normal life cycle of galaxies and alter the amount of molecular gas within them, it is of particular interest to scientists studying the life, maturation, and death of galaxies.
- "We've seen in simulations that not all of the gas being pushed by ram pressure stripping escapes the galaxy because it has to reach escape velocity in order to actually escape and not fall back. The re-accretion that we're seeing, we believe is from clouds of gas that were pushed out of the galaxy by ram pressure stripping, and didn't achieve escape velocity, so they're falling back," said Jeff Kenney, an astronomer at Yale University, and the co-author on the study. "If you're trying to predict how fast a galaxy is going to stop forming stars over time and transform into a red, or dead galaxy, then you want to understand how effective ram pressure is at stripping the gas out. If you don't know that gas can fall back onto the galaxy and continue to recycle and form new stars, you're going to overpredict the quenching of the stars. Having proof of this process means more accurate timelines for the lifecycle of galaxies."
Figure 20: Shown here in composite view, ALMA data (red/orange) reveals filament structures left behind by ram pressure stripping in a Hubble Space Telescope optical view of NGC4921. Scientists believe that these filaments are formed as magnetic fields in the galaxy prevent some matter from being stripped away [image credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), NASA/ESA/Hubble/K. Cook (LLNL), L. Shatz]
Figure 21: This side-by-side composite shows ALMA (red/orange) data laid over Hubble Space Telescope (optical) images of NGC4921. A new study of the spiral bar galaxy revealed filament structures similar to the Pillars of Creation but significantly larger. These structures are caused by a process known as ram pressure stripping, which pushes gas out of galaxies, leaving them without the material needed to form new stars [image credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), NASA/ESA/Hubble/K. Cook (LLNL), L. Shatz]
- The new study focuses on NGC 4921—a barred spiral galaxy and the largest spiral galaxy in the Coma Cluster—located roughly 320 million light-years from Earth in the constellation Coma Berenices. NGC 4921 is of particular interest to scientists studying the effects of ram pressure stripping because evidence of both the process and its aftermath is abundant.
- "Ram pressure triggers star formation on the side where it is having the greatest impact on the galaxy," said Cramer. "It's easy to identify in NGC 4921 because there are many young blue stars on the side of the galaxy where it's occurring."
- Kenney added that ram pressure stripping in NGC 4921 has created a strong, visible line between where dust still exists in the galaxy and where it doesn't. "There is a strong dust line present, and beyond that there's almost no gas in the galaxy. We think that that part of the galaxy has been almost completely cleaned out by ram pressure."
- Using ALMA's Band 6 receiver, scientists were able to resolve carbon monoxide, the key to "seeing" both those areas of the galaxy devoid of gas, as well as those areas where it is re-accreting. "We know that the majority of molecular gas in galaxies is in the form of hydrogen, but molecular hydrogen is very difficult to observe directly," said Cramer. "Carbon monoxide is commonly used as a proxy for studying molecular gas in galaxies because it is much easier to observe."
- The ability to see more of the galaxy, even at its faintest, unveiled interesting structures likely created in the process of gas displacement, and further immune to its effects. "Ram pressure appears to form unique structures, or filaments in galaxies that are clues as to how a galaxy evolves under a ram pressure wind. In the case of NGC 4921, they bear a striking resemblance to the famous nebula, the Pillars of Creation, although on a much more massive scale," said Cramer. "We think that they are supported by magnetic fields which are preventing them from being stripped away with the rest of the gas."
- Observations revealed that the structures are more than just wisps of gas and dust; the filaments have mass and a lot of it. "These filaments are heavier and stickier—they hold on to their material more tightly than the rest of the galaxy's interstellar medium can do—and they seem to be connected to that big dust ridge both in space and in velocity," said Kenney. "They're more like molasses than smoke. If you just blow on something that is smoke, the smoke is light, and it disperses and goes in all directions. But this is much heavier than that."
- Although a significant breakthrough, the results of the study are only a starting point for Cramer and Kenney, who examined one small part of just one galaxy. "If we want to predict the death rate of galaxies, and the birthrate of new stars, we need to understand if and how much of the material that forms stars, originally lost to ram pressure, is actually recycled back," said Cramer. "These observations are of just one quadrant of NGC 4921. There is likely even more gas falling back into other quadrants. While we have confirmed that some stripped gas can 'rain' back down, we need more observations to quantify how much gas falls back and how many new stars form as a result."
- "A fascinating study, demonstrating the power of ALMA and the benefit of combining its observations with those of a telescope at other wavelengths," added Joseph Pesce, NRAO/ALMA program officer at the NSF. "Ram pressure stripping is an important phenomenon for galaxies in clusters, and understanding the process better allows us to understand galaxy evolution—and nature—better."
- The results of the study was presented in the paper "Molecular gas filaments and fallback in the ram pressure stripped Coma spiral NGC 4921" that will be published in an upcoming edition of The Astrophysical Journal. 33)
• July 22, 2021: Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, astronomers have unambiguously detected the presence of a disc around a planet outside our Solar System for the first time. The observations will shed new light on how moons and planets form in young stellar systems. 34)
- "Our work presents a clear detection of a disc in which satellites could be forming," says Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research published today in The Astrophysical Journal Letters. "Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time," she adds.
- The disc in question, called a circumplanetary disc, surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting a star nearly 400 light-years away. Astronomers had found hints of a "moon-forming" disc around this exoplanet before but, since they could not clearly tell the disc apart from its surrounding environment, they could not confirm its detection — until now.
- In addition, with the help of ALMA, Benisty and her team found that the disc has about the same diameter as the distance from our Sun to the Earth and enough mass to form up to three satellites the size of the Moon.
- But the results are not only key to finding out how moons arise. "These new observations are also extremely important to prove theories of planet formation that could not be tested until now," says Jaehan Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science, USA, and author on the study.
- Planets form in dusty discs around young stars, carving out cavities as they gobble up material from this circumstellar disc to grow. In this process, a planet can acquire its own circumplanetary disc, which contributes to the growth of the planet by regulating the amount of material falling onto it. At the same time, the gas and dust in the circumplanetary disc can come together into progressively larger bodies through multiple collisions, ultimately leading to the birth of moons.
- But astronomers do not yet fully understand the details of these processes. "In short, it is still unclear when, where, and how planets and moons form," explains ESO Research Fellow Stefano Facchini, also involved in the research.
Figure 22: Wide and close-up views of a moon-forming disc as seen with ALMA. This image, taken with ALMA, in which ESO is a partner, shows wide (left) and close-up (right) views of the moon-forming disc surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away. The close-up view shows PDS 70c and its circumplanetary disc center-front, with the larger circumstellar ring-like disc taking up most of the right-hand side of the image. The star PDS 70 is at the center of the wide-view image on the left . — Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disc as they gobbled up material from the disc itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disc, which contributes to the growth of the planet and where moons can form. This circumplanetary disc is as large as the Sun-Earth distance and has enough mass to form up to three satellites the size of the Moon (image credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)
- "More than 4000 exoplanets have been found until now, but all of them were detected in mature systems. PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed," explains Miriam Keppler, researcher at the Max Planck Institute for Astronomy in Germany and one of the co-authors of the study [1].
- "This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation," Facchini adds.
- PDS 70b and PDS 70c, the two planets making up the system, were first discovered using ESO's Very Large Telescope (VLT) in 2018 and 2019 respectively, and their unique nature means they have been observed with other telescopes and instruments many times since [2].
- The latest high resolution ALMA observations have now allowed astronomers to gain further insights into the system. In addition to confirming the detection of the circumplanetary disc around PDS 70c and studying its size and mass, they found that PDS 70b does not show clear evidence of such a disc, indicating that it was starved of dust material from its birth environment by PDS 70c.
- An even deeper understanding of the planetary system will be achieved with ESO's Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Chilean Atacama desert. "The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail," says co-author Richard Teague, a researcher at the Center for Astrophysics | Harvard & Smithsonian, USA. In particular, by using the ELT's Mid-infrared ELT Imager and Spectrograph (METIS), the team will be able to look at the gas motions surrounding PDS 70c to get a full 3D picture of the system.
This research was presented in the paper: 35)
Notes:
[1] Despite the similarity with the Jupiter-Saturn pair, note that the disc around PDS 70c is about 500 times larger than Saturn's rings.
[2] PDS 70b was discovered using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument, while PDS 70c was found using the VLT's Multi Unit Spectroscopic Explorer (MUSE). The two-planet system has been investigated using the X-shooter instrument too, also installed on ESO's VLT.
• June 23, 2021: Using data for more than 500 young stars observed with the Atacama Large Millimeter/submillimeter Array (ALMA), scientists have uncovered a direct link between protoplanetary disk structures—the planet-forming disks that surround stars—and planet demographics. The survey proves that higher mass stars are more likely to be surrounded by disks with "gaps" in them and that these gaps directly correlate to the high occurrence of observed giant exoplanets around such stars. These results provide scientists with a window back through time, allowing them to predict what exoplanetary systems looked like through each stage of their formation. 36)
- The original press release was published by the National Radio Astronomy Observatory (NRAO), an ALMA partner on behalf of North America. 37)
- "We found a strong correlation between gaps in protoplanetary disks and stellar mass, which can be linked to the presence of large, gaseous exoplanets," said Nienke van der Marel, a Banting fellow in the Department of Physics and Astronomy at the University of Victoria in British Columbia, and the primary author on the research. "Higher mass stars have relatively more disks with gaps than lower mass stars, consistent with the already known correlations in exoplanets, where higher mass stars more often host gas-giant exoplanets. These correlations directly tell us that gaps in planet-forming disks are most likely caused by giant planets of Neptune mass and above."
- Gaps in protoplanetary disks have long been considered as overall evidence of planet formation. However, there has been some skepticism due to the observed orbital distance between exoplanets and their stars. "One of the primary reasons that scientists have been skeptical about the link between gaps and planets before is that exoplanets at wide orbits of tens of astronomical units are rare. However, exoplanets at smaller orbits, between one and ten astronomical units, are much more common," said Gijs Mulders, assistant professor of astronomy at Universidad Adolfo Ibáñez in Santiago, Chile, and co-author on the research. "We believe that planets that clear the gaps will migrate inwards later on."
