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.
Major ALMA Facilities
ALMA will be the world's most powerful telescope for studying the Universe at submillimetre and millimetre 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. 14)
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 2: 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-metre and 7-metre 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 3: Photo of the ALMA antenna array (image credit: ALMA partnership, Ref. 3)
Figure 4: 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) 15)
Figure 5: 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) 16)
ALMA Front End Integration Centers: A construction project like ALMA, involving several partners in four different continents, requires consensus on several organisational 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 programme 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 6: 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: 17)
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 7). 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°
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. 18)
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 8 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. 19)
Figure 8: 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 9: 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 10: 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 11. 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. 20) 21) 22)
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.
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. 23)
Figure 12: 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
Only a selected few images can be shown here. The interested reader is referred to the ALMA Press Release site for more details. 24)
• March 7, 2018: This spectacular and unusual image of Figure 13 shows part of the famous Orion Nebula, a star formation region lying about 1350 light-years from Earth. It combines a mosaic of millimeter-wavelength images from the ALMA (Atacama Large Millimeter/submillimeter Array) and the IRAM (Institut de Radioastronomie Millimétrique) 30 m telescope, shown in red, with a more familiar infrared view from the HAWK-I (High Acuity Wide-field K-band Imager) instrument on ESO's VLT (Very Large Telescope), shown in blue. The group of bright blue-white stars at the upper-left is the Trapezium Cluster — made up of hot young stars that are only a few million years old. 25)
Figure 13: New data from ALMA and other telescopes have been used to create this stunning image showing a web of filaments in the Orion Nebula. These features appear red-hot and fiery in this dramatic picture, but in reality are so cold that astronomers must use telescopes like ALMA to observe them (image credit: ESO)
- The wispy, fiber-like structures seen in this large image are long filaments of cold gas, only visible to telescopes working in the millimeter wavelength range. They are invisible at both optical and infrared wavelengths, making ALMA one of the only instruments available for astronomers to study them. This gas gives rise to newborn stars — it gradually collapses under the force of its own gravity until it is sufficiently compressed to form a protostar — the precursor to a star.
- The scientists, who gathered the data from which this image was created, were studying these filaments to learn more about their structure and make-up. They used ALMA to look for signatures of diazenylium gas, which makes up part of these structures. Through doing this study, the team managed to identify a network of 55 filaments.
- The Orion Nebula is the nearest region of massive star formation to Earth, and is therefore studied in great detail by astronomers seeking to better understand how stars form and evolve in their first few million years. ESO's telescopes have observed this interesting region multiple times, and you can learn more about previous discoveries here, here, and here.
- This image combines a total of 296 separate individual datasets from the ALMA and IRAM telescopes, making it one of the largest high-resolution mosaics of a star formation region produced so far at millimeter wavelengths.
• February 26, 2018: Space weather emitted by Proxima Centauri, the star closest to our sun, may make that system rather inhospitable to life after all. Using data from ALMA (Atacama Large Millimeter/submillimeter Array), a team of astronomers discovered that a powerful stellar flare erupted from Proxima Centauri in March 2017 (Figure 14). This finding, published in the Astrophysical Journal Letters, raises questions about the habitability of our solar system's nearest exoplanetary neighbor, Proxima b, which orbits Proxima Centauri. 26) 27)
- At its peak, the newly recognized flare was 10 times brighter than our sun's largest flares, when observed at similar wavelengths. Stellar flares have not been well studied at the millimeter and submillimeter wavelengths detected by ALMA, especially around stars of Proxima Centauri's type, called M dwarfs, which are the most common in our galaxy.
- "March 24, 2017, was no ordinary day for Proxima Cen," said Meredith MacGregor, an astronomer at the Carnegie Institution for Science, Department of Terrestrial Magnetism in Washington, D.C., who led the research with fellow Carnegie astronomer Alycia Weinberger. Along with colleagues from the Harvard-Smithsonian Center for Astrophysics, David Wilner and Adam Kowalski, and Steven Cranmer of the University of Colorado Boulder — they discovered the enormous flare when they reanalyzed ALMA observations taken last year.
- The flare increased Proxima Centauri's brightness by 1,000 times over 10 seconds. This was preceded by a smaller flare; taken together, the whole event lasted fewer than two minutes of the 10 hours that ALMA observed the star between January and March of last year (Figure 15).
- Stellar flares happen when a shift in the star's magnetic field accelerates electrons to speeds approaching that of light. The accelerated electrons interact with the highly charged plasma that makes up most of the star, causing an eruption that produces emission across the entire electromagnetic spectrum.
- "It's likely that Proxima b was blasted by high energy radiation during this flare," MacGregor explained, adding that it was already known that Proxima Centauri experienced regular, although smaller, X-ray flares. "Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water."
- An earlier paper that also used the same ALMA data interpreted its average brightness, which included the light output of both the star and the flare together, as being caused by multiple disks of dust encircling Proxima Centauri, not unlike our own solar system's asteroid and Kuiper belts.
- But when MacGregor, Weinberger, and their team looked at the ALMA data as a function of observing time, instead of averaging it all together, they were able to see the transient explosion of radiation emitted from Proxima Centauri for what it truly was.
- "There is now no reason to think that there is a substantial amount of dust around Proxima Cen," Weinberger said. "Nor is there any information yet that indicates the star has a rich planetary system like ours."
Figure 14: Artist impression of a red dwarf star like Proxima Centauri, the nearest star to our sun. New analysis of ALMA observations reveal that Proxima Centauri emitted a powerful flare that would have created inhospitable conditions for planets in that system (image Credit: NRAO/AUI/NSF; D. Berry)
Figure 15: The brightness of Proxima Centauri as observed by ALMA over the two minutes of the event on March 24, 2017. The massive stellar flare is shown in red, with the smaller earlier flare in orange, and the enhanced emission surrounding the flare that could mimic a disk in blue. At its peak, the flare increased Proxima Centauri's brightness by 1,000 times. The shaded area represents uncertainty (image credit: Meredith MacGregor)
Figure 16: An artist's impression of a flare from Proxima Centauri, modeled after the loops of glowing hot gas seen in the largest solar flares. An artist's impression of the exoplanet Proxima b is shown in the foreground. Proxima b orbits its star 20 times closer than the Earth orbits the Sun. A flare 10 times larger than a major solar flare would blast Proxima b with 4,000 times more radiation than the Earth gets from our Sun's flares (image credit: Roberto Molar Candanosa / Carnegie Institution for Science, NASA/SDO, NASA/JPL)
• February 14, 2018: High resolution observations with ALMA imaged a rotating dusty gas torus around an active supermassive black hole. The existence of such rotating donuts-shape structures was first suggested decades ago, but this is the first time one has been confirmed so clearly. This is an important step in understanding the co-evolution of supermassive black holes and their host galaxies. 28) 29)
- Almost all galaxies hold concealed monstrous black holes in their centers. Researchers have known for a long time that the more massive the galaxy is, the more massive the central black hole is. This sounds reasonable at first, but host galaxies are 10 billion times bigger than the central black holes; it should be difficult for two objects of such vastly different scales to directly affect each other. So how could such a relation develop?
- Aiming to solve this shadowy problem, a team of astronomers utilized the high resolution of ALMA to observe the center of spiral galaxy M77. The central region of M77 is an "active galactic nucleus," or AGN, which means that matter is vigorously falling toward the central supermassive black hole and emitting intense light. AGNs can strongly affect the surrounding environment, therefore they are important objects for solving the mystery of the co-evolution of galaxies and black holes.
Figure 17: The central region of the spiral galaxy M77. The NASA/ESA Hubble Space Telescope imaged the distribution of stars. ALMA revealed the distribution of gas in the very center of the galaxy (ALMA image at right). ALMA imaged a horseshoe-like structure with a radius of 700 light-years and a central compact component with a radius of 20 light-years . The latter is the gaseous torus around the AGN (Active Galactic Nucleus). Red indicates emission from formyl ions (HCO+) and green indicates hydrogen cyanide emission (image credit: ALMA (ESO/NAOJ/NRAO), Imanishi et al., NASA/ESA Hubble Space Telescope and A. van der Hoeven)
- The team imaged the area around the supermassive black hole in M77 and resolved a compact gaseous structure with a radius of 20 light-years. And, the astronomers found that the compact structure is rotating around the black hole, as expected.
Figure 18: Motion of gas around the supermassive black hole in the center of M77. The gas moving toward us is shown in blue and that moving away from us is in red. The gas's rotation is centered around the black hole (image credit: ALMA (ESO/NAOJ/NRAO), Imanishi et al.)
- "To interpret various observational features of AGNs, astronomers have assumed rotating donut-like structures of dusty gas around active supermassive black holes. This is called the ‘unified model' of AGN," explained Masatoshi Imanishi of NAOJ (National Astronomical Observatory of Japan), the lead author on a paper published in the Astrophysical Journal Letters. "However, the dusty gaseous donut is very tiny in appearance. With the high resolution of ALMA, now we can directly see the structure."
Figure 19: Artist's impression of the dusty gaseous torus around an active supermassive black hole. ALMA revealed the rotation of the torus very clearly for the first time (image credit: ALMA (ESO/NAOJ/NRAO))
- Many astronomers have observed the center of M77 before, but never has the rotation of the gas donut around the black hole been seen so clearly. Besides the superior resolution of ALMA, the selection of molecular emission lines to observe was key to revealing the structure. The team observed specific microwave emission from hydrogen cyanide molecules (HCN) and formyl ions (HCO+). These molecules emit microwaves only in dense gas, whereas the more frequently observed carbon monoxide (CO) emits microwaves under a variety of conditions . The torus around the AGN is assumed to be very dense, and the team's strategy was right on the mark.
- "Previous observations have revealed the east-west elongation of the dusty gaseous torus. The dynamics revealed from our ALMA data agrees exactly with the expected rotational orientation of the torus," said Imanishi.
