ALMA (Atacama Large Millimeter/submillimeter Array)
ALMA (Atacama Large Millimeter/submillimeter Array)
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 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.
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)
Major ALMA Facilities
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 3: 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 4: Photo of the ALMA antenna array (image credit: ALMA partnership, Ref. 3)
Figure 5: 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 6: 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 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 7: 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 8). 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 is latitude: -23.029° ; longitude: -67.755°
Figure 8: 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.
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 9 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 9: 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 10: 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 11: 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 12. 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 12: Schematic of the ALMA signal processing and data transfer from the Front End to the Correlator (image credit: ALMA partnership)
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. 24)
Figure 13: 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.
Some selected observation imagery provided by ALMA
• May 10, 2019: Astronomers map the substance aluminum monoxide (AlO) in a cloud around a distant young star—Origin Source I. The finding clarifies some important details about how our solar system, and ultimately we, came to be. The cloud's limited distribution suggests AlO gas rapidly condenses to solid grains, which hints at what an early stage of our solar evolution looked like. 27) 28)
- Professor Shogo Tachibana of the University of Tokyo Organization for Planetary and Space Science has a passion for space. From small things like meteorites to enormous things like stars and nebulae—huge clouds of gas and dust in space—he is driven to explore our solar system's origins.
- "I have always wondered about the evolution of our solar system, of what must have taken place all those billions of years ago," he said. "This question leads me to investigate the physics and chemistry of asteroids and meteorites."
- Space rocks of all kinds greatly interest astronomers as these rocks can remain largely unchanged since the time our sun and planets formed from a swirling cloud of gas and dust. They contain records of the conditions at that time—generally considered to be 4.56 billion years ago—and their properties such as composition can tell us about these early conditions.
- "On my desk is a small piece of the Allende meteorite, which fell to Earth in 1969. It's mostly dark but there are some scattered white inclusions (foreign bodies enclosed in the rock), and these are important," continued Tachibana. "These speckles are calcium and aluminum-rich inclusions (CAIs), which were the first solid objects formed in our solar system."
- Minerals present in CAIs indicate that our young solar system must have been extremely hot. Physical techniques for dating these minerals reveal a fairly specific age for the solar system. However, Tachibana and colleagues wished to expand on the details of this stage of evolution.
Figure 14: The white inclusions called CAIs are among the oldest solid matter in the solar system (image credit: Rohan Mehra, Division for Strategic Public Relations)
- "There are no time machines to explore our own past, so we wanted to see a young star that could share traits with our own," said Tachibana. "With the ALMA (Atacama Large Millimeter/submillimeter Array), we found the emission lines—a chemical fingerprint—for AlO in outflows from the circumstellar disk (gas and dust surrounding a star) of the massive young star candidate Orion Source I. It's not exactly like our sun, but it's a good start."
- ALMA was the ideal tool as it offers extremely high resolution and sensitivity to reveal the distribution of AlO around the star. No other instrument can presently make such observations.
- The team now plans to explore gas and solid molecules around other stars to gather data useful to further refine solar system models.
Figure 15: The Orion Nebula where the distant young star Origin Source I resides (image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)
• March 19, 2019: Researchers have detected a radio signal from abundant interstellar dust in MACS0416_Y1, a galaxy 13.2 billion light-years away in the constellation Eridanus. Standard models can't explain this much dust in a galaxy this young, forcing us to rethink the history of star formation. Researchers now think MACS0416_Y1 experienced staggered star formation with two intense starburst periods 300 million and 600 million years after the Big Bang with a quiet phase in between. 29) 30)
- Stars are the main players in the Universe, but they are supported by the unseen backstage stagehands: stardust and gas. Cosmic clouds of dust and gas are the sites of star formation and masterful storytellers of the cosmic history.
- "Dust and relatively heavy elements such as oxygen are disseminated by the deaths of stars," said Yoichi Tamura, an associate professor at Nagoya University and the lead author of the research paper, "Therefore, a detection of dust at some point in time indicates that a number of stars have already formed and died well before that point."
- Using ALMA (Atacama Large Millimeter/submillimeter Array), Tamura and his team observed the distant galaxy MACS0416_Y1. Because of the finite speed of light, the radio waves we observe from this galaxy today had to travel for 13.2 billion years to reach us. In other words, they provide an image of what the galaxy looked like 13.2 billion years ago, which is only 600 million years after the Big Bang.
- The astronomers detected a weak but telltale signal of radio emissions from dust particles in MACS0416_Y1. The HST (Hubble Space Telescope), the Spitzer Space Telescope, and the European Southern Observatory's VLT (Very Large Telescope) have observed the light from stars in the galaxy; and from its color they estimate the stellar age to be 4 million years.
- "It ain't easy," said Tamura half-lost in a moonage daydream. "The dust is too abundant to have been formed in 4 million years. It is surprising, but we need to hang onto ourselves. Older stars might be hiding in the galaxy, or they may have died out and disappeared already."
- "There have been several ideas proposed to overcome this dust budget crisis," said Ken Mawatari, a researcher at the University of Tokyo. "However, no one is conclusive. We made a new model which doesn't need any extreme assumptions diverging far from our knowledge of the life of stars in today's Universe. The model well explains both the color of the galaxy and the amount of dust." In this model, the first burst of star formation started at 300 million years and lasted 100 million years. After that, the star formation activity went quiet for a and then restarted at 600 million years. The researchers think ALMA observed this galaxy at the beginning of its second generation of star formation.
- "Dust is a crucial material for planets like Earth," explains Tamura. "Our result is an important step forward for understanding the early history of the Universe and the origin of dust."
Figure 16: ALMA and Hubble Space Telescope (HST) image of the distant galaxy MACS0416_Y1. Distribution of dust and oxygen gas traced by ALMA are shown in red and green, respectively, while the distribution of stars captured by HST is shown in blue [image credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Tamura, et al.]
Figure 17: Artist's impression of the distant galaxy MACS0416_Y1. Based on the observations with ALMA and HST, researchers assume that this galaxy contains stellar clusters with a mix of old and young stars. The clouds of gas and dust are illuminated by starlight (image credit: National Astronomical Observatory of Japan)
• March 18, 2019: Scientists from the RIKEN Cluster for Pioneering Research in Japan,the Chalmers University of Technology in Sweden,and the University of Virginia in the USA and collaborators used ALMA (Atacama Large Millimeter/submillimeter Array) to observe a molecular cloud that is collapsing to form two massive protostars that will eventually become a binary star system. 31)
- While it is known that most massive stars possess orbiting stellar companions it has been unclear how this comes about – for example, are the stars born together from a common spiraling gas disk at the center of a collapsing cloud, or do they pair up later by chance encounters in a crowded star cluster.
- Understanding the dynamics of forming binaries has been difficult because the protostars in these systems are still enveloped in a thick cloud of gas and dust that prevents most light from escaping. Fortunately, it is possible to see them using radio waves, as long as they can be imaged with sufficiently high spatial resolution.
- In the current research, published in Nature Astronomy, the researchers led by Yichen Zhang of the RIKEN Cluster for Pioneering Research and Jonathan C. Tan at the Chalmers University,and the University of Virginia, used ALMA to observe, at high spatial resolution, a star-forming region known as IRAS07299-1651, which is located 1.68 kpc (kiloparsec), or about 5,500 light years, away.
- The observations showed that already at this early stage, the cloud contains two objects, a massive "primary" central star and another "secondary" forming star, also of high mass. For the first time, the research team was able to use these observations to deduce the dynamics of the system. The observations showed that the two forming stars are separated by a distance of about 180 astronomical units—a unit approximately the distance from the earth to the sun. Hence, they are quite far apart. They are currently orbiting each other with a period of at most 600 years and have a total mass at least 18 times that of our Sun. 32)
Figure 18: ALMA's view of the IRAS-07299 star-forming region and the massive binary system at its center. The background image shows dense, dusty streams of gas (shown in green) that appear to be flowing towards the center. Gas motions, as traced by the methanol molecule, that are towards us are shown in blue; motions away from us in red. The inset image shows a zoom-in view of the massive forming binary, with the brighter, primary protostar moving toward us is shown in blue and the fainter, secondary protostar moving away from us shown in red. The blue and red dotted lines show an example of orbits of the primary and secondary spiraling around their center of mass (marked by the cross) image credit: Riken & Study Team
Figure 19: Movie composed of images taken by ALMA showing the gas streams, as traced by the methanol molecule, with different line-of-sight color-coded velocities, around the massive binary protostar system. The grey background image shows the overall distribution, from all velocities, of dust emission from the dense gas streams (image credit: Riken & Study Team)
• March 13, 2019: Researchers have spotted the formation sites of planets around a young star resembling our Sun. Two rings of dust around the star, at distances comparable to the asteroid belt and the orbit of Neptune in our Solar System, suggest that we are witnessing the formation of a planetary system similar to our own. 33) 34)
- The Solar System is thought to have formed from a cloud of cosmic gas and dust 4.6 billion years ago. By studying young planetary systems forming around other stars, astronomers hope to learn more about our own origins.
- Tomoyuki Kudo, an astronomer at the National Astronomical Observatory of Japan (NAOJ), and his team observed the young star DM Tau using the Atacama Large Millimeter/submillimeter Array (ALMA). Located 470 light-years away in the constellation Taurus, DM Tau is about half the mass of the Sun and estimated to be three to five million years old.
- "Previous observations inferred two different models for the disk around DM Tau," said Kudo. "Some studies suggested the radius of the ring is about where the Solar System's asteroid belt would be. Other observations put the size out where Neptune would be. Our ALMA observations provided a clear answer: both are right. DM Tau has two rings, one at each location."
- The researchers found a bright patch in the outer ring. This indicates a local concentration of dust, which would be a possible formation site for a planet like Uranus or Neptune.
- "We are also interested in seeing the details in the inner region of the disk, because the Earth formed in such an area around the young Sun," commented Jun Hashimoto, a researcher at the Astrobiology Center, Japan. "The distribution of dust in the inner ring around DM Tau will provide crucial information to understand the origin of planets like Earth."
Figure 20: ALMA image of the dusty disk around the young star DM Tau. You can see two concentric rings where planets may be forming (image credit: ALMA (ESO/NAOJ/NRAO), Kudo et al.)