- The new study is the first to show that the number of gapped disks in these regions matches the number of giant exoplanets in a star system. "Previous studies indicated that there were many more gapped disks than detected giant exoplanets," said Mulders. "Our study shows that there are enough exoplanets to explain the observed frequency of the gapped disks at different stellar masses." 38)
Figure 23: Protoplanetary disks are classified into three main categories: transition, ring, or extended. These false-color images from ALMA show these classifications in stark contrast. On left: the ring disk of RU Lup is characterized by narrow gaps thought to be carved by giant planets with masses ranging between a Neptune mass and a Jupiter mass. Middle: the transition disk of J1604.3-2130 is characterized by a large inner cavity thought to be carved by planets more massive than Jupiter, also known as Super-Jovian planets. On right: the compact disk of Sz104 is believed not to contain giant planets, as it lacks the telltale gaps and cavities associated with the presence of giant planets [image credit: ALMA (ESO/NAOJ/NRAO), S. Dagnello (NRAO)]
- The correlation also applies to star systems with low-mass stars, where scientists are more likely to find massive rocky exoplanets, also known as Super-Earths. Van der Marel, who will become an assistant professor at Leiden University in the Netherlands beginning September 2021 said, "Lower mass stars have more rocky Super-Earths—between an Earth mass and a Neptune mass. Disks without gaps, which are more compact, lead to the formation of Super-Earths."
- This link between stellar mass and planetary demographics could help scientists identify which stars to target in the search for rocky planets throughout the Milky Way. "This new understanding of stellar mass dependencies will help to guide the search for small, rocky planets like Earth in the solar neighborhood," said Mulders, who is also a part of the NASA-funded Alien Earths team. "We can use the stellar mass to connect the planet-forming disks around young stars to exoplanets around mature stars. When an exoplanet is detected, the planet-forming material is usually gone. So the stellar mass is a 'tag' that tells us what the planet-forming environment might have looked like for these exoplanets."
- And what it all comes down to is dust. "An important element of planet formation is the influence of dust evolution," said van der Marel. "Without giant planets, dust will always drift inwards, creating the optimal conditions for the formation of smaller, rocky planets close to the star."
- The current research was conducted using data for more than 500 objects observed in prior studies using ALMA's high-resolution Band 6 and Band 7 antennas. At present, ALMA is the only telescope that can image the distribution of millimeter-dust at high enough angular resolution to resolve the dust disks and reveal its substructure, or lack thereof. "Over the past five years, ALMA has produced many snapshot surveys of nearby star-forming regions resulting in hundreds of measurements of disk dust mass, size, and morphology," said van der Marel. "The large number of observed disk properties has allowed us to make a statistical comparison of protoplanetary disks to the thousands of discovered exoplanets. This is the first time that a stellar mass dependency of gapped disks and compact disks has been successfully demonstrated using the ALMA telescope."
- "Our new findings link the beautiful gap structures in disks observed with ALMA directly to the properties of the thousands of exoplanets detected by the NASA Kepler mission and other exoplanet surveys," said Mulders. "Exoplanets and their formation help us place the origins of the Earth and the Solar System in the context of what we see happening around other stars."
• June 8, 2021: A team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) has completed the first census of molecular clouds in the nearby Universe, revealing that contrary to previous scientific opinion, these stellar nurseries do not all look and act the same. In fact, they're as diverse as the people, homes, neighborhoods, and regions that make up our own world. 39)
- Stars are formed out of clouds of dust and gas called molecular clouds, or stellar nurseries. Each stellar nursery in the Universe can form thousands or even tens of thousands of new stars during its lifetime. Between 2013 and 2019, astronomers on the PHANGS— Physics at High Angular Resolution in Nearby GalaxieS— project conducted the first systematic survey of 100,000 stellar nurseries across 90 galaxies in the nearby Universe to get a better understanding of how they connect back to their parent galaxies.
- "We used to think that all stellar nurseries across every galaxy must look more or less the same, but this survey has revealed that this is not the case, and stellar nurseries change from place to place," said Adam Leroy, Associate Professor of Astronomy at Ohio State University (OSU), and lead author of the paper presenting the PHANGS ALMA survey. "This is the first time that we have ever taken millimeter-wave images of many nearby galaxies that have the same sharpness and quality as optical pictures. And while optical pictures show us light from stars, these ground-breaking new images show us the molecular clouds that form those stars."
- The scientists compared these changes to the way that people, houses, neighborhoods, and cities exhibit like-characteristics but change from region to region and country to country.
- "To understand how stars form, we need to link the birth of a single star back to its place in the Universe. It's like linking a person to their home, neighborhood, city, and region. If a galaxy represents a city, then the neighborhood is the spiral arm, the house the star-forming unit, and nearby galaxies are neighboring cities in the region," said Eva Schinnerer, an astronomer at the Max Planck Institute for Astronomy (MPIA) and principal investigator for the PHANGS collaboration "These observations have taught us that the "neighborhood" has small but pronounced effects on where and how many stars are born."
Figure 24: Using ALMA, scientists completed a census of nearly 100 galaxies in the nearby Universe, showcasing their behaviors and appearances. The scientists compared ALMA data to that of the Hubble Space Telescope, shown in composite here. The survey concluded that contrary to popular scientific opinion, stellar nurseries do not all look and act the same. In fact, as shown here, they are as different as the neighborhoods, cities, regions, and countries that make up our own world [image credit: ALMA (ESO/NAOJ/NRAO)/PHANGS, S. Dagnello (NRAO)]
- To better understand star formation in different types of galaxies, the team observed similarities and differences in the molecular gas properties and star formation processes of galaxy disks, stellar bars, spiral arms, and galaxy centers. They confirmed that the location, or neighborhood, plays a critical role in star formation.
- "By mapping different types of galaxies and the diverse range of environments that exist within galaxies, we are tracing the whole range of conditions under which star-forming clouds of gas live in the present-day Universe. This allows us to measure the impact that many different variables have on the way star formation happens," said Guillermo Blanc, an astronomer at the Carnegie Institution for Science, and a co-author on the paper.
- "How stars form, and how their galaxy affects that process, are fundamental aspects of astrophysics," said Joseph Pesce, National Science Foundation's program officer for NRAO/ALMA. "The PHANGS project utilizes the exquisite observational power of the ALMA observatory and has provided remarkable insight into the story of star formation in a new and different way."
- Annie Hughes, an astronomer at L'Institut de Recherche en Astrophysique et Planétologie (IRAP), added that this is the first time scientists have a snapshot of what star-forming clouds are really like across such a broad range of different galaxies. "We found that the properties of star-forming clouds depend on where they are located: clouds in the dense central regions of galaxies tend to be more massive, denser, and more turbulent than clouds that reside in the quiet outskirts of a galaxy. The lifecycle of clouds also depends on their environment. How fast a cloud forms stars and the process that ultimately destroys the cloud both seem to depend on where the cloud lives."
- This is not the first time that stellar nurseries have been observed in other galaxies using ALMA, but nearly all previous studies focused on individual galaxies or part of one. Over a five-year period, PHANGS assembled a full view of the nearby population of galaxies. "The PHANGS project is a new form of cosmic cartography that allows us to see the diversity of galaxies in a new light, literally. We are finally seeing the diversity of star-forming gas across many galaxies and are able to understand how they are changing over time. It was impossible to make these detailed maps before ALMA," said Erik Rosolowsky, Associate Professor of Physics at the University of Alberta, and a co-author on the research. "This new atlas contains 90 of the best maps ever made that reveal where the next generation of stars is going to form."
- For the team, the new atlas doesn't mean the end of the road. While the survey has answered questions about what and where, it has raised others. "This is the first time we have gotten a clear view of the population of stellar nurseries across the whole nearby Universe. In that sense, it's a big step towards understanding where we come from," said Leroy. "While we now know that stellar nurseries vary from place to place, we still do not know why or how these variations affect the stars and planets formed. These are questions that we hope to answer in the near future."
- Ten papers detailing the outcomes of the PHANGS (Physics at High Angular Resolution in Nearby GalaxieS) survey are presented this week at the 238th meeting of the American Astronomical Society. 40)
• April 22, 2021: Using the ALMA Observatory, astronomers found a rotating baby galaxy 1/100th the size of the Milky Way at a time when the Universe was only seven percent of its present age. Assisted by the gravitational lens effect, the team was able to explore for the first time the nature of small and dark "normal galaxies" in the early Universe, representative of the main population of the first galaxies, which greatly advances our understanding of the initial phase of galaxy evolution. 41) 42)
- "Many of the galaxies that existed in the early Universe were so small that their brightness is well below the limit of the current largest telescopes on Earth and in Space, making difficult to study their properties and internal structure," says Nicolas Laporte, a Kavli Senior Fellow at the University of Cambridge. "However, the light coming from the galaxy named RXCJ0600-z6, was highly magnified by gravitational lensing, making it an ideal target for studying the properties and structure of a typical baby galaxies."
- Gravitational lensing is a natural phenomenon in which light emitted from a distant object is bent by the gravity of a massive body such as a galaxy or a galaxy cluster located in the foreground. The name "gravitational lensing" is derived from the fact that the gravity of the massive object acts like a lens. When we look through a gravitational lens, the light of distant objects is magnified and their shapes are stretched. In other words, it is a "natural telescope" floating in space.
- The ALMA Lensing Cluster Survey (ALCS) team used ALMA to search for a large number of galaxies in the early Universe that are enlarged by gravitational lensing. Combining the power of ALMA, with the help of the natural telescopes, the researchers are able to uncover and study fainter galaxies.
- Why is it crucial to explore the faintest galaxies in the early Universe? Theory and simulations predict that the majority of galaxies formed few hundred millions years after the Big-Bang are small, and thus faint. Although several galaxies in the early Universe have been previously observed, those studied were limited to the most massive objects, and therefore the less representative galaxies, in the early Universe, because of telescopes capabilities. The only way to understand the standard formation of the first galaxies, and obtain a complete picture of galaxy formation, is to focus on the fainter and more numerous galaxies.