- Interestingly, the distribution of gas around the supermassive black hole is much more complicated than what a simple unified model suggests. The torus seems to have an asymmetry and the rotation is not just following the gravity of the black hole but also contains highly random motion. These facts could indicate the AGN had a violent history, possibly including a merger with a small galaxy . Nevertheless, the identification of the rotating torus is an important step.
- The Milky Way Galaxy, where we live, also has a supermassive black hole at its center. This black hole is, however, in a very quiet state. Only a tiny amount of gas is accreting onto it. Therefore, to investigate an AGN in detail, astronomers need to observe the centers of distant galaxies. M77 is one of the nearest AGN and a suitable object for peering into the very center in detail.
• November 3, 2017: The ALMA Observatory in Chile has detected dust around the closest star to the Solar System, Proxima Centauri. These new observations reveal the glow coming from cold dust in a region between one to four times as far from Proxima Centauri as the Earth is from the Sun. The data also hint at the presence of an even cooler outer dust belt and may indicate the presence of an elaborate planetary system. These structures are similar to the much larger belts in the Solar System and are also expected to be made from particles of rock and ice that failed to form planets. 30)
- Proxima Centauri is the closest star to the Sun. It is a faint red dwarf lying just four light-years away in the southern constellation of Centaurus (The Centaur). It is orbited by the Earth-sized temperate world Proxima b, discovered in 2016 and the closest planet to the Solar System. But there is more to this system than just a single planet. The new ALMA observations reveal emission from clouds of cold cosmic dust surrounding the star.
- The lead author of the new study, Guillem Anglada, from the Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain, explains the significance of this find: "The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it's the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our Sun." 31)
- Dust belts are the remains of material that did not form into larger bodies such as planets. The particles of rock and ice in these belts vary in size from the tiniest dust grain, smaller than a millimeter across, up to asteroid-like bodies many kilometers in diameter.
- Dust appears to lie in a belt that extends a few hundred million kilometers from Proxima Centauri and has a total mass of about one hundredth of the Earth's mass. This belt is estimated to have a temperature of about –230 degrees Celsius, as cold as that of the Kuiper Belt in the outer Solar System.
- There are also hints in the ALMA data of another belt of even colder dust about ten times further out. If confirmed, the nature of an outer belt is intriguing, given its very cold environment far from a star that is cooler and fainter than the Sun. Both belts are much further from Proxima Centauri than the planet Proxima b, which orbits at just four million kilometers from its parent star.
Figure 20: Artist's impression of the dust belts around Proxima Centauri (image credit: ESO, Release No: eso1735) 32)
• October 2, 2017: Using data captured by ALMA in Chile and from the ROSINA instrument on ESA's Rosetta mission, a team of astronomers has found faint traces of the chemical compound Freon-40 (CH3Cl), also known as methyl chloride and chloromethane, around both the infant star system IRAS 16293-2422 33), about 400 light-years away, and the famous comet 67P/Churyumov-Gerasimenko (67P/C-G) in our own Solar System. The new ALMA observation is the first detection ever of a stable organohalogen in interstellar space. 34) 35)
- Organohalogens consist of halogens, such as chlorine and fluorine, bonded with carbon and sometimes other elements. On Earth, these compounds are created by some biological processes — in organisms ranging from humans to fungi — as well as by industrial processes such as the production of dyes and medical drugs.
- This new discovery of one of these compounds, Freon-40, in places that must predate the origin of life, can be seen as a disappointment, as earlier research had suggested that these molecules could indicate the presence of life.
- "Finding the organohalogen Freon-40 near these young, Sun-like stars was surprising," said Edith Fayolle, a researcher with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts in the USA, and lead author of the new paper. "We simply didn't predict its formation and were surprised to find it in such significant concentrations. It's clear now that these molecules form readily in stellar nurseries, providing insights into the chemical evolution of planetary systems, including our own." 36)
Figure 21: Observations made with ALMA and ESA's Rosetta mission, have revealed the presence of the organohalogen Freon-40 in gas around both an infant star and a comet. Organohalogens are formed by organic processes on Earth, but this is the first ever detection of them in interstellar space. This discovery suggests that organohalogens may not be as good markers of life as had been hoped, but that they may be significant components of the material from which planets form. This result, which appears in the journal Nature Astronomy, underscores the challenge of finding molecules that could indicate the presence of life beyond Earth (image credit: ESO)
• June 26, 2017: Betelgeuse is one of the largest stars currently known — with a radius around 1400 times larger than the Sun's in the millimeter continuum. About 600 light-years away in the constellation of Orion (The Hunter) in the Milky Way, the red supergiant burns brightly, causing it to have only a short life expectancy. The star is just about eight million years old, but is already on the verge of becoming a supernova. When that happens, the resulting explosion will be visible from Earth, even in broad daylight. 37) 38)
The star has been observed in many other wavelengths, particularly in the visible, infrared, and ultraviolet. Using ESO's Very Large Telescope, astronomers discovered a vast plume of gas almost as large as our Solar System. Astronomers have also found a gigantic bubble that boils away on Betelgeuse's surface. These features help to explain how the star is shedding gas and dust at tremendous rates (eso0927, eso1121). In this picture, ALMA observes the hot gas of the lower chromosphere of Betelgeuse at sub-millimeter wavelengths — where localized increased temperatures explain why it is not symmetric. Scientifically, ALMA can help us to understand the extended atmospheres of these hot, blazing stars.
Figure 22: This orange blob shows the nearby star Betelgeuse, as seen by ALMA. This is the first time that ALMA has ever observed the surface of a star and this first attempt has resulted in the highest-resolution image of Betelgeuse available (image credit: ALMA (ESO/NAOJ/NRAO)/E. O'Gorman/P. Kervella)
Figure 23: This image, made with ALMA, shows the red supergiant Betelgeuse — one of the largest stars known. In the millimeter continuum the star is around 1400 times larger than our Sun. The overlaid annotation shows how large the star is compared to the Solar System. Betelgeuse would engulf all four terrestrial planets — Mercury, Venus, Earth and Mars — and even the gas giant Jupiter. Only Saturn would be beyond its surface (image credit: ALMA (ESO/NAOJ/NRAO)/E. O'Gorman/P. Kervella) 39)
• May 18, 2017: An international team of astronomers using the ALMA (Atacama Large Millimeter/submillimeter Array) has made the first complete millimeter-wavelength image of the ring of dusty debris surrounding the young star Fomalhaut. This remarkably well-defined band of rubble and gas is likely the result of exocomets smashing together near the outer edges of a planetary system 25 light-years from Earth. Observations Suggest Chemical Kinship to Comets in Our Own Solar System. 40) 41)
- Earlier ALMA observations of Fomalhaut — taken in 2012 when the telescope was still under construction – revealed only about one half of the debris disk. Though this first image was merely a test of ALMA's initial capabilities, it nonetheless provided tantalizing hints about the nature and possible origin of the disk.
- The new ALMA observations offer a stunningly complete view of this glowing band of debris and suggest that there are chemical similarities between its icy contents and comets in our own solar system.
- "ALMA has given us this staggeringly clear image of a fully formed debris disk," said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA, and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations. "We can finally see the well-defined shape of the disk, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance."
- Fomalhaut is a relatively nearby star system and one of only about 20 in which planets have been imaged directly. The entire system is approximately 440 million years old, or about one-tenth the age of our solar system.
- As revealed in the new ALMA image (Figure 24), a brilliant band of icy dust about 2 billion kilometers wide has formed approximately 20 billion kilometers from the star.
- Debris disks are common features around young stars and represent a very dynamic and chaotic period in the history of a solar system. Astronomers believe they are formed by the ongoing collisions of comets and other planetesimals in the outer reaches of a recently formed planetary system. The leftover debris from these collisions absorbs light from its central star and reradiates that energy as a faint millimeter-wavelength glow that can be studied with ALMA.
- Using the new ALMA data and detailed computer modeling, the researchers could calculate the precise location, width, and geometry of the disk. These parameters confirm that such a narrow ring is likely produced through the gravitational influence of planets in the system, noted MacGregor.
Figure 24: Composite image of the Fomalhaut star system. The ALMA data, shown in orange, reveal the distant and eccentric debris disk in never-before-seen detail. The central dot is the unresolved emission from the star, which is about twice the mass of the Sun. Optical data from the Hubble Space Telescope is in blue; the dark region is a coronagraphic mask, which filtered out the otherwise overwhelming light of the central star [image credit: ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF)]
- The new ALMA observations are also the first to definitively show "apocenter glow," a phenomenon predicted in a 2016 paper by lead author Margaret Pan, a scientist at MIT (Massachusetts Institute of Technology) in Cambridge and co-author on the new ALMA papers. Like all objects with elongated orbits, the dusty material in the Fomalhaut disk travels more slowly when it is farthest from the star. As the dust slows down, it piles up, forming denser concentrations in the more distant portions of the disk. These dense regions can be seen by ALMA as brighter millimeter-wavelength emission.
- Using the same ALMA dataset, but focusing on distinct millimeter-wavelength signals naturally emitted by molecules in space, the researchers also detected vast stores of carbon monoxide gas in precisely the same location as the debris disk.
- "These data allowed us to determine that the relative abundance of carbon monoxide plus carbon dioxide around Fomalhaut is about the same as found in comets in our own solar system," said Luca Matrà with the University of Cambridge, UK, and lead author on the team's second paper. "This chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own." Matrà and his colleagues believe this gas is either released from continuous comet collisions or the result of a single, large impact between supercomets hundreds of times more massive than Hale-Bopp. 42)
- The presence of this well-defined debris disk around Fomalhaut, along with its curiously familiar chemistry, may indicate that this system is undergoing its own version of the Late Heavy Bombardment, a period approximately 4 billion years ago when the Earth and other planets were routinely struck by swarms of asteroids and comets left over from the formation of the Solar System.