• February 28, 2019: Astronomers have detected a stealthy black hole from its effects on an interstellar gas cloud. This intermediate mass black hole is one of over 100 million quiet black holes expected to be lurking in our galaxy. These results provide a new method to search for other hidden black holes and help us understand the growth and evolution of black holes. 35) 36)
- Black holes are objects with such strong gravity that everything, including light, is sucked in and cannot escape. Because black holes do not emit light, astronomers must infer their existence from the effects their gravity produce in other objects. Black holes range in mass from about 5 times the mass of the Sun to supermassive black holes millions of times the mass of the Sun. Astronomers think that small black holes merge and gradually grow into large ones, but no one had ever found an intermediate mass, hundreds or thousands of times the mass of the Sun.
- A research team led by Shunya Takekawa at the National Astronomical Observatory of Japan noticed HCN–0.009–0.044, a gas cloud moving strangely near the center of the Galaxy 25,000 light-years away from Earth in the constellation Sagittarius. They used ALMA (Atacama Large Millimeter/submillimeter Array) to perform high resolution observations of the cloud and found that it is swirling around an invisible massive object.
- Takekawa explains, "Detailed kinematic analyses revealed that an enormous mass, 30,000 times that of the Sun, was concentrated in a region much smaller than our Solar System. This and the lack of any observed object at that location strongly suggests an intermediate-mass black hole. By analyzing other anomalous clouds, we hope to expose other quiet black holes."
- Tomoharu Oka, a professor at Keio University and coleader of the team, adds, "It is significant that this intermediate mass black hole was found only 20 light-years from the supermassive black hole at the Galactic center. In the future, it will fall into the supermassive black hole; much like gas is currently falling into it. This supports the merger model of black hole growth."
Figure 21: Artist's impression of a gas cloud swirling around a black hole [image credit: NAOJ (National Astronomical Observatory of Japan)]
• February 26, 2019: Astronomers have unveiled the enigmatic origins of two different gas streams from a baby star. Using ALMA, they found that the slow outflow and the high speed jet from a protostar have misaligned axes and that the former started to be ejected earlier than the latter. The origins of these two flows have been a mystery, but these observations provide telltale signs that these two streams were launched from different parts of the disk around the protostar. 37)
Figure 22: ALMA image of the protostar MMS5/OMC-3. The protostar is located at the center and the gas streams are ejected to the east and west (left and right). The slow outflow is shown in orange and the fast jet is shown in blue. It is obvious that the axes of the outflow and jet are misaligned (image credit: ALMA (ESO/NAOJ/NRAO) Matsushita et al.)
- Stars in the Universe have a wide range of masses, ranging from hundreds of times the mass of the Sun to less than a tenth of that of the Sun. To understand the origin of this variety, astronomers study the formation process of the stars, that is the aggregation of cosmic gas and dust.
- Baby stars collect the gas with their gravitational pull, however, some of the material is ejected by the protostars. This ejected material forms a stellar birth cry which provides clues to understand the process of mass accumulation.
- Yuko Matsushita, a graduate student at Kyushu University and her team used ALMA to observe the detailed structure of the birth cry from the baby star MMS5/OMC-3 and found two different gaseous flows: a slow outflow and a fast jet. There have been a handful of examples with two flows seen in radio waves, but MMS5/OMC-3 is exceptional.
- "Measuring the Doppler shift of the radio waves, we can estimate the speed and lifetime of the gas flows," said Matsushita, the lead author of the research paper that appeared in the Astrophysical Journal. "We found that the jet and outflow were launched 500 years and 1300 years ago, respectively. These gas streams are quite young."
- More interestingly, the team found that the axes of the two flows are misaligned by 17 degrees. The axis of the flows can be changed over long time periods due to the precession of the central star. But in this case, considering the extreme youth of the gas streams, researchers concluded that the misalignment is not due to precession but is related to the launching process.
Figure 23: Artist's impression of the baby star MMS5/OMC-3. ALMA observations identified two gas streams from the protostar, a collimated fast jet and a wide-angle slow outflow, and found that the axes of the two gas flows are misaligned (image credit: NAOJ)
- There are two competing models for the formation mechanism of the protostellar outflows and jets. Some researchers assume that the two streams are formed independently in different parts of the gas disk around the central baby star, while others propose that the collocated jet is formed first, then it entrains the surrounding material to form the slower outflows. Despite extensive research, astronomers had not yet reached a conclusive answer.
- A misalignment in the two flows could occur in the ‘independent model,' but is difficult in the ‘entrainment model.' Moreover, the team found that the outflow was ejected considerably earlier than the jet. This clearly backs the ‘independent model.'
- "The observation well matches the result of my simulation," said Masahiro Machida, a professor at Kyushu University. A decade ago, he performed pioneering simulation studies using a supercomputer operated by the National Astronomical Observatory of Japan. In the simulation, the wide-angle outflow is ejected from the outer area of the gaseous disk around a protostar, while the collimated jet is launched independently from the inner area of the disk. Machida continues, "An observed misalignment between the two gas streams may indicate that the disk around the protostar is warped."
- "ALMA's high sensitivity and high angular resolution will enable us to find more and more young, energetic outflow-and-jet-systems like MMS 5/OMC-3," said Satoko Takahashi, an astronomer at the National Astronomical Observatory of Japan and the Joint ALMA Observatory and co-author of the paper. "They will provide clues to understand the driving mechanisms of outflows and jets. Moreover studying such objects will also tell us how the mass accretion and ejection processes work at the earliest stage of star formation." 38)
• February 25, 2019: Red giants are old stars that eject gaseous material and solid particles through a stellar wind. Some red giants appeared to lose an exceptionally large amount of mass this way. However, new observations reveal that this is not quite the case. The stellar wind is not more intense than normal, but is affected by a partner that was overlooked until now—a second star that circles the red giant. These are the results of an international study led by Belgian university KU (Katholieke Universiteit) Leuven. 39)
- Humans don't live long enough to realize it, but stars are also born, they age, and they die. It's a process that takes billions of years. As a star gets older, it becomes bigger, colder, and redder - hence the name 'red giants'. Our sun will also become such a red giant in four and a half billion years.
- In the final stage of their life, red giants eject their mass - gas and other matter - in the form of a stellar wind. Earlier observations confirmed that red giants lose a lot of mass this way.
- Twelve mass-loss rate record holders, in particular, have been baffling scientists for decades. These red giants supposedly eject the equivalent of 100 earths per year for 100 to 2,000 years on end. Even astronomically speaking, that's a lot of matter in a short amount of time.
- This was difficult to explain, says Professor Leen Decin from the KU Leuven Institute of Astronomy: "If you look at the mass of such a star in the next phase of its life, the intense stellar wind doesn't last long enough to account for the mass loss that we've seen. It was also statistically improbable that we had discovered twelve of these red giants, knowing that what we were seeing was a phase that lasted only hundreds or thousands of years compared with their billion-year-long life. It's like finding a needle in a haystack twelve times."
- New observations from the ALMA telescope in Chile shed light on what was happening with two of these red giants. "For these stars, the stellar wind forms a spiral. It's an indirect indication that the red giant is not alone, but part of a binary star system. The red giant is the main star with a second star circling it. Both stars affect each other and their environment gravitationally in two ways: on the one hand, the stellar wind is pulled in the direction of the second star and, on the other hand, the red giant itself also wiggles slightly. These movements give the stellar wind a spiral shape."
- The discovery of a partner star made everything fall into place, says Decin: "We believed that these red giants were record holders for mass-loss rate, but that's not the case. It only seemed as though they were losing a lot of mass because there's an area between the two stars where the stellar wind is much more concentrated due to the gravity of the second star. These red giants don't lose the equivalent of 100 earths per year, but rather 10 of them - just like the regular red giants. As such, they also die a bit more slowly than we first assumed. To rephrase in a positive way: these old stars live longer than we thought."
- The astronomers are now investigating whether a system with a binary star could also be the explanation for other special red giants. "We believed that many stars lived alone, but we will probably have to adjust this idea. A star with a partner is likely to be more common than we thought," Decin concludes.
Figure 24: Thanks to new observations from the ALMA telescope in Chile, it became clear that the stellar wind of this red giant forms a spiral. This is an indirect indication that the star is not alone, but part of a binary star (image credit: ALMA (ESO/NAOJ/NRAO)/L. Decin et al.) 40)
• February 7, 2019: A team of astronomers and chemists using ALMA has detected the chemical fingerprints of sodium chloride (NaCl) and other similar salty compounds emanating from the dusty disk surrounding Orion Source I, a massive, young star in a dusty cloud behind the Orion Nebula. 41)
- "It's amazing we're seeing these molecules at all," said Adam Ginsburg, a Jansky Fellow of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, and lead author of a paper accepted for publication in the Astrophysical Journal. "Since we've only ever seen these compounds in the sloughed-off outer layers of dying stars, we don't fully know what our new discovery means. The nature of the detection, however, shows that the environment around this star is very unusual."
- To detect molecules in space, astronomers use radio telescopes to search for their chemical signatures – telltale spikes in the spread-out spectra of radio and millimeter-wavelength light. Atoms and molecules emit these signals in several ways, depending on the temperature of their environments.
- The new ALMA observations contain a bristling array of spectral signatures – or transitions, as astronomers refer to them – of the same molecules. To create such strong and varied molecular fingerprints, the temperature differences where the molecules reside must be extreme, ranging anywhere from 100 kelvin to 4,000 kelvin (about -175 Celsius to 3700 Celsius). An in-depth study of these spectral spikes could provide insights about how the star is heating the disk, which would also be a useful measure of the luminosity of the star.
- "When we look at the information ALMA has provided, we see about 60 different transitions – or unique fingerprints – of molecules like sodium chloride and potassium chloride coming from the disk. That is both shocking and exciting," said Brett McGuire, a chemist at the NRAO in Charlottesville, Virginia, and co-author on the paper.
- The researchers speculate that these salts come from dust grains that collided and spilled their contents into the surrounding disk. Their observations confirm that the salty regions trace the location of the circumstellar disk.
- "Usually when we study protostars in this manner, the signals from the disk and the outflow from the star get muddled, making it difficult to distinguish one from the other," said Ginsburg. "Since we can now isolate just the disk, we can learn how it is moving and how much mass it contains. It also may tell us new things about the star."