- The ALCS team performed a large-scale observation program that took 95 hours, which is a very long time for ALMA observations, to observe the central regions of 33 galaxy clusters that could cause gravitational lensing. One of these clusters, called RXCJ0600-2007, is located in the direction of the constellation of Lepus, and has a mass 1000 trillion times that of the Sun. The team discovered a single distant galaxy that is being affected by the gravitational lens created by this natural telescope. ALMA detected the light from carbon ions and stardust in the galaxy and, together with data taken with the Gemini telescope, determined that the galaxy is seen as it was about 900 million years after the Big Bang (12.9 billion years ago). Further analysis of these data suggested that a part of this source is seen 160 times brighter than it is intrinsically.
- By precisely measuring the mass distribution of the cluster of galaxies, it is possible to "undo" the gravitational lensing effect and restore the original appearance of the magnified object. By combining data from Hubble Space Telescope and the European Southern Observatory's Very Large Telescope with a theoretical model, the team succeeded in reconstructing the actual shape of the distant galaxy RXCJ0600-z6. The total mass of this galaxy is about 2 to 3 billion times that of the Sun, which is about 1/100th of the size of our own Milky Way Galaxy.
- What astonished the team is that RXCJ0600-z6 is rotating. Traditionally, gas in the young galaxies was thought to have random, chaotic motion. Only recently has ALMA discovered several rotating young galaxies that have challenged the traditional theoretical framework [2], but these were several orders of magnitude brighter (larger) than RXCJ0600-z6.
- "Our study demonstrates, for the first time, that we can directly measure the internal motion of such faint (less massive) galaxies in the early Universe and compare it with the theoretical predictions", says Kotaro Kohno, a professor at the University of Tokyo and the leader of the ALCS team.
- "The fact that RXCJ0600-z6 has a very high magnification factor also raises expectations for future research," explains Seiji Fujimoto, a DAWN fellow at the Niels Bohr Institute. "This galaxy has been selected, among hundreds, to be observed by the James Webb Space Telescope (JWST), the next generation space telescope to be launched this autumn. Through joint observations using ALMA and JWST, we will unveil the properties of gas and stars in a baby galaxy and its internal motions. When the Thirty Meter Telescope and the Extremely Large Telescope are completed, they may be able to detect clusters of stars in the galaxy, and possibly even resolve individual stars. There is an example of gravitational lensing that has been used to observe a single star 9.5 billion light-years away, and this research has the potential to extend this to less than a billion years after the birth of the Universe."
Figure 25: Image of the galaxy cluster RXCJ0600-2007 taken by the NASA/ESA Hubble Space Telescope, combined with gravitational lensing images of the distant galaxy RXCJ0600-z6, 12.4 billion light-years away, observed by ALMA (shown in red). Due to the gravitational lensing effect by the galaxy cluster, the image of RXCJ0600-z6 was intensified and magnified, and appeared to be divided into three or more parts (image credit: ALMA (ESO/NAOJ/NRAO), Fujimoto et al., NASA/ESA Hubble Space Telescope)
Notes:
[1] The light emitted from carbon ions was originally infrared light with a wavelength of 156 micrometers, but as the Universe expanded, the wavelength extended and became radio waves with a wavelength of 1.1 millimeters, which were detected with ALMA. The redshift of this object is z=6.07. Using the cosmological parameters measured with Planck (H0=67.3km/s/Mpc, Ωm=0.315, Λ=0.685: Planck 2013 Results), we can calculate the distance to the object to be 12.9 billion light-years. (Please refer to "Expressing the distance to remote objects" for the details.)
[2] Using gravitational lensing, ALMA discovered a rotating galaxy similar in size to the Milky Way at about 12.4 billion years ago. (Please refer to the news article "ALMA sees most distant Milky Way look-alike" issued on August 13, 2020). Also, ALMA discovered a rotating galaxy from 12.4 billion years ago without using gravitational lensing. (Please refer to the news article "ALMA Discovers Massive Rotating Disk in Early Universe.")
• March 24, 2021: The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole, has today revealed a new view of the massive object at the center of the Messier 87 (M87) galaxy: how it looks in polarized light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core. 43)
- "We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy," says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.
- On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region — the black hole's shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarized.
- "This work is a major milestone: the polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before," explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that "unveiling this new polarized-light image required years of work due to the complex techniques involved in obtaining and analyzing the data."
- Light becomes polarized when it goes through certain filters, like the lenses of polarized sunglasses, or when it is emitted in hot regions of space where magnetic fields are present. In the same way that polarized sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their view of the region around the black hole by looking at how the light originating from it is polarized. Specifically, polarization allows astronomers to map the magnetic field lines present at the inner edge of the black hole.
Figure 26: This artist's image shows the polarized view of the black hole in M87. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole (image credit: EHT Collaboration)
- "The newly published polarized images are key to understanding how the magnetic field allows the black hole to 'eat' matter and launch powerful jets," says EHT collaboration member Andrew Chael, a NASA Hubble Fellow at the Princeton Center for Theoretical Science and the Princeton Gravity Initiative in the US.
- The bright jets of energy and matter that emerge from M87's core and extend at least 5000 light-years from its centre are one of the galaxy's most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.
- Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don't know exactly how jets larger than the galaxy are launched from its central region, which is comparable in size to the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarized light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.
- The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetized gas can explain what they are seeing at the event horizon.
- "The observations suggest that the magnetic fields at the black hole's edge are strong enough to push back on the hot gas and help it resist gravity's pull. Only the gas that slips through the field can spiral inwards to the event horizon," explains Jason Dexter, Assistant Professor at the University of Colorado Boulder, US, and Coordinator of the EHT Theory Working Group.
- To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world — including the northern Chile-based Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX), in which the European Southern Observatory (ESO) is a partner — to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.
- "With ALMA and APEX, which through their southern location enhance the image quality by adding geographical spread to the EHT network, European scientists were able to play a central role in the research," says Ciska Kemper, European ALMA Program Scientist at ESO. "With its 66 antennas, ALMA dominates the overall signal collection in polarized light, while APEX has been essential for the calibration of the image."
- "ALMA data were also crucial to calibrate, image and interpret the EHT observations, providing tight constraints on the theoretical models that explain how matter behaves near the black hole event horizon," adds Ciriaco Goddi, a scientist at Radboud University and Leiden Observatory, the Netherlands, who led an accompanying study that relied only on ALMA observations. 44)
- The EHT setup allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarized-light image clearly showing that the ring is magnetized. The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organizations and universities worldwide.
- "The EHT is making rapid advancements, with technological upgrades being done to the network and new observatories being added. We expect future EHT observations to reveal more accurately the magnetic field structure around the black hole and to tell us more about the physics of the hot gas in this region," concludes EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica Institute of Astronomy and Astrophysics in Taipei.
• March 18, 2021: Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, a team of astronomers have directly measured winds in Jupiter's middle atmosphere for the first time. By analyzing the aftermath of a comet collision from the 1990s, the researchers have revealed incredibly powerful winds, with speeds of up to 1450 km/hr, near Jupiter's poles. They could represent what the team have described as a "unique meteorological beast in our Solar System". 45)
- Jupiter is famous for its distinctive red and white bands: swirling clouds of moving gas that astronomers traditionally use to track winds in Jupiter's lower atmosphere. Astronomers have also seen, near Jupiter's poles, the vivid glows known as aurorae, which appear to be associated with strong winds in the planet's upper atmosphere. But until now, researchers had never been able to directly measure wind patterns in between these two atmospheric layers, in the stratosphere.
- Measuring wind speeds in Jupiter's stratosphere using cloud-tracking techniques is impossible because of the absence of clouds in this part of the atmosphere. However, astronomers were provided with an alternative measuring aid in the form of comet Shoemaker–Levy 9, which collided with the gas giant in spectacular fashion in 1994. This impact produced new molecules in Jupiter's stratosphere, where they have been moving with the winds ever since.
- A team of astronomers, led by Thibault Cavalié of the Laboratoire d'Astrophysique de Bordeaux in France, have now tracked one of these molecules — hydrogen cyanide — to directly measure stratospheric "jets" on Jupiter. Scientists use the word "jets" to refer to narrow bands of wind in the atmosphere, like Earth's jet streams.
Figure 27: This image shows an artist's impression of winds in Jupiter's stratosphere near the planet's south pole, with the blue lines representing wind speeds. These lines are superimposed on a real image of Jupiter, taken by the JunoCam imager aboard NASA's Juno spacecraft. - Jupiter's famous bands of clouds are located in the lower atmosphere, where winds have previously been measured. But tracking winds right above this atmospheric layer, in the stratosphere, is much harder since no clouds exist there. By analyzing the aftermath of a comet collision from the 1990s and using the ALMA telescope, in which ESO is a partner, researchers have been able to reveal incredibly powerful stratospheric winds, with speeds of up to 1450 km/hr, near Jupiter's poles (image credit: ESO/L. Calçada & NASA/JPL-Caltech/SwRI/MSSS)
- "The most spectacular result is the presence of strong jets, with speeds of up to 400 m/s, which are located under the aurorae near the poles," says Cavalié. These wind speeds, equivalent to about 1450 km/hr, are more than twice the maximum storm speeds reached in Jupiter's Great Red Spot and over three times the wind speed measured on Earth's strongest tornadoes.
- "Our detection indicates that these jets could behave like a giant vortex with a diameter of up to four times that of Earth, and some 900 km in height," explains co-author Bilal Benmahi, also of the Laboratoire d'Astrophysique de Bordeaux. "A vortex of this size would be a unique meteorological beast in our Solar System," Cavalié adds.
- Astronomers were aware of strong winds near Jupiter's poles, but much higher up in the atmosphere, hundreds of kilometers above the focus area of the new study, which is published today in Astronomy & Astrophysics. Previous studies predicted that these upper-atmosphere winds would decrease in velocity and disappear well before reaching as deep as the stratosphere. "The new ALMA data tell us the contrary," says Cavalié, adding that finding these strong stratospheric winds near Jupiter's poles was a "real surprise". 46)
- The team used 42 of ALMA's 66 high-precision antennas, located in the Atacama Desert in northern Chile, to analyze the hydrogen cyanide molecules that have been moving around in Jupiter's stratosphere since the impact of Shoemaker–Levy 9. The ALMA data allowed them to measure the Doppler shift — tiny changes in the frequency of the radiation emitted by the molecules — caused by the winds in this region of the planet. "By measuring this shift, we were able to deduce the speed of the winds much like one could deduce the speed of a passing train by the change in the frequency of the train whistle," explains study co-author Vincent Hue, a planetary scientist at the Southwest Research Institute in the US.