- "Twenty years ago, the best millimeter-wavelength telescopes gave the first fuzzy maps of sand grains orbiting Fomalhaut. Now with ALMA's full capabilities the entire ring of material has been imaged," concluded Paul Kalas, an astronomer at the University of California at Berkeley and principal investigator on these observations. "One day we hope to detect the planets that influence the orbits of these grains."
Figure 25: ALMA image of the debris disk in the Fomalhaut star system. The ring is approximately 20 billion kilometers from the central star and it is about 2 billion kilometers wide. The central dot is the unresolved emission from the star, which is about twice the mass of the Sun (image credit: ALMA (ESO/NAOJ/NRAO); M. MacGregor)
• April 12, 2017: Using ALMA, astronomers have revealed extraordinary details about a recently discovered far-flung member of our solar system, the planetary body 2014 UZ224, more informally known as DeeDee. 43) 44)
- At about three times the current distance of Pluto from the Sun, DeeDee is the second most distant known trans-Neptunian object (TNO) with a confirmed orbit, surpassed only by the dwarf planet Eris. Astronomers estimate that there are tens-of-thousands of these icy bodies in the outer solar system beyond the orbit of Neptune.
- The new ALMA data reveal, for the first time, that DeeDee is roughly 635 km across, or about two-thirds the diameter of the dwarf planet Ceres, the largest member of our asteroid belt. At this size, DeeDee should have enough mass to be spherical, the criterion necessary for astronomers to consider it a dwarf planet, though it has yet to receive that official designation.
- "Far beyond Pluto is a region surprisingly rich with planetary bodies. Some are quite small but others have sizes to rival Pluto, and could possibly be much larger," said David Gerdes, a scientist with the University of Michigan and lead author on a paper appearing in the Astrophysical Journal Letters. "Because these objects are so distant and dim, it's incredibly difficult to even detect them, let alone study them in any detail. ALMA, however, has unique capabilities that enabled us to learn exciting details about these distant worlds." 45)
- Currently, DeeDee is about 92 astronomical units (AU) from the Sun. An astronomical unit is the average distance from the Earth to the Sun, or about 150 million kilometers. At this tremendous distance, it takes DeeDee more than 1,100 Earth years to complete one orbit. Light from DeeDee takes nearly 13 hours to reach Earth.
- Gerdes and his team announced the discovery of DeeDee in the fall of 2016. They found it using the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile as part of ongoing observations for the Dark Energy Survey, an optical survey of about 12 percent of the sky that seeks to understand the as-yet mysterious force that is accelerating the expansion of the universe.
- The Dark Energy Survey produces vast troves of astronomical images, which give astronomers the opportunity to also search for distant solar system objects. The initial search, which includes nearly 15,000 images, identified more than 1.1 billion candidate objects. The vast majority of these turned out to be background stars and even more distant galaxies. A small fraction, however, were observed to move slowly across the sky over successive observations, the telltale sign of a TNO.
- One such object was identified on 12 separate images. The astronomers informally dubbed it DeeDee, which is short for Distant Dwarf.
Figure 26: Artist concept of the planetary body 2014 UZ224, more informally known as DeeDee. ALMA was able to observe the faint millimeter-wavelength "glow" emitted by the object, confirming it is roughly 635 kilometers across. At this size, DeeDee should have enough mass to be spherical, the criterion necessary for astronomers to consider it a dwarf planet, though it has yet to receive that official designation (image credit: Alexandra Angelich, NRAO/AUI/NSF)
Figure 27: Orbits of objects in our solar system, showing the current location of the planetary body 'DeeDee' (image credit: Alexandra Angelich, NRAO/AUI/NSF)
• March 8, 2017: An international team of astronomers, led by Nicolas Laporte of University College London, have used ALMA to observe A2744_YD4, the youngest and most remote galaxy ever seen by ALMA. They were surprised to find that this youthful galaxy contained an abundance of interstellar dust — dust formed by the deaths of an earlier generation of stars. 46) 47)
- Follow-up observations using the X-shooter instrument on ESO's Very Large Telescope confirmed the enormous distance to A2744_YD4. The galaxy appears to us as it was when the Universe was only 600 million years old, during the period when the first stars and galaxies were forming. This time corresponds to a redshift of z=8.38, during the epoch of reionization. "Not only is A2744_YD4 the most distant galaxy yet observed by ALMA," comments Nicolas Laporte, "but the detection of so much dust indicates early supernovae must have already polluted this galaxy."
- Cosmic dust is mainly composed of silicon, carbon and aluminum, in grains as small as a millionth of a centimeter across. The chemical elements in these grains are forged inside stars and are scattered across the cosmos when the stars die, most spectacularly in supernova explosions, the final fate of short-lived, massive stars. Today, this dust is plentiful and is a key building block in the formation of stars, planets and complex molecules; but in the early Universe — before the first generations of stars died out — it was scarce.
- The observations of the dusty galaxy A2744_YD4 were made possible because this galaxy lies behind a massive galaxy cluster called Abell 2744. Because of a phenomenon called gravitational lensing, the cluster acted like a giant cosmic "telescope" to magnify the more distant A2744_YD4 by about 1.8 times, allowing the team to peer far back into the early Universe.
- The ALMA observations also detected the glowing emission of ionized oxygen from A2744_YD4. This is the most distant, and hence earliest, detection of oxygen in the Universe, surpassing another ALMA result from 2016.
- The detection of dust in the early Universe provides new information on when the first supernovae exploded and hence the time when the first hot stars bathed the Universe in light. Determining the timing of this "cosmic dawn" is one of the holy grails of modern astronomy, and it can be indirectly probed through the study of early interstellar dust.
- The team estimates that A2744_YD4 contained an amount of dust equivalent to 6 million times the mass of our Sun, while the galaxy's total stellar mass — the mass of all its stars — was 2 billion times the mass of our Sun. The team also measured the rate of star formation in A2744_YD4 and found that stars are forming at a rate of 20 solar masses per year — compared to just one solar mass per year in the Milky Way. This rate means that the total mass of the stars formed every year is equivalent to 20 times the mass of the Sun.
- "This rate is not unusual for such a distant galaxy, but it does shed light on how quickly the dust in A2744_YD4 formed," explains Richard Ellis (ESO and University College London), a co-author of the study. "Remarkably, the required time is only about 200 million years — so we are witnessing this galaxy shortly after its formation."
- This means that significant star formation began approximately 200 million years before the epoch at which the galaxy is being observed. This provides a great opportunity for ALMA to help study the era when the first stars and galaxies "switched on" — the earliest epoch yet probed. Our Sun, our planet and our existence are the products — 13 billion years later — of this first generation of stars. By studying their formation, lives and deaths, we are exploring our origins.
Figure 28: Artist's impression of the remote dusty galaxy A2744_YD4 (image credit: ESO)
• February 14, 2017: Astronomers using ALMA have discovered a surprising connection between a supermassive black hole and the galaxy where it resides. Powerful radio jets from the black hole – which normally suppress star formation – are stimulating the production of cold gas in the galaxy's extended halo of hot gas. This newly identified supply of cold, dense gas could eventually fuel future star birth as well as feed the black hole itself. 48)
- The researchers used ALMA to study a galaxy at the heart of the Phoenix Cluster, an uncommonly crowded collection of galaxies about 5.7 billion light-years from Earth.
- The central galaxy in this cluster harbors a supermassive black hole that is in the process of devouring star-forming gas, which fuels a pair of powerful jets that erupt from the black hole in opposite directions into intergalactic space. Astronomers refer to this type of black-hole powered system as an AGN (Active Galactic Nucleus).
- Earlier research with NASA's Chandra X-ray observatory revealed that the jets from this AGN are carving out a pair of giant "radio bubbles," huge cavities in the hot, diffuse plasma that surrounds the galaxy. - These expanding bubbles should create conditions that are too inhospitable for the surrounding hot gas to cool and condense, which are essential steps for future star formation.
- The latest ALMA observations, however, reveal long filaments of cold molecular gas condensing around the outer edges of the radio bubbles. These filaments extend up to 82,000 light-years from either side of the AGN. They collectively contain enough material to make about 10 billion suns.
- "With ALMA we can see that there's a direct link between these radio bubbles inflated by the supermassive black hole and the future fuel for galaxy growth," said Helen Russell, an astronomer with the University of Cambridge, UK, and lead author on a paper appearing in the Astrophysical Journal. "This gives us new insights into how a black hole can regulate future star birth and how a galaxy can acquire additional material to fuel an active black hole." 49)
- The AGN and Galaxy Growth Connection: The new ALMA observations reveal previously unknown connections between an AGN and the abundance of cold molecular gas that fuels star birth. "To produce powerful jets, black holes must feed on the same material that the galaxy uses to make new stars," said Michael McDonald, an astrophysicist at the Massachusetts Institute of Technology in Cambridge and coauthor on the paper. "This material powers the jets that disrupt the region and quenches star formation. This illustrates how black holes can slow the growth of their host galaxies."