- The detection of salts around a young star is also of interest to astronomers and astrochemists because some of constituent atoms of salts are metals – sodium and potassium. This suggests there may be other metal-containing molecules in this environment. If so, it may be possible to use similar observations to measure the amount of metals in star-forming regions. "This type of study is not available to us at all presently. Free-floating metallic compounds are generally invisible to radio astronomy," noted McGuire.
Figure 25: Artist impression of Orion Source I, a young, massive star about 1,500 light-years away. New ALMA observations detected a ring of salt — sodium chloride, ordinary table salt — surrounding the star. This is the first detection of salts of any kind associated with a young star. The blue region (about 1/3 the way out from the center of the disk) represents the region where ALMA detected the millimeter-wavelength "glow" from the salts (image credit: NRAO/AUI/NSF; S. Dagnello)
- The salty signatures were found about 30 to 60 astronomical units (AU, or the average distance between the Earth and the Sun) from the host stars. Based on their observations, the astronomers infer that there may be as much as one sextillion (a one with 21 zeros after it) kilograms of salt in this region, which is roughly equivalent to the entire mass of Earth's oceans.
- "Our next step in this research is to look for salts and metallic molecules in other regions. This will help us understand if these chemical fingerprints are a powerful tool to study a wide range of protoplanetary disks, or if this detection is unique to this source," said Ginsburg. "In looking to the future, the planned Next Generation VLA would have the right mix of sensitivity and wavelength coverage to study these molecules and perhaps use them as tracers for planet-forming disks."
- Orion Source I formed in the Orion Molecular Cloud I, a region of explosive starbirth previously observed with ALMA. "This star was ejected from its parent cloud with a speed of about 10 km/s around 550 years ago," said John Bally, an astronomer at the University of Colorado and co-author on the paper. "It is possible that solid grains of salt were vaporized by shock waves as the star and its disk were abruptly accelerated by a close encounter or collision with another star. It remains to be seen if salt vapor is present in all disks surrounding massive protostars, or if such vapor traces violent events like the one we observed with ALMA." 42)
• February 4, 2019: Astronomers using ALMA have detected various complex organic molecules around the young star V883 Ori. A sudden outburst from this star is releasing molecules from the icy compounds in the planet forming disk. The chemical composition of the disk is similar to that of comets in the modern Solar System. Sensitive ALMA observations enable astronomers to reconstruct the evolution of organic molecules from the birth of the Solar System to the objects we see today. 43)
- The research team led by Jeong-Eun Lee (Kyung Hee University, Korea) used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect complex organic molecules including methanol (CH3OH), acetone (CH3COCH3), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and acetonitrile (CH3CN). This is the first time that acetone was unambiguously detected in a planet forming region or protoplanetary disk.
- Various molecules are frozen in ice around micrometer-sized dust particles in protoplanetary disks. V883 Ori's sudden flare-up is heating the disk and sublimating the ice, which releases the molecules into gas. The region in a disk where the temperature reaches the sublimation temperature of the molecules is called the "snow line." The radii of snow lines are about a few astronomical units (au) around normal young stars, however, they are enlarged almost 10 times around bursting stars.
- "It is difficult to image a disk on the scale of a few au with current telescopes," said Lee. "However, around an outburst star, ice melts in a wider area of the disk and it is easier to see the distribution of molecules. We are interested in the distribution of complex organic molecules as the building blocks of life."
- Ice, including frozen organic molecules, could be closely related to the origin of life on planets. In our Solar System, comets are the focus of attention because of their rich icy compounds. For example, the European Space Agency's legendary comet explorer Rosetta found rich organic chemistry around the comet Churyumov-Gerasimenko. Comets are thought to have been formed in the outer colder region of the proto-Solar System, where the molecules were contained in ice. Probing the chemical composition of ice in protoplanetary disks is directly related to probing the origin of organic molecules in comets, and the origin of the building blocks of life.
- Thanks to ALMA's sharp vision and the enlarged snow line due to the flare-up of the star, the astronomers obtained the spatial distribution of methanol and acetaldehyde. The distribution of these molecules has a ring-like structure with a radius of 60 au, which is twice the size of Neptune's orbit. The researchers assume that inside of this ring the molecules are invisible because they are obscured by thick dusty material, and are invisible outside of this radius because they are frozen in ice.
- "Since rocky and icy planets are made from solid material, the chemical composition of solids in disks is of special importance. An outburst is a unique chance to investigate fresh sublimates, and thus the composition of solids." says Yuri Aikawa at the University of Tokyo, a member of the research team.
Figure 26: The distribution of dust is shown in orange and the distribution of methanol, an organic molecule, is shown in blue (image credit: ALMA (ESO/NAOJ/NRAO), Lee et al. V883Ori)
- V883 Ori is a young star located at 1300 light-years away from the Earth. This star is experiencing a so-called FU Orionis type outburst, a sudden increase of luminosity due to a bursting torrent of material flowing from the disk to the star. These outbursts last only on the order of 100 years, therefore the chance to observe a burst is rather rare. However, since young stars with a wide range of ages experience FU Ori bursts, astronomers expect to be able to trace the chemical composition of ice throughout the evolution of young stars. 44)
• January 21, 2019: Including the powerful ALMA into an array of telescopes for the first time, astronomers have found that the emission from the supermassive black hole Sagittarius A* (Sgr A*) at the center of our Galaxy comes from a smaller region than previously thought. This may indicate that a radio jet from Sgr A* is pointed almost directly towards the Earth. 45) 46) 47)
- So far, a foggy cloud of hot gas has prevented astronomers from making sharp images of the supermassive black hole Sgr A* and causing doubt on its true nature. They have now included for the first time the powerful ALMA telescope in northern Chile into a global network of radio telescopes to peer through this fog, but the source keeps surprising them: its emission region is so small that the source may actually have to point directly at the direction of the Earth.
- Observing at a frequency of 86 GHz with the technique of VLBI (Very Long Baseline Interferometry), which combines many telescopes to form a virtual telescope the size of the Earth, the team succeeded in mapping out the exact properties of the light scattering blocking our view of Sgr A*. To remove the scattering and obtain the image, the team used a technique developed by Michael Johnson of the Harvard-Smithsonian Center for Astrophysics (CfA). "Even though scattering blurs and distorts the image of Sgr A*, the incredible resolution of these observations allowed us to pin down the exact properties of the scattering," says Johnson. "We could then remove most of the effects from scattering and begin to see what things look like near the black hole".
- The high quality of the unscattered image (Figure 28) has allowed the team to constrain theoretical models for the gas around Sgr A*. The bulk of the radio emission is coming from a region of a small size: a mere 300 millionth of an arc degree. "This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet," explains Issaoun, who has tested several computer models against the data. "However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us".
- The German astronomer Heino Falcke, Professor of Radio Astronomy at Radboud University and PhD supervisor of Issaoun, calls this statement very unusual, but he also no longer rules it out. Last year, Falcke would have considered this a contrived model, but recently the GRAVITY team came to a similar conclusion using ESO's Very Large Telescope Interferometer of optical telescopes and an independent technique. "Maybe this is true after all", concludes Falcke, "and we are looking at this beast from a very special vantage point."
Figure 27: The Global Millimeter VLBI Array (GMVA), with ALMA added (image credit: S. Issaoun, Radboud University / D. Pesce, CfA)
Legend to Figure 27: The data were correlated at the Max Planck Institute for Radio Astronomy (MPIfR), which also operates the Global Millimeter-VLBI Array (GMVA). Data analysis software was developed at the MIT Haystack Observatory and the Smithsonian Astrophysical Observatory. Several members of the team worked in this project as part of the European Research Council funded BlackHoleCam (BHC) team. The research team is also part of the Event Horizon Telescope (EHT) consortium, an international partnership of thirteen institutes from ten countries: Germany, the Netherlands, France & Spain (via IRAM), USA, Mexico, Japan, Taiwan, Canada and China (via EAO). The participation of ALMA (Atacama Large Millimeter/submillimeter Array) through the ALMA Phasing Project has been decisive for the success of this project. The GMVA is partially supported by the European Union's Horizon 2020 research and innovation program under grant agreement No. 730562.
- Supermassive black holes are common in the centers of galaxies and may generate the most energetic phenomena in the known universe. It is believed that, around these black holes, matter falls in a rotating disk and part of this matter is expelled in opposite directions along two narrow beams, called jets, at speeds close to the speed of light, which typically produces a lot of radio light. "Whether the radio emission seen from SgrA* originates from a symmetrical underlying structure, or is intrinsically asymmetric is a matter of intense discussion", explains Thomas Krichbaum, member of the team.
- Sgr A* is the nearest supermassive black hole and 'weighs' about 4 million solar masses. Its apparent size on the sky is less than a 100 millionth of an arc degree, which corresponds to the size of a tennis ball on the moon as seen from the Earth. "The black hole is so small that only VLBI can provide the angular resolution needed to resolve its structure", says Pablo Torne, astronomer in support on the observations from the IRAM 30-meter telescope. "The first observations of Sgr A* at 86 GHz date from 26 years ago, with only a handful of telescopes. Over the years, the quality of the data has improved steadily as more telescopes join," adds J. Anton Zensus, director of the Max Planck Institute for Radio Astronomy and head of its Radio Astronomy/VLBI division.
- The findings of Issaoun and her international team including scientists from two research departments (Kramer & Zensus) at MPIfR describe the first observations at 86 GHz in which ALMA also participated, by far the most sensitive telescope at this frequency. ALMA became part of the Global Millimeter VLBI Array (GMVA), which is operated by the Max Planck Institute for Radio Astronomy, in April 2017. The participation of ALMA, made possible by the ALMA Phasing Project effort, has been decisive for the success of this project.
- The participation of ALMA in mm-VLBI is important because of its sensitivity and its location in the southern hemisphere. In addition to ALMA, twelve radio telescopes in North America and Europe also participated in the network. The resolution achieved was twice as large as in previous observations at this frequency and produced the first image of Sgr A* that is considerably reduced in interstellar scattering (an effect caused by density irregularities in the ionized material along the line of sight between Sgr A* and the Earth).