- In addition to the surprising polar winds, the team also used ALMA to confirm the existence of strong stratospheric winds around the planet's equator, by directly measuring their speed, also for the first time. The jets spotted in this part of the planet have average speeds of about 600 km/hr.
- The ALMA observations required to track stratospheric winds in both the poles and equator of Jupiter took less than 30 minutes of telescope time. "The high levels of detail we achieved in this short time really demonstrate the power of the ALMA observations," says Thomas Greathouse, a scientist at the Southwest Research Institute in the US and co-author of the study. "It is astounding to me to see the first direct measurement of these winds."
- These ALMA results open a new window for the study of Jupiter's auroral regions, which was really unexpected just a few months back," says Cavalié. "They also set the stage for similar yet more extensive measurements to be made by the JUICE mission and its Submillimeter Wave Instrument," Great house adds, referring to the European Space Agency's JUICY (JUpiter ICy moons Explorer), which is expected to launch into space next year.
- ESO's ground-based Extremely Large Telescope (ELT), set to see first light later this decade, will also explore Jupiter. The telescope will be capable of making highly detailed observations of the planet's aurorae, giving us further insight into Jupiter's atmosphere.
• January 12, 2021: An international team of astronomers has discovered the most distant quasar yet found — a cosmic monster more than 13 billion light-years from Earth powered by a supermassive black hole more than 1.6 billion times more massive than the Sun and more than 1,000 times brighter than our entire Milky Way Galaxy. 47)
- The quasar, called J0313–1806, is seen as it was when the Universe was only 670 million years old and is providing astronomers with valuable insight on how massive galaxies — and the supermassive black holes at their cores — formed in the early Universe. The scientists presented their findings to the American Astronomical Society's meeting, now underway virtually, and in a paper accepted to the Astrophysical Journal Letters. 48)
- Quasars occur when the powerful gravity of a supermassive black hole at a galaxy's core draws in surrounding material that forms an orbiting disk of superheated material around the black hole. The process releases tremendous amounts of energy, making the quasar extremely bright, often outshining the rest of the galaxy.
- The black hole at the core of J0313–1806 is twice as massive as that of the previous record holder and that fact provides astronomers with a valuable clue about such black holes and their affect on their host galaxies.
Figure 28: Artist's rendition of the most distant quasar found. The new discovery beats the previous distance record for a quasar set three years ago. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile confirmed the distance measurement to high precision (image credit: NOIRLab/NSF/AURA/J. da Silva)
- "This is the earliest evidence of how a supermassive black hole is affecting the galaxy around it," said Feige Wang, a Hubble Fellow at the University of Arizona's Steward Observatory and leader of the research team. "From observations of less distant galaxies, we know that this has to happen, but we have never seen it happening so early in the Universe."
- The huge mass of J0313–1806's black hole at such an early time in the Universe's history rules out two theoretical models for how such objects formed, the astronomers said. In the first of these models, individual massive stars explode as supernovae and collapse into black holes that then coalesce into larger black holes. In the second, dense clusters of stars collapse into a massive black hole. In both cases, however, the process takes too long to produce a black hole as massive as the one in J0313-1806 by the age at which we see it.
- "This tells you that no matter what you do, the seed of this black hole must have formed by a different mechanism," said Xiaohui Fan, also of the University of Arizona. "In this case, it's a mechanism that involves vast quantities of primordial, cold hydrogen gas directly collapsing into a seed black hole."
- The ALMA observations of J0313–1806 provided tantalizing details about the quasar host galaxy, which is forming new stars at a rate 200 times that of our Milky Way. "This is a relatively high star formation rate in galaxies of similar age, and it indicates that the quasar host galaxy is growing very fast," said Jinyi Yang, the second author of the report, who is a Peter A. Strittmatter Fellow at the University of Arizona.
- The quasar's brightness indicates that the black hole is swallowing the equivalent of 25 Suns every year. The energy released by that rapid feeding, the astronomers said, probably is powering a powerful outflow of ionized gas seen moving at about 20 percent of the speed of light.
- Such outflows are thought to be what ultimately stops star formation in the galaxy.
- "We think those supermassive black holes were the reason why many of the big galaxies stopped forming stars at some point," Fan said. "We observe this 'quenching' at later times, but until now, we didn't know how early this process began in the history of the Universe. This quasar is the earliest evidence that quenching may have been happening at very early times."
- This process also will leave the black hole with nothing left to eat and halt its growth, Fan pointed out.
- In addition to ALMA, the astronomers used the 6.5-meter Magellan Baade telescope, the Gemini North telescope and W.M. Keck Observatory in Hawaii, and the Gemini South telescope in Chile.
- The astronomers plan to continue studying J0313-1806 and other quasars with ground-based and space-based telescopes.
• January 11, 2021: "This is the first time we have observed a typical massive star-forming galaxy in the distant Universe about to 'die' because of a massive cold gas ejection," says Annagrazia Puglisi, lead researcher on the new study, from the Durham University, UK, and the Saclay Nuclear Research Centre (CEA-Saclay), France. The galaxy, ID2299, is distant enough that its light takes some 9 billion years to reach us; we see it when the Universe was just 4.5 billion years old. 49)
- The gas ejection is happening at a rate equivalent to 10,000 Suns per year, and is removing an astonishing 46% of the total cold gas from ID2299. Because the galaxy is also forming stars very rapidly, hundreds of times faster than our Milky Way, the remaining gas will be quickly consumed, shutting down ID2299 in just a few tens of million years.
- The event responsible for the spectacular gas loss, the team believes, is a collision between two galaxies, which eventually merged to form ID2299. The elusive clue that pointed the scientists towards this scenario was the association of the ejected gas with a "tidal tail". Tidal tails are elongated streams of stars and gas extending into interstellar space that result when two galaxies merge, and they are usually too faint to see in distant galaxies. However, the team managed to observe the relatively bright feature just as it was launching into space, and were able to identify it as a tidal tail.
- Most astronomers believe that winds caused by star formation and the activity of black holes at the centers of massive galaxies are responsible for launching star-forming material into space, thus ending galaxies' ability to make new stars. However, the new study published today in Nature Astronomy suggests that galactic mergers can also be responsible for ejecting star-forming fuel into space.
Figure 29: Artist's representation of the ID2299 galaxy. Galaxies begin to "die" when they stop forming stars, but until now astronomers had never clearly glimpsed the start of this process in a far-away galaxy. Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, astronomers have seen a galaxy ejecting nearly half of its star-forming gas. This ejection is happening at a startling rate, equivalent to 10 000 Suns-worth of gas a year — the galaxy is rapidly losing its fuel to make new stars. The team believes that this spectacular event was triggered by a collision with another galaxy, which could lead astronomers to rethink how galaxies stop bringing new stars to life (image credit: ESO, M. Kornmesser)
- "Our study suggests that gas ejections can be produced by mergers and that winds and tidal tails can appear very similar," says study co-author Emanuele Daddi of CEA-Saclay. Because of this, some of the teams that previously identified winds from distant galaxies could in fact have been observing tidal tails ejecting gas from them. "This might lead us to revise our understanding of how galaxies 'die'," Daddi adds. 50)
- Puglisi agrees about the significance of the team's finding, saying: "I was thrilled to discover such an exceptional galaxy! I was eager to learn more about this weird object because I was convinced that there was some important lesson to be learned about how distant galaxies evolve."
- This surprising discovery was made by chance, while the team were inspecting a survey of galaxies made with ALMA, designed to study the properties of cold gas in more than 100 far-away galaxies. ID2299 had been observed by ALMA for only a few minutes, but the powerful observatory, located in northern Chile, allowed the team to collect enough data to detect the galaxy and its ejection tail.
- "ALMA has shed new light on the mechanisms that can halt the formation of stars in distant galaxies. Witnessing such a massive disruption event adds an important piece to the complex puzzle of galaxy evolution," says Chiara Circosta, a researcher at the University College London, UK, who also contributed to the research.
- In the future, the team could use ALMA to make higher-resolution and deeper observations of this galaxy, enabling them to better understand the dynamics of the ejected gas. Observations with the future ESO's Extremely Large Telescope could allow the team to explore the connections between the stars and gas in ID2299, shedding new light on how galaxies evolve.
• December 16, 2020: On Friday 11 December, ESO Director General Xavier Barcons received the Order of Bernardo O'Higgins Grand Cross, at a ceremony chaired by Carolina Valdivia, the Chilean Undersecretary of Foreign Affairs. 51)
Figure 30: Xavier Barcons received the Order of Bernardo O'Higgins Grand Cross (photo credit: MINREL)
- The Order Bernardo O'Higgins is a recognition by the Chilean State for foreign citizens' outstanding work in the fields of art, science, education, industry, commerce or humanitarian or social cooperation.
- During the ceremony, held at the Ministry of Foreign Affairs, Barcons received the award for paving new ways towards a deeper understanding between Chile and ESO.
- "On behalf of the Ministry of Foreign Affairs, I would like to extend our sincere thanks for your collaboration and for helping us boost the establishment of a new astronomical paradigm in our country that is based on a regional, fair, inclusive and citizen-oriented approach. This includes cutting edge cooperation agreements, through which Chile –with its National Research and Development Agency– and ESO can equally contribute to highlight the scientific and technological work of our fellow nationals worldwide", said Valdivia during the ceremony.
- ESO's Director General received the award on behalf of the entire organization. "Shortly after I took office as ESO Director General, we embarked on this joint journey towards new and more ambitious horizons, promoting true cooperation to help build towards a future that capitalizes on the knowledge society and the fourth industrial revolution", said Barcons.