Figure 29: Composite image showing how powerful radio jets from the supermassive black hole at the center of a galaxy in the Phoenix Cluster inflated huge "bubbles" in the hot, ionized gas surrounding the galaxy (the cavities inside the blue region imaged by NASA's Chandra X-ray observatory). Hugging the outside of these bubbles, ALMA discovered an unexpected trove of cold gas, the fuel for star formation (red). The background image is from the Hubble Space Telescope [image credit: ALMA (ESO/NAOJ/NRAO) H. Russell, et al.; NASA/ESA Hubble; NASA/CXC/MIT/M. McDonald et al.; B. Saxton (NRAO/AUI/NSF)]
Figure 30: ALMA image of cold molecular gas at the heart of the Phoenix Cluster. The filaments extending from the center hug enormous radio bubbles created by jets from a supermassive black hole. This discovery sheds light on the complex relationship between a supermassive black hole and its host galaxy [image credit: ALMA (ESO/NAOJ/NRAO), H. Russell et al.; B. Saxton (NRAO/AUI/NSF)]
Figure 31: Artist's impression of galaxy at the center of the Phoenix Cluster. Powerful radio jets from the supermassive black hole at the center of the galaxy are creating giant radio bubbles (blue) in the ionized gas surrounding the galaxy. ALMA has detected cold molecular gas (red) hugging the outside of the bubbles. This material could eventually fall into the galaxy where it could fuel future star birth and feed the supermassive black hole (image credit: B. Saxton (NRAO/AUI/NS)]
• January 17, 2017: Astronomers have harnessed ALMA's capabilities to image the millimeter-wavelength radiation emitted by the Sun's chromosphere — the region that lies just above the photosphere, which forms the visible surface of the Sun. The solar campaign team, an international group of astronomers with members from Europe, North America and East Asia, produced the images as a demonstration of ALMA's ability to study solar activity at longer wavelengths of light than are typically available to solar observatories on Earth. 50) 51)
- Astronomers have studied the Sun and probed its dynamic surface and energetic atmosphere in many ways through the centuries. But, to achieve a fuller understanding, astronomers need to study it across the entire electromagnetic spectrum, including the millimeter and submillimeter region of the spectrum that ALMA can observe.
- Since the Sun is many billions of times brighter than the faint objects ALMA typically observes, the ALMA antennas were specially designed to allow them to image the Sun in exquisite detail using the technique of radio interferometry — and avoid damage from the intense heat of the focussed sunlight. The result of this work is a series of images that demonstrate ALMA's unique vision and ability to study our Sun. The data from the solar observing campaign are being released this week to the worldwide astronomical community for further study and analysis.
- The team observed an enormous sunspot at wavelengths of 1.25 mm and of 3 mm (Figures 32 and 33), using two of ALMA's receiver bands. The images reveal differences in temperature between parts of the Sun's chromosphere . Understanding the heating and dynamics of the chromosphere are key areas of research that will be addressed in the future using ALMA.
- Sunspots are transient features that occur in regions where the Sun's magnetic field is extremely concentrated and powerful. They are lower in temperature than the surrounding regions, which is why they appear relatively dark.
- The difference in appearance between the two images is due to the different wavelengths of emitted light being observed. Observations at shorter wavelengths are able to probe deeper into the Sun, meaning the 1.25 mm images show a layer of the chromosphere that is deeper, and therefore closer to the photosphere, than those made at a wavelength of 3 mm.
- ALMA is the first facility where ESO is a partner that allows astronomers to study the nearest star, our own Sun. All other existing and past ESO facilities need to be protected from the intense solar radiation to avoid damage. The new ALMA capabilities will expand the ESO community to include solar astronomers.
- During a 30-month campaign period beginning in 2014, an international team of astronomers harnessed ALMA's single-antenna and array capabilities to detect and image the millimeter-wavelength light emitted by the Sun's chromosphere — the region that lies just above the photosphere, the visible surface of the Sun.
Figure 32: ALMA observed a giant sunspot with the band 6 receiver at the wavelength of 1.25 mm, acquired on Dec. 18, 2015. The sunspot is nearly twice the diameter of the Earth (image credit: ALMA, ESO, NAOJ, NRAO)
Legend to Figure 32: Sunspots are transient features that occur in regions where the Sun's magnetic field is extremely concentrated and powerful. They are lower in temperature than their surrounding regions, which is why they appear relatively dark in visible light. The ALMA image is essentially a map of temperature differences in a layer of the Sun's atmosphere known as the chromosphere, which lies just above the visible surface of the Sun (the photosphere). The chromosphere is considerably hotter than the photosphere. Understanding the heating and dynamics of the chromosphere are key areas of research that will be addressed by ALMA.
The ALMA Solar Development Team includes the following participants: Shin'ichiro Asayama, Miroslav Barta, Ondrejov, Tim Bastian, Roman Brajsa, Bin Chen, Bart De Pontieu, Gregory Fleishman, Dale Gary, Antonio Hales, Akihiko Hirota, Hugh Hudson, Richard Hills, Kazumasa Iwai, Sujin Kim, Neil Philips, Tsuyoshi Sawada, Masumi Shimojo, Giorgio Siringo, Ivica Skokic, Sven Wedemeyer, Stephen White,Pavel Yagoubov, Yihua Yan.
• December 12, 2016: Astronomers now know that our galaxy is teeming with planets, from rocky worlds roughly the size of Earth to gas giants bigger than Jupiter. Nearly every one of these exoplanets has been discovered in orbit around a mature star with a fully evolved planetary system. — New observations with the ALMA (Atacama Large Millimeter/submillimeter Array) contain compelling evidence that two newborn planets, each about the size of Saturn, are in orbit around a young star known as HD 163296. These planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star's protoplanetary disk. 52)
- Previous observations of other young star systems have helped to reshape our understanding of planet formation. For example, ALMA's images of HL Tauri and TW Hydrae revealed striking gaps and prominent ring structures in the stars' dusty disks. These features may be the tantalizing first signs that planets are being born. Remarkably, these signs appeared around much younger stars than astronomers thought possible, suggesting that planet formation can begin soon after the formation of a protoplanetary disk.
- "ALMA has shown us amazing images and never-before-seen views of the rings and gaps around young stars that could be the hallmarks of planet formation. However, since we were only looking at the dust in the disks with sufficient detail, we couldn't be sure what created these features," said Andrea Isella, an astronomer at Rice University in Houston, Texas, and lead author on a paper published in Physical Review Letters. 53)
- In studying HD 163296, the research team used ALMA to trace, for the first time, the distribution of both the dust and the carbon monoxide (CO) gas components of the disk at roughly the same level of detail.
- These observations revealed three distinct gaps in HD 163296's dust-filled protoplanetary disk. The first gap is located approximately 60 astronomical units from the central star, which is about twice the distance from our Sun to Neptune. (An astronomical unit – AU – is the average distance from the Earth to the Sun.) The other two gaps are 100 AU and 160 AU from the central star, well beyond the extent of our solar system's Kuiper Belt, the region of icy bodies beyond the orbit of Neptune.
- Using ALMA's ability to detect the faint millimeter-wavelength "glow" emitted by gas molecules, Isella and his team discovered that there was also an appreciable dip in the amount of CO in the outer two dust gaps.
- By seeing the same features in both the gas and the dust components of the disk, the astronomers believe they have found compelling evidence that there are two planets coalescing remarkably far from the central star. The width and depth of the two CO gaps suggest that each potential planet is roughly the same mass as Saturn, the astronomers said.
- In the gap nearest to the star, the team found little to no difference in the concentration of CO gas compared to the surrounding dusty disk. This means that the innermost gap could have been produced by something other than an emerging planet.
- "Dust and gas behave very differently around young stars," said Isella. "We know, for example, that there are certain chemical and physical process that can produce ringed structures in the dust like the ones we have seen previously. We certainly believe these structures could be the work of a nascent planet plowing through the dust, but we simply can't rule out other possible explanations. Our new observations provide intriguing evidence that planets are indeed forming around this one young star."
- HD 163296 is roughly 5 million years old and about twice the mass of the Sun. It is located approximately 400 light-years from Earth in the direction of the constellation Sagittarius.
Figure 34: ALMA image of the protoplanetary disk surrounding the young star HD 163296 as seen in dust. New observations suggested that two planets, each about the size of Saturn, are in orbit around the star. These planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star's protoplanetary disk [image credit: ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)]
Figure 35: Composite image of the protoplanetary disk surrounding the young star HD 163296. The inner red area shows the dust of the protoplanetary disk. The broader blue disk is the carbon monoxide gas in the system. ALMA observed that in the outer two gaps in the dust, there was a significant dip in the concentration of carbon monoxide, suggesting two planets are forming there [image credit: ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)]
Figure 36: Artist's impression of the protoplanetary disk surrounding the young star HD 163296. By studying the dust (ruddy brown) and carbon monoxide gas (light blue) profiles of the disk, astronomers discovered tantalizing evidence that two planets are forming in the outer two dust gaps in the disk (image credit: B. Saxton, NRAO/AUI/NSF)
• October 4, 2016: Astronomers have discovered a 'hot molecular core,' a cocoon of molecules surrounding a newborn massive star, for the first time outside our Galaxy. The discovery, which marks the first important step for observational studies of extragalactic hot molecular cores and challenges the hidden chemical diversity of our universe, appears in a paper in The Astrophysical Journal Volume 827. 54) 55)
- The scientists from Tohoku University, the University of Tokyo, the National Astronomical Observatory of Japan, and the University of Tsukuba, used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe a newborn star located in the Large Magellanic Cloud, one of the closest neighbors of our Galaxy. As a result, a number of radio emission lines from various molecular gas are detected, which indicates the presence of a hot molecular core associated with the observed newborn star (Figures 37 and 38).
- The observations have revealed that the hot molecular core in the Large Magellanic Cloud shows significantly different chemical compositions as compared to similar objects in our Galaxy. In particular, the results suggest that simple organic molecules such as methanol are deficient in this galaxy, suggesting a potential difficulty in producing large organic species indispensable for the birth of life. The research team suggests that the unique galactic environment of the Large Magellanic Cloud affects the formation processes of molecules around a newborn star, and this results in the observed unique chemical compositions.
- "This is the first detection of an extragalactic hot molecular core, and it demonstrates the great capability of new generation telescopes to study astrochemical phenomena beyond our Galaxy," said Dr. Takashi Shimonishi, an astronomer at Tohoku University, Japan, and the paper's lead author. "The observations have suggested that the chemical compositions of materials that form stars and planets are much more diverse than we expected. " It is known that various complex organic molecules, which have a connection to prebiotic molecules formed in space, are detected from hot molecular cores in our Galaxy. It is, however, not yet clear if such large and complex molecules exist in hot molecular cores in other galaxies. The newly discovered hot molecular core is an excellent target for such a study, and further observations of extragalactic hot molecular cores will shed light on the chemical complexities of our universe.