Figure 28: Sgr A* at 86 GHz: simulation (top left), added effects of scattering (top right), scattered image from observations (bottom right), unscattered image, after removing the effects of scattering in the line of sight (bottom left), image credit: S. Issaoun, M. Mościbrodzka, Radboud University (Nijmegen, The Netherlangs) / M. D. Johnson, CfA
• January 24, 2019: Using the ALMA observatory in Chile, a group of astronomers led by MPIA's (Max Planck Institute for Astronomy, Heidelberg), Henrik Beuther has made the most detailed observation yet of the way that a giant gas cloud fragments into dense cores, which then act as the birthplaces of stars. The astronomers found that the mechanisms for fragmentation are fairly straightforward, resulting from the combination of the cloud's pressure and gravity. More complex features, such as magnetic lines or turbulence, play a smaller role than previously thought. 48) 49)
- Stars are born when giant clouds of gas and dust collapse. Whenever one of the collapsing regions becomes hot and dense enough for nuclear fusion to set in, a star is born. For massive stars, i.e. those stars that exhibit more than eight times the mass of the Sun, that is only part of the picture, though. The biggest stars in the Universe are not born singly. They are born from massive clouds of molecular gas, which then form a cascade of fragments, with many of the fragments giving birth to a star.
- Astronomers have long wondered whether this fragmentation-mode of forming stars requires different physical mechanisms than for lower-mass stars. Proposals include turbulent gas motion, which could destabilize a region and lead to quicker collapse, or magnetic fields that could stabilize and thus delay collapse.
- The different mechanisms should leave tell-tale traces in regions where multiple stars are forming. The collapse that leads to the formation of high-mass stars takes place on a hierarchy of different levels. On the largest scales, star formation involves giant molecular clouds, which consist mostly of hydrogen gas and can reach sizes between a few dozen and more than a hundred light-years across. Within those clouds are slightly denser clumps, typically a few light-years across. Each clump contains one or more dense cores, less than a fifth of a light-year in diameter. Within each core, collapse leads to the formation of either a single star or multiple stars. Together, the stars produced in the cores of a single clump will form a star cluster.
Tell-tale scales of fragmentation
- The scales of this fragmentation at multiple levels depend on the mechanisms involved. The simplest model can be written down using no more than high school physics: An ideal gas has a pressure that depends on its temperature and density. In a simplified gas cloud, assumed to have constant density, that pressure must be strong enough everywhere to balance the force of gravity (given by Newton's law of gravity) – even in the center of the cloud, where the inward gravitation-induced push of all the surrounding matter is strongest. Write this condition down, and you will find that any such constant-density cloud can only have a maximum size. If a cloud is larger than this maximum, which is called the Jeans length, the cloud will fragment and collapse.
- Is the fragmentation of young massive clusters really dominated by these comparatively straightforward processes? It doesn't need to be, and some astronomers have constructed much more complex scenarios, which include the influence of turbulent gas motion and magnetic field lines. These additional mechanisms change the conditions for cloud stability, and typically increase the scales of the different types of fragment.
- Different predictions for cloud sizes offer a way of testing the simple physics scenario against its more complex competitors. That is what Henrik Beuther and his colleagues set out to do when they observed the star formation region G351.77-0.54 in the Southern constellation Scorpius (The Scorpion). Previous observations had indicated that in this region, fragmentation could be caught in the act. But none of these observations had been powerful enough to show the smallest scale of interest for answering the question of fragmentation scales: the protostellar cores, let alone their sub-structure.
ALMA takes the most detailed look yet
- Beuther and his colleagues were able to do more. They used the ALMA Observatory in the Atacama Desert in Chile. ALMA combines the simultaneous observations of up to 66 radio telescopes to achieve a resolution of down to 20 milli-arcseconds, which allows astronomers to discern details more than ten times smaller than with any previous radio telescope, and at unrivalled sensitivity – a combination that has already led to a number of breakthrough observations also in other fields.
- Beuther and his colleagues used ALMA to study the high-mass star-forming region G351.77-0.54 down to sub-core scales smaller than 50 astronomical units (in other words, less than 50 times the average distance between the Earth and the Sun). As Beuther says: "This is a prime example of how technology drives astronomical progress. We could not have obtained our results without the unprecedented spatial resolution and sensitivity of ALMA."
Figure 29: Image of the massive star cluster NGC 3603, obtained with the VLT (Very Large Telescope). It probably has evolved in the same way as the one just forming in G351.77-0.54, the object depicted in this work (image credit: MPIA, ESO)
- Their results, together with earlier studies of the same cloud at larger scales, indicate that thermal gas physics is winning the day, even when it comes to very massive stars: Both the sizes of clumps within the cloud and, as the new observations show, of cores within the clumps and even of some core substructures are as predicted by Jeans length calculations, with no need for additional ingredients. Beuther comments: "In our case, the same physics provides a uniform description. Fragmentation from the largest to the smallest scales seems to be governed by the same physical processes."
Small accretion disks: a new challenge
- Simplicity is always a boon for scientific descriptions. However, the same observations also provided a discovery that will keep astronomers on their collective toes. In addition to studying fragmentation, Beuther et al. had been looking to unravel the structure of nascent stars ("protostars") within the cloud. Astronomers expect such a protostar to be surrounded by a swirling disk of gas, called the accretion disk. From the inner disk of the rim, gas falls onto the growing star, increasing its mass. In addition, magnetic fields produced by the motion of ionized gas and the gas itself interact to produce tightly focused streams called jets, which shoot out some of the matter into space perpendicular to that disk. Submillimeter light from those regions carries tell-tale signs ("Doppler-broadening of spectral lines") of the motion of dust, which in turn traces the motion of gas. But where Beuther and his collaborators had hoped for a clear signature from an accretion disk, instead, he found mainly the signature of jets, cutting a comparatively smooth path through the surrounding gas. Evidently, the accretion disks are even smaller than astronomers had expected – a challenge for future observations at even greater spatial resolution.
• January 15, 2019: New research led by an astronomer at the University of Warwick has found the first confirmed example of a double star system that has flipped its surrounding disc to a position that leaps over the orbital plane of those stars. The international team of astronomers used ALMA (Atacama Large Millimeter/sub-millimeter Array) to obtain high-resolution images of the Asteroid belt-sized disc. 50)
- The overall system presents the unusual sight of a thick hoop of gas and dust circling at right angles to the binary star orbit. Until now this setup only existed in theorists' minds, but the ALMA observation proves that polar discs of this type exist, and may even be relatively common.
- The new research is published 14 January by Royal Society University Research Fellow Dr Grant M. Kennedy of the University of Warwick's Department of Physics and Center for Exoplanets and Habitability in Nature Astronomy in a paper entitled "A circumbinary protoplanetary disc in a polar configuration". 51)
- Dr Grant M. Kennedy of the University of Warwick said: "Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so we've been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form."
- Dr Kennedy and his fellow researchers used ALMA to pin down the orientation of the ring of gas and dust in the binary star system HD 98800 . The orbit of the binary was previously known, from observations that quantified how the stars move in relation to each other. By combining these two pieces of information they were able to establish that the dust ring was consistent with a perfectly polar orbit. This means that while the stellar orbits orbit each other in one plane, like two horses going around on a carousel, the disc surrounds these stars at right angles to their orbits, like a giant ferris wheel with the carousel at the center
- Kennedy added: "Perhaps the most exciting thing about this discovery is that the disc shows some of the same signatures that we attribute to dust growth in discs around single stars. We take this to mean planet formation can at least get started in these polar circumbinary discs. If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover, and things like weird seasonal variations to consider."
- If there were a planet or planetoid present at the inner edge of the dust ring, the ring itself would appear from the surface as a broad band rising almost perpendicularly from the horizon. The polar configuration means that the stars would appear to move in and out of the disc plane, giving objects two shadows at times. Seasons on planets in such systems would also be different. On Earth they vary throughout the year as we orbit the Sun. A polar circumbinary planet would have seasons that also vary as different latitudes receive more or less illumination throughout the binary orbit.
- Future studies at different wavelengths will provide complementary information and further observational constraints for this source, which holds the key to a better understanding of black holes, the most exotic objects in the known universe.
Figure 30: An artist's impression of a view of the HD 98800BaBb binary star system and the surrounding disk (image credit: Mark Garlick, University of Warwick)
- Co-author Dr Daniel Price of Monash University's Center for Astrophysics (MoCA) and School of Physics and Astronomy added:"We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun. But with the new images we see a swirling disc of gas and dust orbiting around two stars. It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars. Incredibly, two more stars were seen orbiting that disc. So if planets were born here there would be four suns in the sky! ALMA is just a fantastic telescope, it is teaching us so much about how planets in other solar systems are born."
• January 10, 2019: Galaxies come in a wide variety of shapes and sizes. Some of the most significant differences among galaxies, however, relate to where and how they form new stars. Compelling research to explain these differences has been elusive, but that is about to change. 52)
- A vast, new research project with the ALMA (Atacama Large Millimeter/submillimeter Array), known as PHANGS-ALMA (Physics at High Angular Resolution in Nearby GalaxieS), delves into this question with far greater power and precision than ever before by measuring the demographics and characteristics of a staggering 100,000 individual stellar nurseries spread throughout 74 galaxies.
- PHANGS-ALMA, an unprecedented and ongoing research campaign, has already amassed a total of 750 hours of observations and given astronomers a much clearer understanding of how the cycle of star formation changes, depending on the size, age, and internal dynamics of each individual galaxy. This campaign is 10 to 100 times more powerful (depending on your parameters) than any prior survey of its kind.
- "Some galaxies are furiously bursting with new stars while others have long ago used up most of their fuel for star formation. The origin of this diversity may very likely lie in the properties of the stellar nurseries themselves," said Erik Rosolowsky, an astronomer at the University of Alberta in Canada and a co-principal investigator of the PHANGS-ALMA research team.
- He presented initial findings of this research at the 233rd meeting of the American Astronomical Society being held this week in Seattle, Washington. Several papers based on this campaign have also been published in the Astrophysical Journal and the Astrophysical Journal Letters. 53) 54) 55)
- "Previous observations with earlier generations of radio telescopes provide some crucial insights about the nature of cold, dense stellar nurseries," Rosolowsky said. - "These observations, however, lacked the sensitivity, fine-scale resolution, and power to study the entire breadth of stellar nurseries across the full population of local galaxies. This severely limited our ability to connect the behavior or properties of individual stellar nurseries to the properties of the galaxies that they live in."