• December 8, 2020: Linda Tacconi, a senior astronomer at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has been elected as the next president of ESO's main governing body, the Council. 52)
Figure 31: This photo was taken on the occasion of the publication of the press release First Successful Test of Einstein's General Relativity Near Supermassive Black Hole in July 2018 (photo credit: ESO, M. Zamani)
- "I am honored to have been chosen as president of the ESO Council," Tacconi says. "The coming years represent a very exciting time for the organization, as the first light of ESO's Extremely Large Telescope approaches and current facilities, such as ESO's Very Large Telescope, continue at the forefront of astronomical research. The various ESO observatories will work in synergy, furthering our knowledge of the Universe and strengthening ESO's position as a world-leader in ground-based astronomy. I look forward to sharing in this experience and working with Council to support ESO in maintaining this leadership role."
- Tacconi completed her PhD at the University of Massachusetts, USA, in 1988 and later worked at the Netherlands Foundation for Research in Astronomy, in Dwingeloo, before starting her career at the MPE in 1991. In 2012, she received the Lancelot M. Berkeley New York Community Trust Prize in recognition of her contributions to the field of astronomy, in particular for her work on cold gas in massive star-forming galaxies in the young universe.
- Tacconi has been strongly involved with ESO for a number of years, including in Council where she has served as the German Scientific Delegate since 2016. In addition to her role in Council, she was the chair of ESO's Scientific Technical Committee in 2006–2008. She has also served on several other international committees, including as chair of the Programme Committee and of the Science Advisory Committee of IRAM, an international research institute for millimeter astronomy. She currently serves on the ALMA Board and chairs the Senior Committee for the European Space Agency's (ESA's) Voyage 2050, a programme to define ESA's space science roadmap for 2035–2050.
- Tacconi succeeds Willy Benz from the University of Bern, Switzerland, in leading the ESO Council. ESO's Director General, Xavier Barcons, expresses his thanks to the former president: "I would like to thank Willy on behalf of all ESO staff for his tireless work as Council President, particularly during these unprecedented times. I look forward to continuing working with Linda in her new role as Council President."
• November 30, 2020: Astronomers have detected fast-moving carbon monoxide gas flowing away from a young, low-mass star: a unique stage of planetary system evolution which may provide insight into how our own solar system evolved and suggests that the way systems develop may be more complicated than previously thought. 53)
- Although it remains unclear how the gas is being ejected so fast, the team of researchers, led by the University of Cambridge, believe it may be produced from icy comets being vaporised in the star's asteroid belt. The results have been accepted for publication in the Monthly Notices of the Royal Astronomical Society and will be presented at the Five Years After HL Tau virtual conference.
- The detection was made with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, as part of a survey of young 'class III' stars, reported in an earlier paper. Some of these class III stars are surrounded by debris discs, which are believed to be formed by the ongoing collisions of comets, asteroids and other solid objects, known as planetesimals, in the outer reaches of recently formed planetary systems. The leftover dust and debris from these collisions absorbs light from their central stars and re-radiate that energy as a faint glow that can be studied with ALMA.
- In the inner regions of planetary systems, the processes of planet formation are expected to result in the loss of all the hottest dust, and class IIII stars are those that are left with - at most - dim, cold dust. These faint belts of cold dust are similar to the known debris discs seen around other stars, similar to the Kuiper belt in our own solar system, which is known to host much larger asteroids and comets.
- In the survey, the star in question, 'NO Lup', which is about 70% the mass of our sun, was found to have a faint, low-mass dusty disc, but it was the only class III star where carbon monoxide gas was detected, a first for this type of young star with ALMA. While it is known that many young stars still host the gas-rich planet-forming discs they are born with, NO Lup is more evolved, and might have been expected to have lost this primordial gas after its planets had formed.
Figure 32: A unique stage of planetary system evolution has been imaged by astronomers, showing fast-moving carbon monoxide gas flowing away from a star system over 400 light years away, a discovery that provides an opportunity to study how our own solar system developed (image credit: ALMA, University of Cambridge)
- While the detection of carbon monoxide gas is rare, what made the observation unique was the scale and speed of the gas, which prompted a follow-up study to explore its motion and origins.
- "Just detecting carbon monoxide gas was exciting, since no other young stars of this type had been previously imaged by ALMA," said first author Joshua Lovell, a PhD student from the Cambridge's Institute of Astronomy. "But when we looked closer, we found something even more unusual: given how far away the gas was from the star, it was moving much faster than expected. This had us puzzled for quite some time." 54)
- Grant Kennedy, Royal Society University Research Fellow at the University of Warwick, who led the modelling work on the study, came up with a solution to the puzzle. "We found a simple way to explain it: by modelling a gas ring, but giving the gas an extra kick outward," he said. "Other models have been used to explain young discs with similar mechanisms, but this disc is more like a debris disc where we haven't witnessed winds before. Our model showed the gas is entirely consistent with a scenario in which it's being launched out of the system at around 22 km/s, which is much higher than any stable orbital speed."
- Further analysis also showed that the gas may be produced during collisions between asteroids, or during periods of sublimation – the transition from a solid to a gaseous phase – on the surface of the star's comets, expected to be rich in carbon monoxide ice.
- There has been recent evidence of this same process in our own solar system from NASA's New Horizons mission, when it observed the Kuiper Belt object Ultima Thule in 2019 and found sublimation evolution on the surface of the comet, which happened around 4.5 billion years ago. The same event that vaporised comets in our own solar system billions of years ago may have therefore been captured for the first time over 400 light years away, in a process that may be common around planet-forming stars, and have implications for how all comets, asteroids, and planets evolve.
- "This fascinating star is shedding light on what kind of physical processes are shaping planetary systems shortly after they are born, just after they have emerged from being enshrouded by their protoplanetary disk," said co-author Professor Mark Wyatt, also from the Institute of Astronomy. "While we have seen gas produced by planetesimals in older systems, the shear rate at which gas is being produced in this system and its outflowing nature are quite remarkable, and point to a phase of planetary system evolution that we are witnessing here for the first time."
- While the puzzle isn't fully solved, and further detailed modelling will be required to understand how the gas is being ejected so quickly, what is sure is that this system is set to be the target of more intense follow-up measurements.
- "We're hoping that ALMA will be back online next year, and we'll be making the case to observe this system again in greater detail," said Lovell. "Given how much we have learned about this early stage of planetary system evolution with only a short 30-minute observation, there is still so much more that this system can tell us."
• November 23, 2020: Stellar systems like our own form inside interstellar clouds of gas and dust that collapse producing young stars surrounded by protoplanetary disks. Planets form within these protoplanetary disks, leaving clear gaps, which have been recently observed in evolved systems, at the time when the mother cloud has been cleared out. ALMA has now revealed an evolved protoplanetary disk with a large gap still being fed by the surrounding cloud via large accretion filaments. This shows that accretion of material onto the protoplanetary disk is continuing for times longer than previously thought, affecting the evolution of the future planetary system. 55)
- A team of astronomers led by Dr. Felipe Alves from the Center for Astrochemical Studies (CAS) at the Max Planck Institute for Extraterrestrial Physics (MPE) used the Atacama Large Millimeter/submillimeter Array (ALMA) to study the accretion process in the stellar object [BHB2007] 1, a system located at the tip of the Pipe Molecular Cloud. The ALMA data reveal a disk of dust and gas around the protostar, and large filaments of gas around this disk. The scientists interpret these filaments as accretion streamers feeding the disk with material extracted from the ambient cloud. The disk reprocesses the accreted material, delivering it to the protostar. The structure observed is very unusual for stellar objects at this stage of evolution — with an estimated age of 1,000,000 years — when circumstellar disks are already formed and matured for planet formation. "We were quite surprised to observe such prominent accretion filaments falling into the disk", said Alves. "The accretion filament activity demonstrates that the disk is still growing while simultaneously nurturing the protostar." 56)
Figure 33: This false-color image shows the filaments of accretion around the protostar [BHB2007] 1. The large structures are inflows of molecular gas (CO) nurturing the disk surrounding the protostar. The inset shows the dust emission from the disk, which is seen edge-on. The "holes" in the dust map represent an enormous ringed cavity seen (sideways) in the disk structure (image credit: MPE)
- The team also reports the presence of an enormous cavity within the disk. The cavity has a width of 70 astronomical units, and it encompasses a compact zone of hot molecular gas. In addition, supplementary data at radio frequencies by the Very Large Array (VLA) point to the existence of non-thermal emission in the same spot where the hot gas was detected. These two lines of evidence indicate that a substellar object — a young giant planet or brown dwarf — is present within the cavity. As this companion accretes material from the disk, it heats up the gas and possibly powers strong ionized winds and/or jets. The team estimates that an object with a mass between 4 and 70 Jupiter masses is needed to produce the observed gap in the disk.
- "We present a new case of star and planet formation happening in tandem," states Paola Caselli, director at MPE and head of the CAS group. "Our observations strongly indicate that protoplanetary disks keep accreting material also after planet formation has started. This is important because the fresh material falling onto the disk will affect both the chemical composition of the future planetary system and the dynamical evolution of the whole disk." These observations also put new time constraints for planet formation and disk evolution, shedding light on how stellar systems like our own are sculpted from the original cloud.
Figure 34: Two different observations of the protoplanetary disk show signatures of the formation of a companion to the protostar . The grey scale represents the dust thermal emission from the disk, same as in the inset of Figure 33. The red/blue contours show the molecular CO brightness emission levels from the northern/southern side of the dust cavity observed with ALMA. The brighter CO emission from the south indicates that the gas is hotter there. This location coincides with a zone of non-thermal emission tracing ionized gas (green contours) observed with the VLA (middle), which is observed in addition to the protostar (center of the image). The team proposes that both the ionized gas and the hot molecular gas are due to the presence of a protoplanet or a brown dwarf in the cavity. The configuration of such a system is shown in the sketch on the right (image credit: MPE, illustration: Gabriel A. P. Franco)
• October 21, 2020: New radio images from the Atacama Large Millimeter/submillimeter Array (ALMA) show for the first time the direct effect of volcanic activity on the atmosphere of Jupiter's moon Io. 57)
- Io is the most volcanically active moon in our solar system. It hosts more than 400 active volcanoes, spewing out sulfur gases that give Io its yellow-white-orange-red colors when they freeze out on its surface.
- Although it is extremely thin – about a billion times thinner than Earth's atmosphere – Io has an atmosphere that can teach us about Io's volcanic activity and provide us a window into the exotic moon's interior and what is happening below its colorful crust.