Figure 37: Artist's concept image of the hot molecular core discovered in the Large Magellanic Cloud [image credit: RIS/Tohoku University. The figure is a derivative work of the following sources (ESO/M. Kornmesser; NASA, ESA, and S. Beckwith (STScI) and the HUDF Team; NASA/ESA and the Hubble Heritage Team (AURA/STScI)/HEI]
Figure 38: Left: Distributions of molecular line emission from a hot molecular core in the Large Magellanic Cloud observed with ALMA. Emissions from dust, sulfur dioxide (SO2), nitric oxide (NO), and and formaldehyde (H2CO) are shown as examples. Right: An infrared image of the surrounding star-forming region (based on the 8 µm data provided by the NASA/Spitzer Space Telescope), image credit: T. Shimonishi/Tohoku University, ALMA (ESO/NAOJ/NRAO)
• September 22, 2016: International teams of astronomers have used ALMA to explore the distant corner of the Universe first revealed in the iconic images of the Hubble Ultra Deep Field (HUDF). These new ALMA observations are significantly deeper and sharper than previous surveys at millimeter wavelengths. They clearly show how the rate of star formation in young galaxies is closely related to their total mass in stars. They also trace the previously unknown abundance of star-forming gas at different points in time, providing new insights into the "Golden Age" of galaxy formation approximately 10 billion years ago. — The new ALMA results will be published in a series of papers appearing in the Astrophysical Journal and Monthly Notices of the Royal Astronomical Society. 56)
- In 2004 the Hubble Ultra Deep Field images — pioneering deep-field observations with the NASA/ESA Hubble Space Telescope — were published. These spectacular pictures probed more deeply than ever before and revealed a menagerie of galaxies stretching back to less than a billion years after the Big Bang. The area was observed several times by Hubble and many other telescopes, resulting in the deepest view of the Universe to date.
- Astronomers using ALMA have now surveyed this seemingly unremarkable, but heavily studied, window into the distant Universe for the first time both deeply and sharply in the millimeter range of wavelengths. This allows them to see the faint glow from gas clouds and also the emission from warm dust in galaxies in the early Universe.
- ALMA has observed the HUDF for a total of around 50 hours up to now. This is the largest amount of ALMA observing time spent on one area of the sky so far. One team, led by Jim Dunlop (University of Edinburgh, United Kingdom) used ALMA to obtain the first deep, homogeneous ALMA image of a region as large as the HUDF. This data allowed them to clearly match up the galaxies that they detected with objects already seen with Hubble and other facilities.
- This study showed clearly for the first time that the stellar mass of a galaxy is the best predictor of star formation rate in the high redshift Universe. They detected essentially all of the high-mass galaxies and virtually nothing else. 57)
- The second team, led by Manuel Aravena of the Núcleo de Astronomía, Universidad Diego Portales, Santiago, Chile, and Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany, conducted a deeper search across about one sixth of the total HUDF. 58)
- "We conducted the first fully blind, three-dimensional search for cool gas in the early Universe," said Chris Carilli, an astronomer with the NRAO (National Radio Astronomy Observatory) in Socorro, New Mexico, USA and member of the research team. "Through this, we discovered a population of galaxies that is not clearly evident in any other deep surveys of the sky." 59)
- The new ALMA observations of the HUDF include two distinct, yet complementary types of data: continuum observations, which reveal dust emission and star formation, and a spectral emission line survey, which looks at the cold molecular gas fueling star formation. The second survey is particularly valuable because it includes information about the degree to which light from distant objects has been redshifted by the expansion of the Universe. Greater redshift means that an object is further away and seen farther back in time. This allows astronomers to create a three-dimensional map of star-forming gas as it evolves over cosmic time.
Figure 39: This image combines a background picture taken by the NASA/ESA Hubble Space Telescope (blue/green) with a new very deep ALMA view of this field (orange, marked with circles). All the objects that ALMA sees appear to be massive star-forming galaxies (image credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA/J. Dunlop et al. and S. Beckwith (STScI) and the HUDF Team)
• July 2016: A Chalmers-led team of astronomers (Chalmers University of Technology, Gothenburg, Sweden) have used the Alma telescope to make the surprising discovery of a jet of cool, dense gas in the center of a galaxy located 70 million light years from Earth. The jet, with its unusual, swirling structure, gives new clues to a long-standing astronomical mystery – how supermassive black holes grow. 60) 61)
- A team of astronomers led by Susanne Aalto, professor of radio astronomy at Chalmers, has used the Alma telescope (Atacama Large Millimeter/submillimeter Array) to observe a remarkable structure in the center of the galaxy NGC 1377, located 70 million light years from Earth in the constellation Eridanus (the River). "We were curious about this galaxy because of its bright, dust-enshrouded center. What we weren't expecting was this: a long, narrow jet streaming out from the galaxy nucleus", says Susanne Aalto.
- The observations with Alma reveal a jet which is 500 light years long and less than 60 light years across, travelling at speeds of at least 800 000 km/hour.
- Most galaxies have a supermassive black hole in their centers; these black holes can have masses of between a few million to a billion solar masses. How they grew to be so massive is a long-standing mystery for scientists.
- A black hole's presence can be seen indirectly by telescopes when matter is falling into it – a process which astronomers call "accretion". Jets of fast-moving material are typical signatures that a black hole is growing by accreting matter. The jet in NGC 1377 reveals the presence of a supermassive black hole. But it has even more to tell us, explains Francesco Costagliola (Chalmers and ORA-INAF, Italy), co-author on the paper. "The jets we usually see emerging from galaxy nuclei are very narrow tubes of hot plasma. This jet is very different. Instead it's extremely cool, and its light comes from dense gas composed of molecules", he says.
- The jet has ejected molecular gas equivalent to two million times the mass of the Sun over a period of only around half a million years - a very short time in the life of a galaxy. During this short and dramatic phase in the galaxy's evolution, its central, supermassive black hole must have grown fast.
- "Black holes that cause powerful narrow jets can grow slowly by accreting hot plasma. The black hole in NGC1377, on the other hand, is on a diet of cold gas and dust, and can therefore grow – at least for now – at a much faster rate", explains team member Jay Gallagher (University of Wisconsin-Madison).
- The motion of the gas in the jet also surprised the astronomers. The measurements with Alma are consistent with a jet that is precessing – swirling outwards like water from a garden sprinkler.
- "The jet's unusual swirling could be due to an uneven flow of gas towards the central black hole. Another possibility is that the galaxy's center contains two supermassive black holes in orbit around each other", says Sebastien Muller, Chalmers, also a member of the team.
- The discovery of the remarkable cool, swirling jet from the center of this galaxy would have been impossible without Alma, concludes Susanne Aalto.
Figure 40: Alma's close-up view of the center of galaxy NGC 1377 (upper left) reveals a swirling jet. In this color-coded image, reddish gas clouds are moving away from us, bluish clouds towards us, relative to the galaxy's center. The Alma image shows light with wavelength around one millimeter from molecules of carbon monoxide (CO). A cartoon view (lower right) shows how these clouds are moving, this time seen from the side. The background color image of NGC 1377 and its surroundings is a composite made from a visible light images taken at the CTIO 1.5 meter telescope in Chile by H. Roussel et al. [image credit: CTIO/H. Roussel et al./ESO (left panel); Alma/ESO/NRAO/S. Aalto (top right panel); S. Aalto (lower right panel)]
• June 16, 2016: Astronomers using the ALMA (Atacama Large Millimeter/submillimeter Array) in Chile, detected a clear signal from oxygen in a galaxy located 13.1 billion light-years away from us. This is the most distant oxygen ever detected. Oxygen in this galaxy seems to be ionized by a number of young giant stars, and this detection is a key step to understand the enigmatic "cosmic reionization" in the early history of the Universe. These observations have opened a new window to probe the early Universe with ALMA. 62) 63)
- The research team from Japan, Sweden, the United Kingdom and ESO have used ALMA to observe one of the most distant galaxies known. SXDF-NB1006-2 lies at a redshift of 7.2, meaning that we see it only 700 million years after the Big Bang.
- The astronomers hoped to find out about the heavy chemical elements present in the galaxy, as they can tell us about the level of star formation, and hence provide clues about the period in the history of the Universe known as cosmic reionization.
- "Seeking heavy elements in the early Universe is an essential approach to explore the star formation activity in that period," said Akio Inoue of Osaka Sangyo University, Japan, the lead author of the research paper published in Science. He added, "Studying heavy elements also gives us a hint to understand how the galaxies were formed and what caused the cosmic reionization".
- Various elements are found around us in the present Universe, but just after the Big Bang, 13.8 billion years ago, only the lightest elements (hydrogen, helium, and lithium) existed. Heavier elements, such as carbon and oxygen, have been formed in stars and accumulated in the Universe over time.
- Before the first celestial objects formed, the Universe was filled with electrically neutral gas. Celestial objects emitted strong radiation and started to ionize the neutral gas a few hundred million years after the Big Bang. This is known as cosmic reionization. The state of the whole Universe changed dramatically during this period. But, the process is deeply shrouded in darkness. It has been under debate what kind of objects caused the reionization.
- "We expected that the light from ionized oxygen is strong enough to be observed, even 13 billion light-years away," explained Hiroshi Matsuo at the NAOJ (National Astronomy Observatory, Japan), "because the Japanese infrared astronomy satellite AKARI has found that this emission is very bright in the Large Magellanic Cloud, which has an environment similar to the early Universe."