- For decades, astronomers have speculated that there are fundamental differences in the way disk galaxies of various sizes convert hydrogen into new stars. Some astronomers theorize that larger, and generally older galaxies, are not as efficient at stellar production as their smaller cousins.
- The most logical explanation would be that these big galaxies have less efficient stellar nurseries. But testing this idea with observations has been difficult.
Figure 31: The ALMA telescope is conducting an unprecedented survey of nearby disk galaxies to study their stellar nurseries. With it, astronomers are beginning to unravel the complex and as-yet poorly understood relationship between star-forming clouds and their host galaxies [image credit: ALMA (ESO/NAOJ/NRAO); NRAO/AUI/NSF, B. Saxton]
• January 1, 2019: Using observations from the ALMA radio observatory in Chile, researchers have observed, for the first time, a warped disk around an infant protostar that formed just several tens of thousands of years ago. This implies that the misalignment of planetary orbits in many planetary systems —including our own— may be caused by distortions in the planet-forming disk early in their existence. 56)
- The planets in our Solar System orbit the Sun in planes that are at most about seven degrees offset from the equator of the Sun itself. It has been known for some time that many extrasolar systems have planets that are not lined up in a single plane or with the equator of the star. One explanation for this is that some of the planets might have been affected by collisions with other objects in the system or by stars passing by the system, ejecting them from their initial orbital plane.
- However, the possibility remained that the formation of planets out of the normal plane was actually caused by a warping of the star-forming cloud out of which the planets were born. Recently, images of protoplanetary disks—rotating disks where planets form around a star—have in fact showed such warping. But it was still unclear how early this happened.
- In the latest findings, published in Nature, the group from the RIKEN Cluster for Pioneering Research (CPR) and Chiba University in Japan have discovered that L1527; an infant protostar still embedded within a cloud, has a disk that has two parts —an inner one rotating in one plane, and an outer one in a different plane. The disk is very young and still growing. L1527, which is about 450 light-years away in the Taurus Molecular Cloud, is a good object for study as it has a disk that is nearly edge-on to our view. 57)
- According to Nami Sakai, who led the research group, "This observation shows that it is conceivable that the misalignment of planetary orbits can be caused by a warp structure formed in the earliest stages of planetary formation. We will have to investigate more systems to find out if this is a common phenomenon or not."
- The remaining question is what caused the warping of the disk. Sakai suggests two reasonable explanations. "One possibility," she says, "is that irregularities in the flow of gas and dust in the protostellar cloud are still preserved and manifest themselves as the warped disk. A second possibility is that the magnetic field of the protostar is in a different plane from the rotational plane of the disk, and that the inner disk is being pulled into a different plane from the rest of the disk by the magnetic field." She says they plan further work to determine which is responsible for the warping of the disk.
- The ALMA observatory in Chile is managed by an international consortium including the National Astronomical Observatory of Japan (NAOJ).
Figure 32: Artist's impression of a warped disk around a protostar. ALMA observed the protostar IRAS04368+2557 in the dark cloud L1527 and discovered that the protostar has a disk with two misaligned parts (image credit: RIKEN, Japan)
• December 20, 2018: As comet 46P/Wirtanen neared Earth on December 2, astronomers using ALMA took a remarkably close look the innermost regions of the comet's coma, the gaseous envelope around its nucleus. ALMA imaged the comet when it was approximately 16.5 million kilometers from Earth. At its closet on 16 December, the comet – one of the brightest in years — was approximately 11.6 million kilometers from Earth, or about 30 times the distance from the Earth to the moon. 58)
- "This comet is causing a stir in the professional and amateur astronomy communities due to its combined brightness and proximity, which allows us to study it in unprecedented detail" said NASA's Martin Cordiner, who led the team that made the ALMA observations. "As the comet drew nearer to the Sun, its icy body heated up, releasing water vapor and various other particles stored inside, forming the characteristic puffed-up coma and elongated tail."
- The ALMA image of comet 46P/Wirtanen zooms-in to very near its nucleus – the solid "dirty snowball" of the comet itself — to image the natural millimeter-wavelength "glow" emitted by molecules of hydrogen cyanide (HCN), a simple organic molecule that forms an ethereal atmosphere around the comet. ALMA, using its remarkable ability to see fine details, was able to detect and image the fine-scale distribution of this particular molecule.
- The HCN image shows a compact region of gas and an extended, diffuse, and somewhat asymmetrical, pattern in the inner portion of the coma. Due to the extreme proximity of this comet, most of the extended coma is resolved out, so these observations are only sensitive to the innermost regions, in the immediate vicinity of the nucleus.
- The astronomers also performed observations of more complex molecules on 9 December, when the comet was 13.6 million kilometers from Earth.
- Comet 46P/Wirtanen orbits the Sun once every 5.5 years, which is remarkably brisk compared to its more famous cousin Halley's Comet, which has an orbital period of about 75 years. Other bright comets can have periods that are on the order of hundreds and even thousands of years. The comet may yet be visible to the naked eye.
- For comparison, an optical view of the comet taken by an amateur astrophotographer is shown (Figure 33 right). Though they appear to be similar, the ALMA image spans an area of the sky only about 5 arcseconds – about 1000 times smaller than the optical image – meaning ALMA is looking at the very fine-scale features in the coma.
- This and previous observations of comets with ALMA confirm that they are rich in organic molecules, and may therefore have seeded the early Earth with the chemical building blocks of life.
- The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Figure 33: The side-by-side comparison shows an ALMA image of comet 46P/Wirtanen (left) and an optical image (right). The ALMA image has approximately 1000 times the resolution of the optical image and zooms in on the inner portion of the comet's diffuse coma (image credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner, NASA/CUA; Derek Demeter, Emil Buehler Planetarium)
• December 17, 2018: Researchers from RIKEN (Institute of Physical and Chemical Research, Tokyo, Japan) have used observations from ALMA (Atacama Large Millimeter/submillimeter Array) to measure the strength of magnetic fields near two supermassive black holes at the center of an important group of active galaxies. Surprisingly, the strengths of the magnetic fields do not appear sufficient to power the coronae, clouds of superheated plasma, that are observed around the black holes at the centers of those galaxies. 59)
- It has long been known that the supermassive black holes that lie at the centers of galaxies, sometimes outshining their host galaxies, have coronae of superheated plasma around them, like the Sun. For black holes, these coronae can be heated to a phenomenal temperature of one billion degrees Celsius. It was long assumed that, like that of the Sun, the coronae were heated by magnetic field energies. However, these magnetic fields had never been measured, leaving uncertainty regarding the exact mechanism.
- In a 2014 paper, the research group predicted that electrons in the plasma surrounding the black holes would emit a special kind of light, known as synchrotron radiation, as they exist together with the magnetic forces in the coronae. Specifically, this radiation would be in the radio band, meaning light with a very long wavelength and low frequency. And the group set out to measure the fields.
- They decided to look at data from two "nearby"—in astronomical terms—active galactic nuclei—IC 4329A, which is about 200 million light years away, and NGC 985, which is approximately 580 million light years away. They began by taking measurements from the ALMA observatory in Chile, and then compared them to observations from two other radio telescopes: the VLA (Very Large Array) observatory in the United States, and the ATCA (Australia Telescope Compact Array) observatory in Australia, which measure slightly different bands; and found indeed that there was an excess of radio emission originating from synchrotron radiation, in addition to emissions from the jets case out by the black holes.
- Through the observations, the team deduced that the coronaehad a size of about 40 Schwartzchild radii (the radius of a black hole from which not even light can escape), and a strength of about 10 gauss, a figure that is a bit more than the magnetic field at the surface of the earth but quite a bit less than that given out by a typical refrigerator magnet.
- "The surprise," says Yoshiyuki Inoue, the first author of the paper, "is that although we confirmed the emission of radio synchrotron radiation from the coronaein both objects, it turns out that the field of the magnetic field we measured is much too weak to be able to drive the intense heating of the coronaearound these black holes." He also notes that the same phenomenon was observed in both galaxies, implying that it could be a general phenomenon. 60)
- Looking to the future, Inoue says that the group plans to look for signs of powerful gamma rays that should accompany the radio emissions, to further understand what is happening in the environment near supermassive black holes.
Figure 34: Artist's rendering of the corona around a black hole (image credit: RIKEN)
• December 12, 2018: Astronomers have cataloged nearly 4,000 exoplanets in orbit around distant stars. Though the discovery of these newfound worlds has taught us much, there is still a great deal we do not know about the birth of planets and the precise cosmic recipes that spawn the wide array of planetary bodies we have already uncovered, including so-called hot Jupiters, massive rocky worlds, icy dwarf planets, and – hopefully someday soon – distant analogs of Earth. 61)
- To help answer these and other intriguing questions, a team of astronomers has conducted ALMA‘s first large-scale, high-resolution survey of protoplanetary disks, the belts of dust and gas around young stars.
- Known as the DSHARP (Disk Substructures at High Angular Resolution Project), this "Large Program" of ALMA (Atacama Large Millimeter/submillimeter Array) has yielded stunning, high-resolution images of 20 nearby protoplanetary disks and given astronomers new insights into the variety of features they contain and the speed with which planets can emerge.
- The results of this survey will appear in a special focus issue of the Astrophysical Journal Letters.
- According to the researchers, the most compelling interpretation of these observations is that large planets, likely similar in size and composition to Neptune or Saturn, form quickly, much faster than current theory would allow. Such planets also tend to form in the outer reaches of their solar systems at tremendous distances from their host stars.
- Such precocious formation could also help explain how rocky, Earth-size worlds are able to evolve and grow, surviving their presumed self-destructive adolescence.
- "The goal of this months-long observing campaign was to search for structural commonalities and differences in protoplanetary disks. ALMA's remarkably sharp vision has revealed previously unseen structures and unexpectedly complex patterns," said Sean Andrews, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) and a leader of the ALMA observing campaign along with Andrea Isella of Rice University, Laura Pérez of the University of Chile, and Cornelis Dullemond of Heidelberg University. "We are seeing distinct details around a wide assortment of young stars of various masses. The most compelling interpretation of these highly diverse, small-scale features is that there are unseen planets interacting with the disk material."