- Previous research has shown that Io's atmosphere is dominated by sulfur dioxide gas, ultimately sourced from volcanic activity. "However, it is not known which process drives the dynamics in Io's atmosphere," said Imke de Pater of the University of California, Berkeley. "Is it volcanic activity, or gas that has sublimated (transitioned from solid to gaseous state) from the icy surface when Io is in sunlight?"
- To distinguish between the different processes that give rise to Io's atmosphere, a team of astronomers used ALMA to make snapshots of the moon when it passed in and out of Jupiter's shadow (they call this an "eclipse").
- "When Io passes into Jupiter's shadow, and is out of direct sunlight, it is too cold for sulfur dioxide gas, and it condenses onto Io's surface. During that time we can only see volcanically-sourced sulfur dioxide. We can therefore see exactly how much of the atmosphere is impacted by volcanic activity," explained Statia Luszcz-Cook from Columbia University, New York.
- Thanks to ALMA's exquisite resolution and sensitivity, the astronomers could, for the first time, clearly see the plumes of sulfur dioxide (SO2) and sulfur monoxide (SO) rise up from the volcanoes. Based on the snapshots, they calculated that active volcanoes directly produce 30-50 percent of Io's atmosphere.
Figure 35: Composite image showing Jupiter's moon Io in radio (ALMA), and optical light (Voyager 1 and Galileo). The ALMA images of Io show for the first time plumes of sulfur dioxide (in yellow) rise up from its volcanoes. Jupiter is visible in the background (Hubble), image credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello; NASA/ESA
- The ALMA images also showed a third gas coming out of volcanoes: potassium chloride (KCl). "We see KCl in volcanic regions where we do not see SO2 or SO," said Luszcz-Cook. "This is strong evidence that the magma reservoirs are different under different volcanoes."
- Io is volcanically active due to a process called tidal heating. Io orbits Jupiter in an orbit that is not quite circular and, like our Moon always faces the same side of Earth, so does the same side of Io always face Jupiter. The gravitational pull of Jupiter's other moons Europa and Ganymede causes tremendous amounts of internal friction and heat, giving rise to volcanoes such as Loki Patera, which spans more than 200 kilometers (124 miles) across. "By studying Io's atmosphere and volcanic activity we learn more about not only the volcanoes themselves, but also the tidal heating process and Io's interior," added Luszcz-Cook.
- A big unknown remains the temperature in Io's lower atmosphere. In future research, the astronomers hope to measure this with ALMA. "To measure the temperature of Io's atmosphere, we need to obtain a higher resolution in our observations, which requires that we observe the moon for a longer period of time. We can only do this when Io is in sunlight since it does not spend much time in eclipse," said de Pater. "During such an observation, Io will rotate by tens of degrees. We will need to apply software that helps us make un-smeared images. We have done this previously with radio images of Jupiter made with ALMA and the Very Large Array (VLA)."
- Imke de Pater and Statia Luszcz-Cook worked with Patricio Rojo of the Universidad de Chile, Erin Redwing of the University of California, Berkeley, Katherine de Kleer of the California Institute of Technology (Caltech), and Arielle Moullet of SOFIA/USRA in California.
- This research titled "ALMA Observations of Io Going into and Coming out of Eclipse" has been accepted for publication in The Planetary Science Journal. Preprint: https://arxiv.org/abs/2009.07729
• October 7, 2020: Astronomers have found compelling evidence that planets start to form while infant stars are still growing. The high-resolution image obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) shows a young proto-stellar disk with multiple gaps and rings of dust. This new result, just published in Nature, shows the youngest and most detailed example of dust rings acting as cosmic cradles, where the seeds of planets form and take hold. 58) 59)
- An international team of scientists led by Dominique Segura-Cox at the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany targeted the proto-star IRS 63 with the ALMA radio observatory. This system is 470 light years from Earth and located deep within the dense L1709 interstellar cloud in the Ophiuchus constellation. Proto-stars as young as IRS 63 are still swaddled in a large and massive blanket of gas and dust called an envelope, and the proto-star and disk feed from this reservoir of material.
Figure 36: The dense L1709 region of the Ophiuchus Molecular Cloud, mapped by the Herschel Space Telescope, which surrounds and ... [more], image credit: MPE, D. Segura-Cox, Herschel data from ESA/Herschel/SPIRE/PACS/D. Arzoumanian)
- In systems older than 1,000,000 years, after the proto-stars have finished gathering most of their mass, rings of dust have been previously detected in great numbers. IRS 63 is different: at under 500,000 years old, it is less than half the age of other young stars with dust rings and the proto-star will still grow significantly in mass. "The rings in the disk around IRS 63 are so young," emphasizes Segura-Cox. "We used to think that stars entered adulthood first and then were the mothers of planets that came later. But now we see that proto-stars and planets grow and evolve together from early times, like siblings."
- Planets face some serious obstacles during their earliest stages of formation. They have to grow from tiny dust particles, smaller than household dust here on Earth. "The rings in the disk of IRS 63 are vast pile-ups of dust, ready to combine into planets," notes co-author Anika Schmiedeke at MPE. However, even after the dust clumps together to form a planet embryo, the still-forming planet could disappear by spiraling inwards and being consumed by the central proto-star. If planets do start to form very early and at large distances from the proto-star, they may better survive this process.
Figure 37: The ALMA image of the young planet-forming dust rings surrounding the IRS 63 proto-star, which is less than 500,000 years old (image credit: MPE, D. Segura-Cox)
- The team of researchers found that there is about 0.5 Jupiter masses of dust in the young disk of IRS 63 further than 20 au from its center (at a distance similar to the Uranus orbit in our solar system). That is not counting the amount of gas, which could add up to 100 times more material. It takes at least 0.03 Jupiter masses of solid material to form a planet core that will efficiently accrete gas and grow to form a giant gas planet. Team member Jaime Pineda at MPE adds, "These results show that we must focus on the youngest systems to truly understand planet formation." For example, there is growing evidence that Jupiter may have actually formed much farther out in the Solar System, beyond the Neptune orbit, and then migrated inwards to its present location. Similarly, the dust surrounding IRS 63 shows that there is enough material far from the proto-star and at a stage young enough that there is a chance for this Solar System analogue to form planets in the way that Jupiter is suspected to have formed.
- "The size of the disk is very similar to our own Solar System," Segura-Cox explains. "Even the mass of the proto-star is just a little less than our Sun's. Studying such young planet-forming disks around proto-stars can give us important insights into our own origins."
Figure 38: The rings and gaps in the IRS 63 dust disk compared to a sketch of orbits in our own Solar System at the same scale ... [more], image credit: MPE, D. Segura-Cox)
• September 18, 2020: Astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe a set of stellar winds around aging stars and present an explanation for the mesmerizing shapes of planetary nebulae. Contrary to common consensus, the team found that stellar winds are not spherical but have a form similar to that of planetary nebulae. The team concludes that interaction with an accompanying star or exoplanet shapes both the stellar winds and planetary nebulae. The findings were published in Science. 60) 61) 62)
- Dying stars swell and cool to eventually become red giants. They produce stellar winds, flows of particles that the star expels, which causes them to lose mass. Because detailed observations were lacking, astronomers have always assumed that these winds were spherical, like the stars they surround. As the star evolves further, it heats up again, and the stellar radiation causes the expanding ejected layers of stellar material to glow, forming a planetary nebula.
- For centuries, astronomers were in the dark about the extraordinary variety of colorful shapes observed in planetary nebulas. The nebulae all seem to have a certain symmetry but are rarely round. "The Sun – which will ultimately become a red giant – is as round as a billiard ball, so we wondered: how can such a star produce all these different shapes?" says corresponding author Leen Decin (KU Leuven).
- Her team observed stellar winds around cool red giant stars with the ALMA observatory in Chile, the world's largest radio telescope. For the first time, they gathered an extensive, detailed collection of observations. Each of them made using the same method, crucial to compare the data, and exclude biases directly.
- What the astronomers saw surprised them. "We noticed these winds are anything but symmetrical or round," Professor Decin says. "Some of them are quite similar in shape to planetary nebulae."
Figure 39: This image gallery of stellar winds around cool ageing stars shows a variety of morphologies, including disks, cones, and spirals. The blue color represents material that is coming towards you; red is material that is moving away from you (image credit: L. Decin, ESO/ALMA)
- The researchers could even identify different categories of shapes. "Some stellar winds were disk-shaped, others contained spirals, and we identified cones in a third group." This is a clear indication that the shapes weren't created randomly. The team realized that other low-mass stars or even heavy planets in the dying star's vicinity were causing the different patterns. These companions are too small and dim to detect directly. "Just like how a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind," Decin explains.
- The team put this theory into models, and indeed: the shape of the stellar winds can be explained by the companions surrounding them. The rate at which the cool evolved star is losing its mass due to the stellar wind is an important parameter.
- Up until now, calculations about the evolution of stars were based on the assumption that aging Sun-like stars have spherical stellar winds. "Our findings change a lot. Since the complexity of stellar winds was not accounted for in the past, any previous mass-loss rate estimate of old stars could be wrong by up to a factor of 10." The team is now doing further research to see how this might impact calculations of other crucial characteristics of stellar and galactic evolution.
- The study also helps envision what the Sun might look like when it dies in 7000 million years. "Jupiter or even Saturn – because they have such a big mass – are going to influence whether the Sun spends its last millennia at the heart of a spiral, a butterfly, or any of the other entrancing shapes we see in planetary nebulae today," Decin notes. "Our calculations now indicate that a weak spiral will form in the stellar wind of the old dying Sun."
- "We were very excited when we explored the first images," says co-author Miguel Montargès (KU Leuven). "Each star, which was only a number before, became an individual by itself. Now, to us, they have their own identity. This is the magic of having high-precision observations: stars are no longer just points anymore."
- The study is part of the ATOMIUM project, which aims to learn more about the physics and chemistry of old stars. "Cool aging stars are considered boring, old and simple, but we now prove that they are not: they tell the story of what comes after. It took us some time to realize that stellar winds can have the shape of rose petals (see, for example, the stellar wind of R Aquilae). But, as Antoine de Saint-Exupéry said in his book Le Petit Prince: 'C'est le temps que tu as perdu pour ta rose, qui fait ta rose si importante' – 'It's the time you spent on your rose that makes your rose so important,'" Decin concludes.