- Nevertheless, the detection of light from ionized oxygen in very distant galaxies was a new challenge for ALMA. To secure the competitive observation time with ALMA, the researchers first performed large-scale computer simulations of the cosmic evolution to predict the emission brightness. "The simulation showed that the light should be particularly bright and easily detected with ALMA," said Ikkoh Shimizu at Osaka University, the main contributor to this simulation.
Figure 41: Schematic diagram of the history of the Universe. The Universe is in a neutral state at 400 thousands years after the Big Bang and light from the first generation stars starts to ionize the hydrogen. After several hundred million years, the gas in the Universe is completely ionized (image credit: NAOJ)
• April 14, 2016: Subtle distortions hidden in ALMA's stunning image of the gravitational lens SDP.81 are telltale signs that a dwarf dark galaxy is lurking in the halo of a much larger galaxy nearly 4 billion light-years away (Figure 42). This discovery paves the way for ALMA to find many more such objects and could help astronomers address important questions on the nature of dark matter. 64)
- In 2014, as part of ALMA's Long Baseline Campaign, astronomers studied a variety of astronomical objects to test the telescope's new, high-resolution capabilities. One of these experimental images was that of an Einstein ring, which was produced by the gravity of a massive foreground galaxy bending the light emitted by another galaxy nearly 12 billion light-years away.
- This phenomenon, called gravitational lensing, was predicted by Einstein's general theory of relativity and it offers a powerful tool for studying galaxies that are otherwise too distant to observe. It also sheds light on the properties of the nearby lensing galaxy because of the way its gravity distorts and focuses light from more distant objects.
- In a new paper accepted for publication in the Astrophysical Journal, astronomer Yashar Hezaveh at Stanford University in California and his team explain how detailed analysis of this widely publicized image uncovered signs of a hidden dwarf dark galaxy in the halo of the more nearby galaxy. 65)
- "We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects," explained Hezaveh. In the case of a rain drop, the image distortions are caused by refraction. In this image, similar distortions are generated by the gravitational influence of dark matter.
- Current theories suggest that dark matter, which makes up about 80 percent of the mass of the Universe, is made of as-yet-unidentified particles that don't interact with visible light or other forms of electromagnetic radiation. Dark matter does, however, have appreciable mass, so it can be identified by its gravitational influence.
- For their analysis, the researchers harnessed thousands of computers working in parallel for many weeks, including the National Science Foundation's most powerful supercomputer, Blue Waters, to search for subtle anomalies that had a consistent and measurable counterpart in each "band" of radio data. From these combined computations, the researchers were able to piece together an unprecedented understanding of the lensing galaxy's halo, the diffuse and predominantly star-free region around the galaxy, and discovered a distinctive clump less than one-thousandth the mass of the Milky Way.
- Because of its relationship to the larger galaxy, estimated mass, and lack of an optical counterpart, the astronomers believe this gravitational anomaly may be caused by an extremely faint, dark-matter dominated satellite of the lensing galaxy. According to theoretical predictions, most galaxies should be brimming with similar dwarf galaxies and other companion objects. Detecting them, however, has proven challenging. Even around our own Milky Way, astronomers can identify only 40 or so of the thousands of satellite objects that are predicted to be present.
- "This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades, even called a 'crisis' by some researchers," said Neal Dalal of the University of Illinois, a member of the team. "If these dwarf objects are dominated by dark matter, this could explain the discrepancy while offering new insights into the true nature of dark matter," he added.
- Computer models of the evolution of the Universe indicate that by measuring the "clumpiness" of dark matter, it's possible to measure its temperature. So by counting the number of small dark matter clumps around distant galaxies, astronomers can infer the temperature of dark matter, which has an important bearing on the smoothness of our Universe.
- "If these halo objects are simply not there," notes co-author Daniel Marrone of the University of Arizona, "then our current dark matter model cannot be correct and we will have to modify what we think we understand about dark matter particles."
- This study suggests, however, that the majority of dwarf galaxies may simply not be seen because they're mainly composed of invisible dark matter and emit little if any light. "Our current measurements agree with the predictions of cold dark matter," said team member Gilbert Holder of McGill University in Montreal, Canada. "In order to increase our confidence we will need to look at many more lenses."
- "This is an amazing demonstration of the power of ALMA," said Hezaveh. "We are now confident that ALMA can efficiently discover these dwarf galaxies. Our next step is to look for more of them and to have a census of their abundance to figure out if there is any possibility of a warm temperature for dark matter particles."
Figure 42: Composite image of the gravitational lens SDP.81 showing the distorted ALMA image of the more distant galaxy (red arcs) and the Hubble optical image of the nearby lensing galaxy (blue center object). By analyzing the distortions in the ring, astronomers have determined that a dark dwarf galaxy (data indicated by white dot near left lower arc segment) is lurking nearly 4 billion light-years away (image credit: Y. Hezaveh, Stanford University; ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble Space Telescope)
• March 31, 2016: This new image of ALMA shows the finest detail ever seen in the planet-forming disc around the nearby Sun-like star TW Hydrae. It reveals a tantalizing gap at the same distance from the star as the Earth is from the Sun, which may mean that an infant version of our home planet, or possibly a more massive super-Earth, is beginning to form there. 66) 67)
- The star TW Hydrae is a popular target of study for astronomers because of its proximity to Earth (only about 175 light-years away) and its status as an infant star (about 10 million years old). It also has a face-on orientation as seen from Earth. This gives astronomers a rare, undistorted view of the complete protoplanetary disc around the star.
- "Previous studies with optical and radio telescopes confirm that TW Hydrae hosts a prominent disc with features that strongly suggest planets are beginning to coalesce," said Sean Andrews with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA and lead author on a paper published today in the Astrophysical Journal Letters. "The new ALMA images show the disc in unprecedented detail, revealing a series of concentric dusty bright rings and dark gaps, including intriguing features that may indicate that a planet with an Earth-like orbit is forming there."
- Other pronounced gaps that show up in the new images are located three billion and six billion kilometers from the central star, similar to the average distances from the Sun to Uranus and Pluto in the Solar System. They too are likely to be the results of particles that came together to form planets, which then swept their orbits clear of dust and gas and shepherded the remaining material into well-defined bands.
Figure 43: ALMA's best image of a protoplanetary disc to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system [image credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)]
Legend to Figure 43: The long baseline ALMA observations of the 870 µm continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that trace mm-sized particles down to spatial scales as small as 1 au (20 mas). These data reveal a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli (1–6 au) with modest contrasts (5%–30%). These features are associated with concentrations of solids that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima. No significant non-axisymmetric structures are detected. Some of the observed features occur near temperatures that may be associated with the condensation fronts of major volatile species, but the relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the so-called zonal flows). Other features, particularly a narrow dark annulus located only 1 au from the star, could indicate interactions between the disk and young planets. These data signal that ordered substructures on ~au scales can be common, fundamental factors in disk evolution and that high-resolution microwave imaging can help characterize them during the epoch of planet formation.
Figure 44: This ALMA image shows the planet-forming disc around TW Hydrae. The inset image zooms in on the gap nearest to the star, which is at the same distance as the Earth is from the Sun, suggesting an infant version of an Earth-like exoplanet could be emerging from the dust and gas. The additional concentric light and dark features represent other planet-forming regions farther out in the disc (image credit: S. Andrews, Harvard-Smithsonian Center for Astrophysics / ALMA / ESO / NAOJ / NRAO) 68)
• Dec. 16, 2015: Astronomers using ALMA have found the clearest indications yet that planets with masses several times that of Jupiter have recently formed in the discs of gas and dust around four young stars. Measurements of the gas around the stars also provide additional clues about the properties of those planets. 69) 70)
- Planets are found around nearly every star, but astronomers still do not fully understand how — and under what conditions — they form. To answer such questions, they study the rotating discs of gas and dust present around young stars from which planets are built. But these discs are small and far from Earth, and the power of ALMA was needed for them to reveal their secrets.
- A special class of discs, called transitional discs, have a surprising absence of dust in their centers, in the region around the star. Two main ideas have been put forward to explain these mysterious gaps. Firstly, the strong stellar winds and intense radiation could have blown away or destroyed the encircling material (this process, which clears the dust and gas from the inside out, is known as photoevaporation). Alternatively, massive young planets in the process of formation could have cleared the material as they orbit the star (Figure 46).
- The unparalleled sensitivity and image sharpness of ALMA have now allowed the team of astronomers, led by Nienke van der Marel from the Leiden Observatory in the Netherlands to map the distribution of gas and dust in four of these transitional discs better than ever before. This in turn has allowed them to choose between the two options as the cause of the gaps for the first time.
- The new images show that there are significant amounts of gas within the dust gaps. But to the team's surprise, the gas also possessed a gap, up to three times smaller than that of the dust. This could only be explained by the scenario in which newly formed massive planets have cleared the gas as they travelled around their orbits, but trapped the dust particles further out.
- "Previous observations already hinted at the presence of gas inside the dust gaps," explains Nienke van der Marel. "But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc."
- Remarkably, these observations were conducted utilizing just one tenth of the current resolving power of ALMA, as they were performed whilst half of the array was still under construction on the Chajnantor Plateau in northern Chile. — Further studies are now needed to determine whether more transitional discs also point towards this planet-clearing scenario, although ALMA's observations have, in the meantime, provided astronomers with a valuable new insight into the complex process of planetary formation.