- The leading models for planet formation hold that planets are born by the gradual accumulation of dust and gas inside a protoplanetary disk, beginning with grains of icy dust that coalesce to form larger and larger rocks, until asteroids, planetesimals, and planets emerge. This hierarchical process should take many millions of years to unfold, suggesting that its impact on protoplanetary disks would be most prevalent in older, more mature systems. Mounting evidence, however, indicates that is not always the case.
- ALMA's early observations of young protoplanetary disks, some only about one million years old, reveal surprisingly well-defined structures, including prominent rings and gaps, which appear to be the hallmarks of planets. Astronomers were initially cautious to ascribe these features to the actions of planets since other natural process could be at play.
- "It was surprising to see possible signatures of planet formation in the very first high-resolution images of young disks. It was important to find out whether these were anomalies or if those signatures were common in disks," said Jane Huang, a graduate student at CfA and a member of the research team.
Figure 35: Four of the twenty disks that comprise ALMA's highest resolution survey of nearby protoplanetary disks. This image shows the millimeter-wavelength light emitted by the dust in the disk, giving astronomers a clearer understanding of the similarities and differences among the disks and what that has to say about planet formation (image credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello)
- Since the initial sample of disks that astronomers could study was so small, however, it was impossible to draw any overarching conclusions. It could have been that astronomers were observing atypical systems. More observations on a variety of protoplanetary disks were needed to determine the most likely causes of the features they were seeing.
- The DSHARP campaign was designed to do precisely that by studying the relatively small-scale distribution of dust particles around 20 nearby protoplanetary disks. These dust particles naturally glow in millimeter-wavelength light, enabling ALMA to precisely map the density distribution of small, solid particles around young stars.
- Depending on the star's distance from Earth, ALMA was able to distinguish features as small as a few Astronomical Units. (An Astronomical Unit is the average distance of the Earth to the Sun – about 150 million kilometers, which is a useful scale for measuring distances on the scale of star systems). Using these observations, the researchers were able to image an entire population of nearby protoplanetary disks and study their AU-scale features.
- The researchers found that many substructures – concentric gaps, narrow rings – are common to nearly all the disks, while large-scale spiral patterns and arc-like features are also present in some of the cases. Also, the disks and gaps are present at a wide range of distances from their host stars, from a few AU to more than 100 AU, which is more than three times the distance of Neptune from our Sun.
- These features, which could be the imprint of large planets, may explain how rocky Earth-like planets are able to form and grow. For decades, astronomers have puzzled over a major hurdle in planet-formation theory: Once dusty bodies grow to a certain size – about one centimeter in diameter – the dynamics of a smooth protoplanetary disk would induce them to fall in on their host star, never acquiring the mass necessary to form planets like Mars, Venus, and Earth.
- The dense rings of dust we now see with ALMA would produce a safe haven for rocky worlds to fully mature. Their higher densities and the concentration of dust particles would create perturbations in the disk, forming zones where planetesimals would have more time to grow into fully fledged planets.
- "When ALMA truly revealed its capabilities with its iconic image of HL Tau, we had to wonder if that was an outlier since the disk was comparatively massive and young," noted Laura Perez with the University of Chile and a member of the research team. "These latest observations show that, though striking, HL Tau is far from unusual and may actually represent the normal evolution of planets around young stars."
- The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Figure 36: ALMA's high-resolution images of nearby protoplanetary disks, which are results of the Disk Substructures at High Angular Resolution Project (DSHARP), image credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello
This research is presented in the following papers accepted to the Astrophysical Journal Letters.
1) Sean M. Andrews, Jane Huang, Laura M. Pérez, Andrea Isella, Cornelis P. Dullemond, Nicolás T. Kurtovic, Viviana V. Guzmán, John M. Carpenter, David J. Wilner, Shangjia Zhang, Zhaohuan Zhu, Tilman Birnstiel, Xue-Ning Bai, Myriam Benisty, A. Meredith Hughes, Karin I. Öberg, Luca Ricci, "The Disk Substructures at High Angular Resolution Project (DSHARP): I. Motivation, Sample, Calibration, and Overview," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04040.pdf
2) Jane Huang, Sean M. Andrews, Cornelis P. Dullemond, Andrea Isella, Laura M. Pérez, Viviana V. Guzmán, Karin I. "Öberg, Zhaohuan Zhu, Shangjia Zhang, Xue-Ning Bai, Myriam Benisty, Tilman Birnstiel, John M. Carpenter, A. Meredith Hughes, Luca Ricci, Erik Weaver, David J. Wilner, "The Disk Substructures at High Angular Resolution Project (DSHARP): II. Characteristics of Annular Substructures," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04041.pdf
3) Jane Huang, Sean M. Andrews, Laura M. Pérez, Zhaohuan Zhu, Cornelis P. Dullemond, Andrea Isella, Myriam Benisty, Xue-Ning Bai, Tilman Birnstiel, John M. Carpenter, Viviana V. Guzmán, A. Meredith Hughes, Karin I. Öberg, Luca Ricci, David J. Wilner, Shangjia Zhang, "The Disk Substructures at High Angular Resolution Project (DSHARP): III. Spiral Structures in the Millimeter Continuum of the Elias 27, IM Lup, and WaOph 6 Disks," Astrophysical Journal Letters, 11 December 2018, URL: https://arxiv.org/pdf/1812.04193.pdf
4) Nicolás Kurtovic, Laura Pérez, Myriam Benisty, Zhaohuan Zhu, Shangjia Zhang, Jane Huang, Sean M. Andrews, Cornellis P. Dullemond, Andrea Isella, Xue-Ning Bai, John M. Carpenter, Viviana V. Guzmán, Luca Ricci, David J. Wilner, "The Disk Substructures at High Angular Resolution Project (DSHARP): IV. Characterizing substructures and interactions in disks around multiple star systems," 11 December, 2018, URL: https://arxiv.org/pdf/1812.04536.pdf
5) Tilman Birnstiel, Cornelis P. Dullemond, Zhaohuan Zhu, Sean M. Andrews, Xue-Ning Bai, David J. Wilner, John M. Carpenter, Jane Huang, Andrea Isella, Myriam Benisty, Laura M. Pérez, Shangjia Zhang, "The Disk Substructures at High Angular Resolution Project (DSHARP): V. Interpreting ALMA maps of protoplanetary disks in terms of a dust model," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04043.pdf
6) Cornelis P. Dullemond, Tilman Birnstiel, Jane Huang, Nicolás T. Kurtovic, Sean M. Andrews, Viviana V. Guzmán, Laura M. Pérez, Andrea Isella, Zhaohuan Zhu, Myriam Benisty, David J. Wilner, Xue-Ning Bai, John M. Carpenter, Shangjia Zhang, Luca Ricci, "The Disk Substructures at High Angular Resolution Project (DSHARP): VI. Dust Trapping in Thin-Ringed Protoplanetary Disks," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04044.pdf
7) Shangjia Zhang, Zhaohuan Zhu, Jane Huang, Viviana V. Guzmán, Sean M. Andrews, Tilman Birnstiel, Cornelis P. Dullemond, John M. Carpenter, Andrea Isella, Laura M. Pérez, Myriam Benisty, David J. Wilner, Clément Baruteau, Xue-Ning Bai, Luca Ricci, "The Disk Substructures at High Angular Resolution Project (DSHARP): VII. The Planet-Disk Interactions Interpretation," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04045.pdf
8) Viviana V. Guzmán, Jane Huang, Sean M. Andrews, Andrea Isella, Laura M. Pérez, John M. Carpenter, Cornelis P. Dullemond, Luca Ricci, Tilman Birnstiel, Shangjia Zhang, Zhaohuan Zhu, Xue-Ning Bai, Myriam Benisty, Karin I. Öberg, David J. Wilner"The Disk Substructures at High Angular Resolution Project (DSHARP): VIII. The Rich Ringed Substructures in the AS 209 Disk," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04046.pdf
9) Andrea Isella, Jane Huang, Sean M. Andrews, Cornelis P. Dullemond, Tilman Birnstiel, Shangjia Zhang, Zhaohuan Zhu, Viviana V. Guzmán, Laura M. Pérez, Xue-Ning Bai, Myriam Benisty, John M. Carpenter, Luca Ricci, David J. Wilner, "The Disk Substructures at High Angular Resolution Project (DSHARP): IX. A High Definition Study of the HD 163296 Planet Forming Disk," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04047.pdf
10) Laura M. Pérez, Myriam Benisty, Sean M. Andrews, Andrea Isella, Cornelis P. Dullemond, Jane Huang, Nicolás T. Kurtovic, Viviana V. Guzmán, Zhaohuan Zhu, Tilman Birnstiel, Shangjia Zhang, John M. Carpenter, David J. Wilner, Luca Ricci, Xue-Ning Bai, Erik Weaver, Karin I. Öberg, "The Disk Substructures at High Angular Resolution Project (DSHARP): X. Multiple Rings, a Misaligned Inner Disk, and a Bright Arc in the Disk around the T Tauri Star HD 143006," Astrophysical Journal Letters, 10 December 2018, URL: https://arxiv.org/pdf/1812.04049.pdf
• November 30, 2018: Based on computer simulations and new observations from the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have found that the rings of gas surrounding active supermassive black holes are not simple donut shapes. Instead, gas expelled from the center interacts with infalling gas to create a dynamic circulation pattern, similar to a water fountain in a city park. 62) 63)
- Most galaxies host a supermassive black hole, millions or billions of times as heavy as the Sun, in their centers. Some of these black holes swallow material quite actively. But astronomers have believed that rather than falling directly into the black hole, matter instead builds up around the active black hole forming a donut structure.
- Takuma Izumi, a researcher at the National Astronomical Observatory of Japan (NAOJ), led a team of astronomers that used ALMA to observe the supermassive black hole in the Circinus Galaxy located 14 million light-years away from the Earth in the direction of the constellation Circinus. The team then compared their observations to a computer simulation of gas falling towards a black hole made with the Cray XC30 ATERUI supercomputer operated by NAOJ. This comparison revealed that the presumptive "donut" is not actually a rigid structure, but instead a complex collection of highly dynamic gaseous components. First, cold molecular gas falling towards the black hole forms a disk near the plane of rotation. As it approaches the black hole, this gas is heated until the molecules break down into the component atoms and ions. Some of these atoms are then expelled above and below the disk, rather than being absorbed by the black hole. This hot atomic gas falls back onto the disk creating a turbulent three dimensional structure. These three components circulate continuously, similar to a water fountain in a city park.