• August 12, 2020: Astronomers using the ALMA (Atacama Large Millimeter/submillimeter Array), in which the European Southern Observatory (ESO) is a partner, have revealed an extremely distant and therefore very young galaxy that looks surprisingly like our Milky Way. The galaxy is so far away its light has taken more than 12 billion years to reach us: we see it as it was when the Universe was just 1.4 billion years old. It is also surprisingly unchaotic, contradicting theories that all galaxies in the early Universe were turbulent and unstable. This unexpected discovery challenges our understanding of how galaxies form, giving new insights into the past of our Universe. 63)
- "This result represents a breakthrough in the field of galaxy formation, showing that the structures that we observe in nearby spiral galaxies and in our Milky Way were already in place 12 billion years ago," says Francesca Rizzo, PhD student from the Max Planck Institute for Astrophysics in Germany, who led the research published today in Nature. While the galaxy the astronomers studied, called SPT0418-47, doesn't appear to have spiral arms, it has at least two features typical of our Milky Way: a rotating disc and a bulge, the large group of stars packed tightly around the galactic center. This is the first time a bulge has been seen this early in the history of the Universe, making SPT0418-47 the most distant Milky Way look-alike.
- "The big surprise was to find that this galaxy is actually quite similar to nearby galaxies, contrary to all expectations from the models and previous, less detailed, observations," says co-author Filippo Fraternali, from the Kapteyn Astronomical Institute, University of Groningen in the Netherlands. In the early Universe, young galaxies were still in the process of forming, so researchers expected them to be chaotic and lacking the distinct structures typical of more mature galaxies like the Milky Way.
- Studying distant galaxies like SPT0418-47 is fundamental to our understanding of how galaxies formed and evolved. This galaxy is so far away we see it when the Universe was just 10% of its current age because its light took 12 billion years to reach Earth. By studying it, we are going back to a time when these baby galaxies were just beginning to develop.
Figure 40: Astronomers using ALMA, in which the ESO is a partner, have revealed an extremely distant galaxy that looks surprisingly like our Milky Way. The galaxy, SPT0418-47, is gravitationally lensed by a nearby galaxy, appearing in the sky as a near-perfect ring of light (image credit: ALMA (ESO/NAOJ/NRAO), Rizzo et al.)
- Because these galaxies are so far away, detailed observations with even the most powerful telescopes are almost impossible as the galaxies appear small and faint. The team overcame this obstacle by using a nearby galaxy as a powerful magnifying glass — an effect known as gravitational lensing — allowing ALMA to see into the distant past in unprecedented detail. In this effect, the gravitational pull from the nearby galaxy distorts and bends the light from the distant galaxy, causing it to appear misshapen and magnified.
- The gravitationally lensed, distant galaxy appears as a near-perfect ring of light around the nearby galaxy, thanks to their almost exact alignment. The research team reconstructed the distant galaxy's true shape and the motion of its gas from the ALMA data using a new computer modelling technique. "When I first saw the reconstructed image of SPT0418-47 I could not believe it: a treasure chest was opening," says Rizzo.
- "What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disc ever observed in the early Universe," stated co-author Simona Vegetti, also from the Max Planck Institute for Astrophysics. "This result is quite unexpected and has important implications for how we think galaxies evolve." The astronomers note, however, that even though SPT0418-47 has a disc and other features similar to those of spiral galaxies we see today, they expect it to evolve into a galaxy very different from the Milky Way, and join the class of elliptical galaxies, another type of galaxies that, alongside the spirals, inhabit the Universe today.
- This unexpected discovery suggests the early Universe may not be as chaotic as once believed and raises many questions on how a well-ordered galaxy could have formed so soon after the Big Bang. This ALMA finding follows the earlier discovery announced in May of a massive rotating disc seen at a similar distance. SPT0418-47 is seen in finer detail, thanks to the lensing effect, and has a bulge in addition to a disc, making it even more similar to our present-day Milky Way than the one studied previously.
- Future studies, including with ESO's Extremely Large Telescope, will seek to uncover how typical these 'baby' disc galaxies really are and whether they are commonly less chaotic than predicted, opening up new avenues for astronomers to discover how galaxies evolved. 64)
• July 9, 2020: At this time, the COVID-19 pandemic continues to affect the lives of ALMA staff and users around the world. Although in some of the ALMA regions the situation is slowly improving, in other regions, including Chile, the evolution of the outbreak remains highly uncertain. 65)
- Because of the on-going situation in Chile, ALMA operations unfortunately remain suspended. ALMA staff continue to monitor the situation very carefully and work on the development of detailed plans for the return to operations, which will be initiated when the situation allows. We will keep updating the user community on the developments.
- Northern Chile was recently hit by a magnitude 6.8 earthquake. Fortunately this caused no injuries to ALMA staff and no serious damage at the ALMA site. This was followed by extremely high winds, that led to some minor damage at the ALMA OSF (Operations Support Facility).
- As always, the ALMA Regional Centers provide support to their respective communities, and can assist in the analysis of your data and help with archive research projects. If you have any questions on this, or comments or concerns related to the situation at ALMA, please contact the ALMA helpdesk.
• July 2, 2020: Astronomers created a stunning new image showing celestial fireworks in star cluster G286.21+0.17. 66)
- Most stars in the universe, including our Sun, were born in massive star clusters. These clusters are the building blocks of galaxies, but their formation from dense molecular clouds is still largely a mystery.
- The image of cluster G286.21+0.17 (Figure 41) is a multiwavelength mosaic. The cluster is located in the Carina region of our galaxy, about 8000 light-years away.
Figure 41: Star cluster G286.21+0.17, caught in the act of formation. This is a multiwavelength mosaic of more than 750 ALMA radio images, and 9 Hubble infrared images. ALMA shows molecular clouds (purple) and Hubble shows stars and glowing dust (yellow and red), image credit: ALMA (ESO/NAOJ/NRAO), Y. Cheng et al.; NRAO/AUI/NSF, S. Dagnello; NASA/ESA Hubble)
- Dense clouds made of molecular gas (purple 'fireworks streamers') are revealed by ALMA. The telescope observed the motions of turbulent gas falling into the cluster, forming dense cores that ultimately create individual stars.
- The stars in the image are revealed by their infrared light, as seen by Hubble, including a large group of stars bursting out from one side of the cloud. The powerful winds and radiation from the most massive of these stars are blasting away the molecular clouds, leaving faint wisps of glowing, hot dust (shown in yellow and red).
- "This image shows stars in various stages of formation within this single cluster," said Yu Cheng of the University of Virginia in Charlottesville, Virginia, and lead author of two papers published in The Astrophysical Journal. 67) 68)
- Hubble revealed about a thousand newly-formed stars with a wide range of masses. Additionally, ALMA showed that there is a lot more mass present in dense gas that still has to undergo collapse. "Overall the process may take at least a million years to complete," Cheng added.
- "This illustrates how dynamic and chaotic the process of star birth is," said co-author Jonathan Tan of Chalmers University in Sweden and the University of Virginia and principal investigator of the project. "We see competing forces in action: gravity and turbulence from the cloud on one side, and stellar winds and radiation pressure from the young stars on the other. This process sculpts the region. It is amazing to think that our own Sun and planets were once part of such a cosmic dance."
- "The phenomenal resolution and sensitivity of ALMA are evident in this stunning image of star formation," said Joe Pesce, NSF Program Officer for NRAO/ALMA. "Combined with the Hubble Space Telescope data we can clearly see the power of multiwavelength observations to help us understand these fundamental universal processes."
• July 6, 2020: The molecular gas in galaxies is organized into a hierarchy of structures. The molecular material in giant molecular gas clouds travels along intricate networks of filamentary gas lanes towards the congested centers of gas and dust where it is compressed into stars and planets, much like the millions of people commuting to cities for work around the world. To better understand this process, a team of astronomers led by Jonathan Henshaw at Max Planck Institute for Astronomy (MPIA) have measured the motion of gas flowing from galaxy scales down to the scales of the gas clumps within which individual stars form. Their results show that the gas flowing through each scale is dynamically interconnected: while star and planet formation occurs on the smallest scales, this process is controlled by a cascade of matter flows that begin on galactic scales. These results are published today in the scientific journal Nature Astronomy. 69) 70)
- The scientists use data from the following observatories: Atacama Large Millimeter/submillimeter Array (ALMA), Morita Atacama Compact Array, Five College Radio Astronomy Observatory (FCRAO), Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer, Mopra Radio Telescope, and HSO (Herschel Space Observatory).
- The molecular gas in galaxies is set into motion by physical mechanisms such as galactic rotation, supernova explosions, magnetic fields, turbulence, and gravity, shaping the structure of the gas. Understanding how these motions directly impact star and planet formation is difficult, because it requires quantifying gas motion over a huge range in spatial scale, and then linking this motion to the physical structures we observe. Modern astrophysical facilities now routinely map huge areas of the sky, with some maps containing millions of pixels, each with hundreds to thousands of independent velocity measurements. As a result, measuring these motions is both scientifically and technologically challenging.
- In order to address these challenges, an international team of researchers led by Jonathan Henshaw at the MPIA in Heidelberg set out to measure gas motions throughout a variety of different environments using observations of the gas in the Milky Way and a nearby galaxy. They detect these motions by measuring the apparent change in the frequency of light emitted by molecules caused by the relative motion between the source of the light and the observer; a phenomenon known as the Doppler effect. By applying novel software designed by Henshaw and Ph.D. student Manuel Riener (a co-author on the paper; also at MPIA), the team were able to analyze millions of measurements. "This method allowed us to visualize the interstellar medium in a new way," says Henshaw.
- The researchers found that cold molecular gas motions appear to fluctuate in velocity, reminiscent in appearance of waves on the surface of the ocean. These fluctuations represent gas motion. "The fluctuations themselves weren't particularly surprising, we know that the gas is moving," says Henshaw. Steve Longmore, co-author of the paper, based at Liverpool John Moores University, adds, "What surprised us was how similar the velocity structure of these different regions appeared. It didn't matter if we were looking at an entire galaxy or an individual cloud within our own galaxy, the structure is more or less the same."