Figure 45: Artist's impression of a transitional disc around a young star (image credit: ALMA (ESO/NAOJ/NRAO), M. Kornmesser)
• Dec. 15, 2015: Galaxy clusters are massive congregations of galaxies that host huge reservoirs of hot gas — the temperatures are so high that X-rays are produced. These structures are useful to astronomers because their construction is believed to be influenced by the Universe's notoriously strange components — dark matter and dark energy. By studying their properties at different stages in the history of the Universe, galaxy clusters can shed light on the Universe's poorly understood dark side. 71)
- The team, consisting of over 100 astronomers from around the world, started a hunt for the cosmic monsters in 2011. Although the high-energy X-ray radiation that reveals their location is absorbed by the Earth's atmosphere, it can be detected by X-ray observatories in space. Thus, they combined an ESA XMM-Newton survey — the largest time allocation ever granted for this orbiting telescope — with observations from ESO and other observatories. The result is a huge and growing collection of data across the electromagnetic spectrum, collectively called the XXL survey (Figure 47). "The main goal of the XXL survey is to provide a well-defined sample of some 500 galaxy clusters out to a distance when the Universe was half its current age," explains XXL principal investigator Marguerite Pierre of CEA, Saclay, France.
Figure 47: X-ray image of the XXL-South Field, one of the two fields observed by the XXL survey. The XXL survey has combined archival data as well as new observations of galaxy clusters covering the wavelength range from 1 x 10-4 µm (X-ray, observed with XMM) to more than 1 meter, observed with the GMRT (Giant Meterwave Radio Telescope), image credit: ESA/XMM-Newton/XXL survey consortium, (S. Snowden, L. Faccioli, F. Pacaud)
Legend to Figure 47: XXL is one of the largest quests for galaxy clusters ever undertaken and provides by far the best view of the deep X-ray sky yet obtained. The survey was carried out with ESA's XMM-Newton X-ray observatory. Additional vital observations to measure the distances to the galaxy clusters were made with ESO facilities. 72)
The area shown in this image was obtained with some 220 XMM-Newton pointings and, if viewed on the sky, would have a two dimensional area a hundred times larger than the full Moon (which spans one half degree), and that is without taking into account the depth that the survey explores.
The red circles in this image show the clusters of galaxies detected in the survey. Along with the other field — XXL-North Field (or XXL-N) — around 450 of these clusters were uncovered in the survey, which mapped them back to a time when the Universe was just half of its present age.
The image also reveals some of the 12 000 galaxies that had very bright cores containing supermassive black holes that were detected in the field.
• Nov. 19, 2015: Astronomers using ALMA have discovered that a dim, cool dwarf star is generating a surprisingly powerful magnetic field, one that rivals the most intense magnetic regions of our own Sun. 73)
- The star's extraordinary magnetic field is potentially associated with a constant flurry of solar-flare-like eruptions. As with our Sun, these flares would trace tightly wound magnetic field lines that act like cosmic particle accelerators: warping the path of electrons and causing them to emit telltale radio signals that can be detected with ALMA.
- "If we lived around a star like this one, we wouldn't have any satellite communications. In fact, it might be extremely difficult for life to evolve at all in such a stormy environment," says lead author Peter Williams of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.
- The team used ALMA to study the well-known red dwarf star TVLM 513-46546, which is located about 35 light-years from Earth in the constellation Boötes. The star is a mere 10 % the mass of the Sun and is so small and cool that it's right on the dividing line between stars (which fuse hydrogen) and brown dwarfs (which don't). One of the things that make this small star remarkable is that it spins rapidly, completing a full rotation about every two hours. Our Sun takes about 25 days to rotate once about its equator.
- Previous data from the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array in Socorro, New Mexico, show that this star exhibits a magnetic field that rivals the Sun's most extreme magnetic regions and is several hundred times stronger than the Sun's average magnetic field.
- "This star is a very different beast from our Sun, magnetically speaking," states CfA astronomer and co-author Edo Berger. When the researchers examined the star with ALMA they detected emission at a particularly high frequency (95 GHz or a wavelength of about 3 mm). Such a radio signal is produced by a process known as synchrotron emission, in which electrons zip around powerful magnetic field lines: the more powerful the magnetic field, the higher the frequency.
- This is the first time that flare-like emission at such high frequencies has been detected from a red dwarf star. It is also the first time that such a star has been detected at millimeter wavelengths, opening up a new avenue of study with ALMA.
- Our Sun generates similar emission from solar flares but only intermittently. What's more, the emission from this star is 10,000 times brighter than what our own Sun produces, even though it has less than one-tenth of the Sun's mass. The fact that ALMA detected this emission in a brief 4-hour observation suggests that the red dwarf is continuously active.
- This has important implications for the search for habitable planets outside the Solar system. Red dwarfs are the most common type of star in our Galaxy, which makes them promising targets for planet searches. But because a red dwarf is so cool, a planet would have to orbit very close to the star to be warm enough for liquid water to exist at its surface. That proximity would put the planet right in the bull's-eye for radiation that could strip its atmosphere or destroy any complex molecules on its surface, the astronomers speculate.
Figure 48: Artist's impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field, potentially associated with a flurry of solar-flare-like eruptions (image credit: Dana Berry (NRAO/AUI/NSF) / SkyWorks)
• April 16, 2015: ALMA has revealed an extremely powerful magnetic field, beyond anything previously detected in the core of a galaxy, very close to the event horizon of a supermassive black hole (Figure 49). This new observation helps astronomers to understand the structure and formation of these massive inhabitants of the centers of galaxies, and the twin high-speed jets of plasma they frequently eject from their poles. The results appear in the 17 April 2015 issue of the journal Science. 74)
- Supermassive black holes, often with masses billions of times that of the Sun, are located at the heart of almost all galaxies in the Universe. These black holes can accrete huge amounts of matter in the form of a surrounding disc. While most of this matter is fed into the black hole, some can escape moments before capture and be flung out into space at close to the speed of light as part of a jet of plasma. How this happens is not well understood, although it is thought that strong magnetic fields, acting very close to the event horizon, play a crucial part in this process, helping the matter to escape from the gaping jaws of darkness.
- Up to now only weak magnetic fields far from black holes — several light-years away — had been probed. In this study, however, astronomers from Chalmers University of Technology and Onsala Space Observatory in Sweden have now used ALMA to detect signals directly related to a strong magnetic field very close to the event horizon of the supermassive black hole in a distant galaxy named PKS 1830-211. This magnetic field is located precisely at the place where matter is suddenly boosted away from the black hole in the form of a jet.
- The team measured the strength of the magnetic field by studying the way in which light was polarized, as it moved away from the black hole. "Polarization is an important property of light and is much used in daily life, for example in sun glasses or 3D glasses at the cinema," says Ivan Marti-Vidal, lead author of this work. "When produced naturally, polarization can be used to measure magnetic fields, since light changes its polarization when it travels through a magnetized medium. In this case, the light that we detected with ALMA had been travelling through material very close to the black hole, a place full of highly magnetized plasma."
- The astronomers applied a new analysis technique that they had developed to the ALMA data and found that the direction of polarization of the radiation coming from the center of PKS 1830-211 had rotated. These are the shortest wavelengths ever used in this kind of study, which allow the regions very close to the central black hole to be probed.
Figure 49: This artist's impression shows the surroundings of a supermassive black hole, typical of that found at the heart of many galaxies. The black hole itself is surrounded by a brilliant accretion disc of very hot, infalling material and, further out, a dusty torus. There are also often high-speed jets of material ejected at the black hole's poles that can extend huge distances into space. Observations with ALMA have detected a very strong magnetic field close to the black hole at the base of the jets and this is probably involved in jet production and collimation (image credit: ALMA,ESO/NAOJ/NRAO)
• Nov. 5, 2014: A new image from ALMA reveals extraordinarily fine detail that has never been seen before in the planet-forming disc around a young star. ALMA's new high-resolution capabilities were achieved by spacing the antennas up to 15 km apart. This new result represents an enormous step forward in the understanding of how protoplanetary discs develop and how planets form. 75)
- ALMA has obtained its most detailed image yet showing the structure of the disc around HL Tau, a million-year-old Sun-like star located approximately 450 light-years from Earth in the constellation of Taurus. The image exceeds all expectations and reveals a series of concentric and bright rings, separated by gaps.
- "These features are almost certainly the result of young planet-like bodies that are being formed in the disc. This is surprising since such young stars are not expected to have large planetary bodies capable of producing the structures we see in this image," said Stuartt Corder, ALMA Deputy Director.
- "When we first saw this image we were astounded at the spectacular level of detail. HL Tauri is no more than a million years old, yet already its disc appears to be full of forming planets. This one image alone will revolutionize theories of planet formation," explained Catherine Vlahakis, ALMA Deputy Program Scientist and Lead Program Scientist for the ALMA Long Baseline Campaign.
- Such a resolution can only be achieved with the long baseline capabilities of ALMA and provides astronomers with new information that is impossible to collect with any other facility, even the Hubble Space Telescope. "The logistics and infrastructure required to place antennas at such distant locations required an unprecedented coordinated effort for the international expert team of engineers and scientists" said ALMA Director, Pierre Cox. "These long baselines fulfill one of ALMA's major objectives and mark an impressive technological, scientific and engineering milestone", celebrated Cox.
- Stars like HL Tau and our own Sun form within clouds of gas and dust that collapse under gravity. Over time, the surrounding dust particles stick together, growing into sand, pebbles, and larger-size rocks, which eventually settle into a thin disc where asteroids, comets, and planets form. Once these planetary bodies acquire enough mass, they dramatically reshape the structure of the disc, fashioning rings and gaps as the planets sweep their orbits clear of debris and shepherd dust and gas into tighter and more confined zones.
Figure 50: This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system (image credit: ALMA, ESO/NAOJ/NRAO)
• October 16, 2013: Two international teams of astronomers have used the power of the ALMA to focus on jets from the huge black holes at the centers of galaxies and observe how they affect their surroundings. They have respectively obtained the best view yet of the molecular gas around a nearby, quiet black hole and caught an unexpected glimpse of the base of a powerful jet close to a distant black hole. 76)
- There are supermassive black holes — with masses up to several billion solar masses — at the hearts of almost all galaxies in the Universe, including our own galaxy, the Milky Way. In the remote past, these bizarre objects were very active, swallowing enormous quantities of matter from their surroundings, shining with dazzling brilliance, and expelling tiny fractions of this matter through extremely powerful jets. In the current Universe, most supermassive black holes are much less active than they were in their youth, but the interplay between jets and their surroundings is still shaping galaxy evolution.