- "Previous theoretical models set a priori assumptions of rigid donuts," explains Keiichi Wada, a theoretician at Kagoshima University in Japan, who lead the simulation study and is a member of the research team. "Rather than starting from assumptions, our simulation started from the physical equations and showed for the first time that the gas circulation naturally forms a donut. Our simulation can also explain various observational features of the system."
- "By investigating the motion and distribution of both the cold molecular gas and warm atomic gas with ALMA, we demonstrated the origin of the so-called ‘donut' structure around active black holes," said Izumi. "Based on this discovery, we need to rewrite the astronomy textbooks."
Figure 37: ALMA image of the gas around the supermassive black hole in the center of the Circinus Galaxy. The distributions of CO molecular gas and C atomic gas are shown in orange and cyan, respectively [image credit: ALMA (ESO/NAOJ/NROA), Izumi et al.]
Figure 38: Artist's impression of the gas motion around the supermassive black hole in the center of the Circinus Galaxy. The three gaseous components form the long-theorized "donut" structure: (1) a disk of infalling dense cold molecular gas, (2) outflowing hot atomic gas, and (3) gas returning to the disk (image credit: NAOJ)
Figure 39: Cross section of the gas around a supermassive black hole simulated with NAOJ's supercomputer ATERUI. The different colors represent the density of the gas, and the arrows show the motion of the gas. It clearly shows the three gaseous components forming the "donut" structure (image credit: Wada et al.)
• November 22, 2018: The Atacama Large Millimeter/submillimeter Array (ALMA) has opened another new window to the Universe. Using its highest frequency receivers yet, researchers obtained 695 radio signals from various molecules, including simple sugar, in the direction of a massive star forming region, and revealed a pair of water vapor fountains erupting from the region. These first scientific results from the ALMA Band 10 receivers developed in Japan ensure a promising future for high frequency observations. 64)
Figure 40: Photo of the star forming region NGC 6334I, also known as the Cat's Paw Nebula, taken by the NASA/ESA Hubble Space Telescope (top) and the high frequency radio spectra (bottom). The blue line shows the spectral lines detected by ALMA and the gray line shows the lines detected by the European Space Agency's Herschel Space Observatory. The ALMA observations detected more than ten times as many spectral lines. Note that the Herschel data have been inverted for comparison. Two molecular lines are labeled for reference (image credit: S. Lipinski/NASA & ESA, NAOJ, NRAO/AUI/NSF, B. McGuire et al.)
- The institutes participating in ALMA have shared responsibility for developing dedicated radio receivers for each of ALMA's 10 frequency bands. The National Astronomical Observatory of Japan (NAOJ) developed receivers for three bands; Band 4 (125-163 GHz), Band 8 (385-500 GHz), and Band 10 (787 to 950 GHz). The Band 10 receiver covers the highest frequency range in ALMA, which has not yet been extensively observed with other ground-based telescopes.
Figure 41: ALMA Band 10 receiver [image credit:ALMA (ESO/NAOJ/NRAO)]
- "High-frequency radio observations like in Band 10 are normally not possible from the ground," said Brett McGuire, a chemist at the National Radio Astronomy Observatory in Charlottesville, Virginia, and lead author on a paper in the Astrophysical Journal Letters. "They require the extreme precision and sensitivity of ALMA, along with some of the driest and most stable atmospheric conditions that can be found on Earth." 65)
- ALMA is a supremely sensitive cosmic chemical sensor. As molecules tumble and vibrate in space, they naturally emit electromagnetic radiation at specific frequencies, which appear as spikes on a spectrum. Each of ALMA's receiver bands can detect a different selection of these unique spectral fingerprints. The highest frequencies offer unique insight into lighter, important chemicals, like heavy water (HDO), as well as complex, warm molecules.
- McGuire and his team observed NGC 6634I, a nursery cloud of massive stars, with ALMA in 880 GHz. NGC 6334I is part of the Cat's Paw Nebula located 4,300 light-years away from Earth. "We detected a wealth of complex organic molecules surrounding this massive star-forming region," said McGuire. "These results have been received with excitement by the astronomical community and show once again how ALMA will reshape our understanding of the universe."
- The European Space Agency's Herschel Space Observatory has observed NGC 6334I in the same frequency range and detected 65 molecular emission lines. On the other hand, ALMA detected 695, 10 times as many spectral lines as Herschel. ALMA's prominent sensitivity and resolution offers a new level of astrochemistry research.
- The molecules detected in the direction of NGC 6334I include methanol, ethanol, methylamine, and glycolaldehyde, the simplest sugar-related molecule. Glycolaldehyde has already been detected around small baby stars in the IRAS 16293-2422 system with ALMA at a lower frequency. The difference in frequency reflects a difference in the environment. With Band 10 receivers, astronomers obtained a new tool to investigate warmer, denser regions.
- The other Band 10 result was also one of the most challenging, the direct observation of jets of water vapor streaming away from one of the massive protostars in NGC 6334I. ALMA was able to detect the high frequency radio waves naturally emitted by heavy water (water molecules made up of oxygen, hydrogen, and deuterium atoms, which are hydrogen atoms with a proton and a neutron in their nuclei).
- As a star begins to form out of massive clouds of dust and gas, the material surrounding the star falls onto the mass at the center. A portion of this material, however, is propelled away from the growing protostar as a pair of jets, which carry away gas and molecules, including water.
- The heavy water the researchers observed is flowing away from either a single protostar or a small cluster of protostars. These jets are oriented differently from what appear to be much larger and potentially more-mature jets emanating from the same region. The astronomers speculate that the heavy-water jets seen by ALMA are relatively recent features just beginning to move out into the surrounding nebula.
- "It is with much pleasure that we see the first scientific result from the ALMA Band 10 receiver," said Yoshinori Uzawa, the Director of the NAOJ Advanced Technology Center. He is an engineering researcher specializing in superconducting devices and led the Band 10 receiver development. "I have devoted myself to the research of superconducting devices for more than two decades, and the Band 10 receiver is one of the fruits of my work and the efforts of many staffs, including the Band 10 development team and the commissioning team in Chile. I'd like to express my appreciation to all, and I am looking forward to seeing yet more new insights into the Universe."
- Development of the ALMA receivers was not easy, especially for Band 10. Due to its extreme high frequency, researchers could not use the conventional superconducting devices made of Niobium. The development team made a high quality film from the compound superconductive material NbTiN (niobium-titanium nitrides) in cooperation with the National Institute of Information and Communication Technology to achieve the world's highest performance in the frequency of Band 10 in 2009. The team finished manufacturing and shipping the 73 receiver cartridges in 2014. After extensive commissioning and test observation on site, the Band 10 receivers have been used in ALMA's normal science operation since October 2015.
Figure 42: Composite ALMA image of NGC 6334I, a star-forming region in the Cat's Paw Nebula, taken with the Band 10 receivers, ALMA's highest-frequency vision. The blue component is heavy water (HDO) streaming away from either a single protostar or a small cluster of protostars. The orange region is the "continuum emission" in the same region, which scientists found is extraordinarily rich in molecular fingerprints, including glycoaldehyde, the simplest sugar-related molecule (image credit: ALMA (ESO/NAOJ/NRAO): NRAO/AUI/NSF, B. Saxton)
• November 15, 2018: The most luminous galaxy in the universe has been caught in the act of stripping away nearly half the mass from at least three of its smaller neighbors, according to a new study published in the journal Science. The light from this galaxy, known as W2246-0526, took 12.4 billion years to reach us, so we are seeing it as it was when our universe was only about a tenth of its present age. 66) 67)
- New observations with the ALMA reveal distinct streamers of material being pulled from three smaller galaxies and flowing into the more massive galaxy, which was discovered in 2015 by NASA's spaceborne WISE (Wide-field Infrared Survey Explorer). It is by no means the largest or most massive galaxy we know of, but it is unrivaled in its brightness, emitting as much infrared light as 350 trillion Suns.
- The connecting tendrils between the galaxies contain about as much material as the galaxies themselves. ALMA's amazing resolution and sensitivity allowed the researchers to detect these remarkably faint and distant trans-galactic streamers.
- "We knew from previous data that there were three companion galaxies, but there was no evidence of interactions between these neighbors and the central source," said Tanio Díaz-Santos of the Universidad Diego Portales in Santiago, Chile, lead author of the study. "We weren't looking for cannibalistic behavior and weren't expecting it, but this deep dive with the ALMA observatory makes it very clear." 68)
- Galactic cannibalism is not uncommon, though this is the most distant galaxy in which such behavior has been observed and the study authors are not aware of any other direct images of a galaxy simultaneously feeding on material from multiple sources at those early cosmic times.
- The researchers emphasize that the amount of gas being devoured by W2246-0526 is enough to keep it forming stars and feeding its central black hole for hundreds of millions of years.
- This galaxy's startling luminosity is not due to its individual stars. Rather, its brightness is powered by a tiny, yet fantastically energetic disk of gas that is being superheated as it spirals in on the supermassive black hole. The light from this blazingly bright accretion disk is then absorbed by the surrounding dust, which re-emits the energy as infrared light.
- This extreme infrared radiation makes this galaxy one of a rare class of quasars known as Hot, Dust-Obscured Galaxies or Hot DOGs. Only about one out of every 3,000 quasars observed by WISE belongs to this class.
- Much of the dust and gas being siphoned away from the three smaller galaxies is likely being converted into new stars and feeding the larger galaxy's central black hole. This galaxy's gluttony, however, may lead to its self-destruction. Previous research suggests that the energy of the AGN will ultimately jettison much, if not all of the galaxy's star-forming fuel.
- An earlier work led by co-author Chao-Wei Tsai of UCLA estimates that the black hole at the center of W2246-0526 is about 4 billion times the mass of the Sun. The mass of the black hole directly influences how bright the AGN can become, but — according to this earlier research — W2246-0526 is about 3 times more luminous than what should be possible. Solving this apparent contradiction will require additional observations.