- To better understand the nature of the gas flows, the team selected several regions for close examination, using advanced statistical techniques to look for differences between the fluctuations. By combining a variety of different measurements, the researchers were able to determine how the velocity fluctuations depend on the spatial scale.
Figure 42: Image of the molecular gas (carbon monoxide) distribution in the southern spiral arm of the galaxy NGC 4321 spanning roughly 15,000 light years across. The bright spots indicate giant molecular clouds that are semi-regularly spaced inside the ridge of more dilute gas inside the spiral arm. The cyan circles depict the locations of star forming complexes (image credit: J. Henshaw/MPIA)
- "A neat feature of our analysis techniques is that they are sensitive to periodicity," explains Henshaw. "If there are repeating patterns in your data, such as equally spaced giant molecular clouds along a spiral arm, we can directly identify the scale on which the pattern repeats." The team identified three filamentary gas lanes, which, despite tracing vastly different scales, all seemed to show structure that was roughly equidistantly spaced along their crests, like beads on a string, whether it was giant molecular clouds along a spiral arm or tiny "cores" forming stars along a filament.
- The team discovered that the velocity fluctuations associated with equidistantly spaced structure all showed a distinctive pattern. "The fluctuations look like waves oscillating along the crests of the filaments, they have a well-defined amplitude and wavelength," says Henshaw adding, "The periodic spacing of the giant molecular clouds on large-scales or individual star-forming cores on small-scales is probably the result of their parent filaments becoming gravitationally unstable. We believe that these oscillatory flows are the signature of gas streaming along spiral arms or converging towards the density peaks, supplying new fuel for star formation."
- In contrast, the team found that the velocity fluctuations measured throughout giant molecular clouds, on scales intermediate between entire clouds and the tiny cores within them, show no obvious characteristic scale. Diederik Kruijssen, co-author of the paper based at Heidelberg University explains: "The density and velocity structures that we see in giant molecular clouds are 'scale-free', because the turbulent gas flows generating these structures form a chaotic cascade, revealing ever smaller fluctuations as you zoom in – much like a Romanesco broccoli, or a snowflake. This scale-free behavior takes place between two well-defined extremes: the large scale of the entire cloud, and the small scale of the cores forming individual stars. We now find that these extremes have well-defined characteristic sizes, but in between them chaos rules."
Figure 43: Visualization of the observed velocity flows in the spiral galaxy NGC 4321, measured using the radio emission of the molecular gas (carbon monoxide): along the vertical axis, this image shows the velocities of the gas, while the horizontal axis represents the spatial extent of the galaxy. The wave-like oscillations in gas velocity are visible throughout the galaxy (image credit: T. Müller/J. Henshaw/MPIA)
• June 16, 2020: An international team of astronomers has created the most detailed map yet of the atmosphere of the red supergiant star Antares. The unprecedented sensitivity and resolution of both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation's Karl G. Jansky Very Large Array (VLA) revealed the size and temperature of Antares' atmosphere from just above the star's surface, throughout its chromosphere, and all the way out to the wind region. 71) 72)
- Red supergiant stars, like Antares and its more well-known cousin Betelgeuse, are huge, relatively cold stars at the end of their lifetime. They are on their way to run out of fuel, collapse, and become supernovae. Through their vast stellar winds, they launch heavy elements into space, thereby playing an important role in providing the essential building blocks for life in the universe. But it is a mystery how these enormous winds are launched. A detailed study of the atmosphere of Antares, the closest supergiant star to Earth, provides a crucial step towards an answer.
- The ALMA and VLA map of Antares is the most detailed radio map yet of any star, other than the Sun. ALMA observed Antares close to its surface (its optical photosphere) in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star's atmosphere further out. As seen in visible light, Antares' diameter is approximately 700 times larger than the Sun. But when ALMA and the VLA revealed its atmosphere in radio light, the supergiant turned out to be even more gigantic.
- "The size of a star can vary dramatically depending on what wavelength of light it is observed with," explained Eamon O'Gorman of the Dublin Institute for Advanced Studies in Ireland and lead author of the study published in the June 16 edition of the journal Astronomy & Astrophysics. "The longer wavelengths of the VLA revealed the supergiant's atmosphere out to nearly 12 times the star's radius."
Figure 44: Radio images of Antares with ALMA and the VLA. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star's atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B (image credit: ALMA (ESO/NAOJ/NRAO), E. O'Gorman; NRAO/AUI/NSF, S. Dagnello)
- The radio telescopes measured the temperature of most of the gas and plasma in Antares' atmosphere. Most noticeable was the temperature in the chromosphere. This is the region above the star's surface that is heated up by magnetic fields and shock waves created by the vigorous roiling convection at the stellar surface – much like the bubbling motion in a pot of boiling water. Not much is known about chromospheres, and this is the first time that this region has been detected in radio waves.
Figure 45: Artist impression of the atmosphere of Antares. As seen with the naked eye (up until the photosphere), Antares is around 700 times larger than our sun, big enough to fill the solar system beyond the orbit of Mars (Solar System scale shown for comparison). But ALMA and VLA showed that its atmosphere, including the lower and upper chromosphere and wind zones, reaches out 12 times farther than that (image credit: NRAO/AUI/NSF, S. Dagnello)
- Thanks to ALMA and the VLA, the scientists discovered that the star's chromosphere extends out to 2.5 times the star's radius (our Sun's chromosphere is only 1/200th of its radius). They also found that the temperature of the chromosphere is lower than previous optical and ultraviolet observations have suggested. The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun's chromosphere reaches temperatures of almost 20,000 degrees Celsius.
- "We found that the chromosphere is 'lukewarm' rather than hot, in stellar temperatures," said O'Gorman. "The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star's atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma."
- "We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere," said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile who previously observed Antares' atmosphere in infrared light. "Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can't observe these dots individually, but we want to try that in future studies."
- In the ALMA and VLA data, astronomers for the first time saw a clear distinction between the chromosphere and the region where winds start to form. In the VLA image, a huge wind is visible, ejected from Antares and lit up by its smaller but hotter companion star Antares B.
- "When I was a student, I dreamt of having data like this," said co-author Graham Harper of the University of Colorado, Boulder. "Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected."
- "Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard," said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998.
• June 9, 2020: High-resolution observations of a young star forming system clearly unveil a pair of protostars at their earliest stages of evolution deeply embedded within the source IRAS 16293-2422 in the Ophiuchus molecular cloud. The team led by the MPE (Max Planck Institute for Extraterrestrial Physics) in Garching, Germany, used the ALMA interferometer not only to pin down the source configuration, but also to measure the gas and stellar kinematics, determining the mass of the young binary. The two close protostars are somewhat heavier than previously thought and they revolve around each other once in about 400 years. 73)
- The system called IRAS 16293-2422 is one of the brightest star-forming regions in our neighborhood. It is located in the Ophiuchus molecular cloud at a distance of about 460 light-years and has been widely studied, also because it shows strong emission of numerous complex organic molecules, building blocks of pre-biotic species. However, until now the detailed configuration of the region was unclear, with observations at different wavelengths showing multiple compact sources at slightly different locations. This confusion was due to the large amount of material in front of the nascent protostars, expected at these earliest stages of formation.
- An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now obtained high-resolution radio observations with the ALMA interferometer, which clearly reveals two compact sources A1 and A2 in addition to the well-known protostar B (see Figure 46). "Our observations confirm the location of the two close protostars and reveal that each is surrounded by a very small dust disk. Both, in turn, are in turn embedded in a large amount of material showing complex patterns" remarks María José Maureira from MPE, the lead author of the study. 74)
Figure 46: Zoom into the Ophiuchus molecular cloud, highlighting the star forming system IRAS 16293-2422 with the protostar B in the upper right corner and the now clearly identified protostars A1 and A2 on the bottom left. The binary system is shown also in a further zoom-in panel (image credit: MPE; background: ESO/Digitized Sky Survey 2, Davide De Martin)
Figure 47: Detailed view of the binary protostar system with a size comparison to our solar system. The separation between the sources A1 and A2 is roughly the diameter of the Pluto orbit. The size of the disk around A1 (unresolved) is about the diameter of the asteroid belt. The size of the disk around A2 is about the diameter of the Saturn orbit (image credit: MPE)
- The source A1 has a mass of a bit less than 1 solar mass and is embedded in a small dust disk about the size of the asteroid belt; the source A2 has a mass of about 1.4 solar masses and is embedded in a somewhat larger disk (see Figure 47). Interestingly, this disk around A2 also appears at an angle compared to the overall orientation of the larger cloud structure, while the disk around the source B – at a much larger distance – is seen face-on, indicating a rather chaotic formation history.
- In addition to direct imaging of the dust emission, the team also obtained information on the motion of the gas around the stars through observations of spectral lines of organic molecules, which well trace the high-density region surrounding the discovered binary system. This allowed them to get an independent mass measurement and to confirm that A1 and A2 form a bound pair.
- Combining their latest observations with data collected over the past 30 years, the team found that the two stars orbit each other once every 360 years at a distance similar to the extent of Pluto's orbit, where the orbit is tilted by about 60° (see Figure 48). "This is the first time that we were able to derive the full orbital parameters of a binary system at this early stage of star formation," points out Jaime Pineda from MPE, who contributed to the modelling.
- "With these results we are finally able to dive into one of the most embedded and youngest proto-stellar systems, unveiling its dynamical structure and complex morphology, where we clearly see filamentary material connecting the circumstellar disks to the surrounding region and likely to the cirbumbinary disk. The small disks are probably still being fed and growing!" emphasizes Paola Caselli, director at MPE and head of the Center for Astrochemical Studies. "This was only possible thanks to the great sensitivity of ALMA and the observations of molecules which uniquely trace these dense regions. Molecules send us signals at very specific frequencies, and, following changes of such frequencies across the region (due to internal motions) one can reconstruct the complex kinematics of the system. This is the power of astrochemistry."
Figure 48: Relative motion of A1 (blue) with respect to A2 (red) overlaid on the ALMA continuum observation. The visual impression that A1 orbits around A2 is confirmed through a detailed analysis of the motion of the protostars over a 30-year period (image credit: MPE)
Submillimeter-scale Innovation and Unlock New Design Possibilities
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