- Two new studies, both published on Oct. 16, 2013 in the journal Astronomy & Astrophysics, used ALMA to probe black hole jets at very different scales: a nearby and relatively quiet black hole in the galaxy NGC 1433 (Figure 51) and a very distant and active object called PKS 1830-211 (Figure 52). The discovery of this outflow, which is being dragged along by the jet from the central black hole, shows how such jets can stop star formation and regulate the growth of the central bulges of galaxies.
- "ALMA has revealed a surprising spiral structure in the molecular gas close to the center of NGC 1433," says Françoise Combes (Observatoire de Paris, France), who is the lead author of the first paper. "This explains how the material is flowing in to fuel the black hole. With the sharp new observations from ALMA, we have discovered a jet of material flowing away from the black hole, extending for only 150 light-years. This is the smallest such molecular outflow ever observed in an external galaxy."
- The discovery of this outflow, which is being dragged along by the jet from the central black hole, shows how such jets can stop star formation and regulate the growth of the central bulges of galaxies.
- In PKS 1830-211, Ivan Martí-Vidal (Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden) and his team also observed a supermassive black hole with a jet, but a much brighter and more active one in the early Universe. It is unusual because its brilliant light passes a massive intervening galaxy on its way to Earth, and is split into two images by gravitational lensing.
- From time to time, supermassive black holes suddenly swallow a huge amount of mass, which increases the power of the jet and boosts the radiation up to the very highest energies. And now ALMA has, by chance, caught one of these events as it happens in PKS 1830-211.
Figure 51: Composite view of the galaxy NGC 1433 from ALMA and Hubble. This detailed view shows the central parts of the nearby active galaxy NGC 1433. The dim blue background image, showing the central dust lanes of this galaxy, comes from the NASA/ESA Hubble Space Telescope. The colored structures near the center are from recent ALMA observations that have revealed a spiral shape, as well as an unexpected outflow, for the first time (image credit: ALMA, ESO/NAOJ/NRAO)/NASA/ESA, F. Combes)
Figure 52: This image from the NASA/ESA Hubble Space Telescope shows the distant active galaxy PKG 1830-211. It shows up as an unremarkable looking star-like object, hard to spot among the many much closer real stars in this picture. Recent ALMA observations show both components of this distant gravitational lens and are marked in red on this composite picture (image credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA, I. Martí-Vidal) 77)
• On Oct. 3, 2011, ALMA opened officially for astronomers. The first released image (Figure 53), from a telescope still under construction, reveals a view of the Universe that cannot be seen at all by visible-light and infrared telescopes. Thousands of scientists from around the world competed to be the first few researchers to explore some of the darkest, coldest, farthest, and most hidden secrets of the Cosmos with this new astronomical tool. 78)
- "We are living in a historic moment for science and particularly for astronomy, and perhaps also for the evolution of humanity, because we start to use the greatest observatory under construction at the moment," said Thijs de Graauw, ALMA Director.
- At present, around a third of ALMA's eventual 66 radio antennas make up the growing array on the Chajnantor plateau in northern Chile. And yet, even under construction, ALMA has become the best telescope of its kind – as reflected by the extraordinary number of astronomers who requested to observe with ALMA.
- ALMA is radically different from visible-light and infrared telescopes. It is an array of linked antennas acting as a single giant telescope, and it detects much longer wavelengths than those of visible light. Its images therefore look quite unlike more familiar pictures of the cosmos.
- The ALMA team has been busy testing the observatory's systems over the past few months, in preparation for the first round of scientific observations, known as Early Science. One outcome of their tests is the first image published from ALMA, albeit from what is still very much a growing telescope. Most of the observations used to create this image of the Antennae Galaxies were made using only twelve antennas working together —fewer than will be used for the first science observations — and with the antennas much closer together as well. Both of these factors make the new image just a taster of what is to come. As the observatory grows, the sharpness, efficiency, and quality of its observations will increase dramatically as more antennas become available and the array grows in size. This is nevertheless the best submillimeter-wavelength image ever taken of the Antennae Galaxies and opens a new window on the submillimeter Universe.
- The Antennae Galaxies (Figure 54, also known as NGC 4038 and 4039) are a pair of distorted colliding spiral galaxies about 70 million light-years away, in the constellation of Corvus (The Crow). This view combines ALMA observations, made in two different wavelength ranges during the observatory's early testing phase, with visible-light observations from the NASA/ESA Hubble Space Telescope.
- The Hubble image is the sharpest view of this object ever taken and serves as the ultimate benchmark in terms of resolution. ALMA observes at much longer wavelengths which makes it much harder to obtain comparably sharp images. However, when the full ALMA array is completed its vision will be up to ten times sharper than Hubble.
Figure 53: Multiwavelength composite of interacting galaxies NGC 4038/4039, the Antennae Galaxies, showing their namesake tidal tails in radio (blues), past and recent starbirths in optical (whites and pinks), and a selection of current star-forming regions in mm/submm (orange and yellows). Inset: ALMA's first mm/submm test views, in Bands 3 (orange), 6 (amber), & 7 (yellow), showing detail surpassing all other views in these wavelengths [image credit: (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); HST (NASA, ESA, and B. Whitmore (STScI)); J. Hibbard, (NRAO/AUI/NSF); NOAO/AURA/NSF]
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31) Guillem Anglada, Pedro J. Amado, Jose L. Ortiz, José F. Gómez, Enrique Macías, Antxon Alberdi, Mayra Osorio, José L. Gómez, Itziar de Gregorio-Monsalvo, Miguel A. Pérez-Torres, Guillem Anglada-Escudé, Zaira M. Berdiñas, James S. Jenkins, Izaskun Jimenez-Serra, Luisa M. Lara, Maria J. López-González, Manuel López-Puertas, Nicolas Morales, Ignasi Ribas, Anita M. S. Richards, Cristina Rodríguez-López, Eloy Rodriguez, "ALMA Discovery of Dust Belts around Proxima Centauri," The Astrophysical Journal Letters, Volume 850, Number 1, Nov. 2017, DOI: 10.3847/2041-8213/aa978b, URL: https://arxiv.org/pdf/1711.00578.pdf
33) Note: This protostar is a binary star system surrounded by a molecular cloud in the Rho Ophiuchi star-forming region, which makes it an excellent target for ALMA's millimeter/submillimeter view.
35) The data used were from the ALMA Protostellar Interferometric Line Survey (PILS). The aim of this survey is to chart the chemical complexity of IRAS 16293-2422 by imaging the full wavelength range covered by ALMA in the 0.8-millimetre atmospheric window on very small scales, equivalent to the size of the Solar System.
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41) Meredith A. MacGregor, Luca Matra, Paul Kalas, David J. Wilner, Margaret Pan, Grant M. Kennedy, Mark C. Wyatt, Gaspard Duchene, A. Meredith Hughes, George H. Rieke, Mark Clampin, Michael P. Fitzgerald, James R. Graham, Wayne S. Holland, Olja Panic, Andrew Shannon, Kate Su, "A Complete ALMA Map of the Fomalhaut Debris Disk," The Astrophysical Journal, May 16, 2017, URL: https://arxiv.org/pdf/1705.05867.pdf
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45) David Gerdes, Masao Sako, Stephanie Hamilton, Ke Zhang, Tali Khain, Juliette Becker, James Annis, William Wester, Gary Bernstein, Colin Scheibner, Lynus Zullo, Fred Adams, Edwin Bergin, Alistair Walker, J.H. Mueller, T. Abbott, Filipe Abdalla, Sahar Allam, K. Bechtol, Aurelien Benoit-Lévy, Emmanuel Bertin, David Brooks, David Burke, A. Rosell, M. Kind, Jorge Carretero, Carlos Cunha, Luiz da Costa, S. Desai, H. Thomas Diehl, Tim Eifler, Brenna Flaugher, Joshua Frieman, J. Garc'ia-Bellido, Enrique Gaztanaga, Daniel Goldstein, Daniel Gruen, J. Gschwend, Gaston Gutierrez, Klaus Honscheid, D. James, Stephen M. Kent, E. Krause, Kyler Kuehn, Nikolay Kuropatkin, O. Lahav, T. Li, M. Maia, Marisa Cristina March, Jennifer Marshall, P. Martini, Felipe Menanteau, Ramon Miquel, Robert Nichol, Andres Plazas Malagón, A. Kathy Romer, Aaron Roodman, Eusebio Sanchez, Ignacio Sevilla-Noarbe, Mathew Smith, R. Smith, Marcelle Soares-Santos, Flavia Sobreira, E. Suchyta, M. Swanson, Gregory Tarle, Douglas Tucker, Y. Zhang, "Discovery and Physical Characterization of a Large Scattered Disk Object at 92 AU," Astrophysical Journal Letters, Vol. 839, No 1, L15 DOI: 10.3847/2041-8213/aa64d8, Draft version March 7, 2017, URL: https://arxiv.org/pdf/1702.00731.pdf
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57) In this context "high mass" means galaxies with stellar masses greater than 20 billion times that of the Sun ( 2 x 1010 solar masses). For comparison, the Milky Way is a large galaxy and has a mass of around 100 billion solar masses.
58) This region of sky is about seven hundred times smaller than the area of the disc of the full Moon as seen from Earth. One of the most startling aspects of the HUDF was the vast number of galaxies found in such a tiny fraction of the sky.
59) ALMA's ability to see a completely different portion of the electromagnetic spectrum from Hubble allows astronomers to study a different class of astronomical objects, such as massive star-forming clouds, as well as objects that are otherwise too faint to observe in visible light, but visible at millimeter wavelengths.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).