Figure 43: Artist impression of W2246-0526, the most luminous known galaxy, and three companion galaxies (image credit: NRAO/AUI/NSF, S. Dagnello)
• October 23, 2018: Jupiter's icy moon Europa has a chaotic surface terrain that is fractured and cracked, suggesting a long-standing history of geologic activity. A new series of four images of Europa taken with ALMA (Atacama Large Millimeter/submillimeter Array) has helped astronomers create the first global thermal map of this cold satellite of Jupiter. The new images have a resolution of roughly 200 km, sufficient to study the relationship between surface thermal variations and the moon's major geologic features. 69)
- The researchers compared the new ALMA observations of Europa to a thermal model based on observations from the Galileo spacecraft. This comparison allowed them to analyze the temperature changes in the data and construct the first-ever global map of Europa's thermal characteristics. The new data also revealed an enigmatic cold spot on Europa's northern hemisphere.
- "These ALMA images are really interesting because they provide the first global map of Europa's thermal emission," said Samantha Trumbo, a planetary scientist at the California Institute of Technology and lead author on a paper published in the Astronomical Journal. "Since Europa is an ocean world with potential geologic activity, its surface temperatures are of great interest because they may constrain the locations and extents of any such activity." 70)
- Evidence strongly suggests that beneath its thin veneer of ice, Europa has an ocean of briny water in contact with a rocky core. Europa also has a comparatively young surface, only about 20 to 180 million years old, indicating that there are as-yet-unidentified thermal or geologic processes at work.
- Unlike optical telescopes, which can only detect sunlight reflected by planetary bodies, radio and millimeter-wave telescopes like ALMA can detect the thermal "glow" naturally emitted by even relatively cold object in our Solar System, including comets, asteroids, and moons. At its warmest, Europa's surface temperature never rises above minus 160 degrees Celsius (minus 260 degrees Fahrenheit).
- "Studying Europa's thermal properties provides a unique means of understanding its surface," said Bryan Butler, an astronomer at the NRAO (National Radio Astronomy Observatory) in Socorro, New Mexico, and coauthor on the paper. NRAO is a facility of the National Science Foundation (NSF), operated under cooperative agreement by Associated Universities, Inc.
Figure 44: First spatially resolved, complete thermal data set of Jupiter's Icy Moon, Europa. ALMA was able to map out thermal variations on its surface. Hubble image of Jupiter in the background (image credit: ALMA (ESO/NAOJ/NRAO), S. Trumbo et al.; NRAO/AUI NSF, S. Dagnello; NASA/Hubble)
Figure 45: Series of 4 images of the surface of Europa taken with ALMA, enabling astronomers to create the first global thermal map of Jupiter's icy moon (image credit: ALMA (ESO/NAOJ/NRAO), S. Trumbo et al.)
• October 8, 2018: Using ALMA, an international team of astronomers found evidence that a white dwarf (the elderly remains of a Sun-like star) and a brown dwarf (a failed star without the mass to sustain nuclear fusion) collided in a short-lived blaze of glory that was witnessed on Earth in 1670 as Nova sub Capite Cygni (a New Star below the Head of the Swan), which is now known as CK Vulpeculae. 71)
- In July of 1670, observers on Earth witnessed a "new star," or nova, in the constellation Cygnus. Where previously there was dark sky, a bright pinprick of light appeared, faded, reappeared, and then disappeared entirely from view. Modern astronomers studying the remains of this cosmic event initially thought it heralded the merging of two main sequence stars – stars on the same evolutionary path as our Sun.
- New observations with ALMA point to a more intriguing explanation. By studying the debris from this explosion, which takes the form of dual rings of dust and gas resembling an hourglass with a compact central object, the researchers concluded that a brown dwarf (a so-called failed star without the mass to sustain nuclear fusion) merged with a white dwarf (the elderly, cooling remains of a Sun-like star).
Figure 46: ALMA image of CK Vulpeculae. New research indicates that this hourglass-like object is the result of the collision of a brown dwarf and a white dwarf (image credit: ALMA (ESO/NAOJ/NRAO)/S. P. S. Eyres)
- "It now seems what was observed centuries ago was not what we would today describe as a classic ‘nova.' Instead, it was the merger of two stellar objects, a white dwarf and a brown dwarf. When these two objects collided, they spilled out a cocktail of molecules and unusual isotopes, which gave us new insights into the nature of this object," said Sumner Starrfield, an astronomer at Arizona State University and co-author on a paper appearing in the Monthly Notices of the Royal Astronomical Society.
- According to the researchers, the white dwarf would have been about ten times more massive than the brown dwarf, though much smaller in size. As the brown dwarf spiraled inward, intense tidal forces exerted by the white dwarf would have ripped it apart. "This is the first time such an event has been conclusively identified," remarked Starrfield.
- Since most star systems in the Milky Way are binary, stellar collisions are not that rare, the astronomers note. The new ALMA observations reveal new details about the 1670 event. By studying the light from two, more-distant stars as it shines through the dusty remains of the merger, the researchers were able to detect the telltale signature of the element lithium, which is easily destroyed in the interior of a main sequence star, but not inside a brown dwarf.
- "The presence of lithium, together with unusual isotopic ratios of the elements carbon, nitrogen, and oxygen point to material from a brown dwarf star being dumped on the surface of a white dwarf. The thermonuclear ‘burning' and an eruption of this material resulted in the hourglass we see today," said Stewart Eyres, Deputy Dean of the Faculty of Computing, Engineering and Science at the University of South Wales and lead author on the paper. 72)
- Intriguingly, the hourglass is also rich in organic molecules such as formaldehyde (H2CO) and formamide (NH2CHO), which is derived from formic acid. These molecules would not survive in an environment undergoing nuclear fusion and must have been produced in the debris from the explosion. This lends further support to the conclusion that a brown dwarf met its demise in a star-on-star collision with a white dwarf.
• August 30, 2018: Using ALMA (Atacama Large Millimeter/submillimeter Array), a team of astronomers revealed that molecular clouds in a galaxy called AzTEC/COSMOS-1, an extreme starburst (star-forming) galaxy located 12.4 billion light-years away, are highly unstable, which leads to runaway star formation. Extreme starburst galaxies, also known as ‘monster' galaxies, are thought to be the ancestors of very massive elliptical galaxies in today's Universe, therefore the findings pave the way to understand the formation and evolution of such galaxies. 73)
- ‘Monster' galaxies form stars at a startling pace — 1,000 times higher than the star formation in our Milky Way Galaxy. But why are they so active?
- To tackle this problem, astronomers need to know the environment around the stellar nurseries. Drawing detailed maps of molecular clouds is an important step to scout a cosmic monster.
- "A real surprise is that AzTEC/COSMOS-1 seen 12.4 billion years ago has a massive, ordered gas disk that is in regular rotation instead of what we had expected, which would have been some kind of a disordered train wreck that most theoretical studies had predicted," said University of Massachusetts Amherst's Professor Min Yun, co-author of the current study and a member of the team that discovered this galaxy in 2007.
- "AzTEC/COSMOS-1's gas disk is dynamically unstable now, which means the entire gas disk that makes up this galaxy is fragmenting and undergoing a gigantic episode of starburst, which helps to explain its enormous star formation rate, more than 1,000 times that of the Milky Way."
- Professor Yun and colleagues found that AzTEC/COSMOS-1 is rich in the ingredients of stars, but it was still difficult to figure out the nature of the cosmic gas in the galaxy.
- They used ALMA's high resolution and high sensitivity to observe the galaxy and obtain a detailed map of the distribution and the motion of the gas to make the highest resolution molecular gas map of a distant monster galaxy ever made.
- "We found that there are two distinct large clouds several thousand light-years away from the center," said study lead author Dr. Ken-ichi Tadaki, a postdoctoral researcher at the Japan Society for the Promotion of Science and the National Astronomical Observatory of Japan. "In most distant starburst galaxies, stars are actively formed in the center. So it is surprising to find off-center clouds."
- "How these galaxies have been able to amass such a large quantity of gas in the first place and then essentially turn the entire gas reserve into stars in the blink of an eye, cosmologically speaking, was a completely unknown question about which we could only speculate. We have the first answers now," Professor Yun said. "Until this result came in from ALMA, nobody knew how Nature created massive, young galaxies formed only 1 billion years after the Big Bang."
Figure 47: Artist's impression of the extreme starburst galaxy AzTEC/COSMOS-1 (image credit: National Astronomical Observatory of Japan)
- With the new observations, the researchers now believe that the monster galaxy is powered by ‘an extremely gas-heavy disk that is somehow kept stable until enough gas is amassed.'
- "We still don't know yet how so much gas is collected so quickly and what kept this enormous gas reserve from igniting and turning into stars, as gas is known to do in the local Universe," Professor Yun said.
- The astronomers found that the gas clouds in AzTEC/COSMOS-1 are very unstable, which is unusual. "In a normal situation, the inward gravity and outward pressure are balanced," they said. "Once gravity overcomes pressure, the gas cloud collapses and forms stars at a rapid pace. Then, stars and supernova explosions at the end of the stellar life cycle blast out gases, which increase the outward pressure."
Figure 48: ALMA observations revealed dense gas concentrations in the disk of AzTEC/COSMOS-1 and intense stars formation in those concentrations (image credit: National Astronomical Observatory of Japan)
- "As a result, the gravity and pressure reach a balanced state and star formation continues at a moderate pace. In this way star formation in galaxies is self-regulating."
- "But in AzTEC/COSMOS-1, the pressure is far weaker than gravity and hard to balance. Therefore this galaxy shows runaway star formation and has morphed into an unstoppable monster galaxy."
- The study authors estimate that the gas in AzTEC/COSMOS-1 will be completely consumed in 100 million years, which is 10 times faster than in other star forming galaxies. 74)
- Why the gas in this galaxy is so unstable is not clear yet, but a phenomenon called ‘galaxy merger' is a possible cause.
- Galaxy collision may have efficiently transported the gas into a small area and ignited intense star formation.
- "At this moment, we have no evidence of merger in this galaxy. But by observing other similar galaxies with ALMA, we want to unveil the relation between galaxy mergers and monster galaxies," Dr. Tadaki said.