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News Article | May 11, 2017
Site: phys.org

61 Vir is a G-type, 4.6-billion-year-old main-sequence star about the size of our sun, located approximately 28 light years away. The star is known to be orbited by at least three planets that are five, 18 and 23 times more massive than Earth. One of the most intriguing features of this system is a debris disk extending from 30 to at least 100 AU from the star. Debris disks are clouds of planetesimals and dust found in orbits around many stars. Studying such disks could improve our understanding about planet formation and the migration history of planets in planetary systems. With this aim in mind, a team of astronomers led by Sebastian Marino of the University of Cambridge in the U.K., has performed observations of 61 Vir's debris disk using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. These observations were complemented by data from the Submillimetre Common-User Bolometer Array 2 (SCUBA2) installed in the James Clerk Maxwell Telescope (JCMT) at Mauna Kea Observatory in Hawaii. "In this paper, we present the first observations of 61 Vir with ALMA at 0.86 mm, obtained with the aim of studying its debris disc to reveal the location of the parent planetesimals, and place constraints on the presence of planets at large separations that can shape the mass distribution in the disc. (…) In order to obtain the best disc constraints, in our analysis we combine new ALMA band 7 observations and new data at 0.85 mm from SCUBA2 installed on JCMT, thus, incorporating information from small and large angular scale structure," the researchers wrote in the paper. The new study reveals that the debris disk is larger than previously thought. Marino's team found that it extends from 30 to at least 150 AU. Combined ALMA and SCUBA2/JMCT observations also show that at 0.86 mm the total disc emission is about 3.7 mJy and the disk has a surface density distribution of millimeter sized grains with a power law slope of approximately 0.1. Moreover, the researchers assume that a yet unseen fourth planet may lurk somewhere in the system between 61 Vir d at 0.5 AU and the inner edge of the disc. They argue that if the disc was stirred at 150 AU by an additional planet, that unseen alien world should have a mass of at least 10 Earth masses and should orbit its host at a distance between 10 and 20 AU. "We found that in order to have stirred the disc out to 150 AU, the planet must be more massive than 10 Earth masses and a semi-major axis between 10 and 20 AU if it has an eccentricity lower than 0.1. Otherwise, for higher eccentricities, it could have a lower mass and a semi-major axis between 4 and 20 AU," the team concluded. More information: ALMA observations of the multiplanet system 61 Vir: What lies outside super-Earth systems? arXiv:1705.01944 [astro-ph.EP] arxiv.org/abs/1705.01944 Abstract A decade of surveys has hinted at a possible higher occurrence rate of debris discs in systems hosting low mass planets. This could be due to common favourable forming conditions for rocky planets close in and planetesimals at large radii. In this paper we present the first resolved millimetre study of the debris disc in the 4.6 Gyr old multiplanet system 61 Vir, combining ALMA and JCMT data at 0.86 mm. We fit the data using a parametric disc model, finding that the disc of planetesimals extends from 30 AU to at least 150 AU, with a surface density distribution of millimetre sized grains with a power law slope of 0.1+1.1−0.8. We also present a numerical collisional model that can predict the evolution of the surface density of millimetre grains for a given primordial disc, finding that it does not necessarily have the same radial profile as the total mass surface density (as previous studies suggested for the optical depth), with the former being flatter. Finally, we find that if the planetesimal disc was stirred at 150 AU by an additional unseen planet, that planet should be more massive than 10 M⊕ and lie between 10-20 AU. Lower planet masses and semi-major axes down to 4 AU are possible for eccentricities ≫ 0.1.


Artists' impression of the gas and dust disk around the planet-like object OTS44. First radio observations indicate that OTS44 has formed in the same way as a young star. Credit: Johan Olofsson (U Valparaiso & MPIA) First radio observations of the lonely, planet-like object OTS44 reveal a dusty protoplanetary disk that is very similar to disks around young stars. This is unexpected, given that models of star and planet formation predict that formation from a collapsing cloud, forming a central object with surrounding disk, should not be possible for such low-mass objects. Apparently, stars and planet-like objects are more similar than previously thought. The finding, by an international team led by Amelia Bayo and including several astronomers from the Max Planck Institute for Astronomy, has been published in Astrophysical Journal Letters. A new study of the lonely, planet-like object OTS44 has provided evidence that this object has formed in a similar way as ordinary stars and brown dwarfs – a surprising result that challenges current models of star and planet formation. The study by a group of astronomers, led by Amelia Bayo of the University of Valparaiso and involving several astronomers from the Max Planck Institute for Astronomy, used the ALMA observatory in Chile to detect dust from the disk surrounding OTS44. This detection yielded mass estimates for the dust contained in the disk, which place OTS44 in a row with stars and brown dwarfs (that is, failed stars with too little mass for sustained nuclear fusion): All these objects, it seems, have rather similar properties, including a similar ratio between the mass of dust in the disk and the mass of the central object. The findings supplement earlier research that found OTS44 is still growing by drawing matter from its disk onto itself – another tell-tale similarity between the object and young stars. Taken together, this is compelling evidence that OTS44 formed in the same way as stars and brown dwarfs, namely by the collapse of a cloud of gas and dust. But going by current models of star and planet formation, it should not be possible for an object as low-mass as OTS44 to form in this way. An alternative way, the formation of multiple objects in one go, with low-mass objects like OTS44 among them, is contradicted by the observations, which show no such companion objects anywhere near OTS44. The strength of the radiation received from the dust at millimetre wavelength also suggests the presence of large, millimetre sized dust grains. This, too, is surprising. Under the conditions in the disk of a low-mass object, dust is not expected to clump together to reach this size (or beyond). Instead, the OTS44 dust grains appear to be growing – and might even be on the way of forming a mini-moon around the object; another similarity with stars and their planetary systems. Amelia Bayo (University of Valparaiso), who led this research effort, says: "The more we know about OTS44, the greater its similarities with a young star. But its mass is so low that theory tells us it cannot have formed like a star!" Thomas Henning of the Max Planck Institute for Astronomy adds: "It is amazing how an observatory like ALMA allows us to see half an Earth mass worth of dust orbiting an object with ten times the mass of Jupiter at a distance of 500 light-years. But the new data also shows the limit of our understanding. Clearly, there is still a lot to learn about the formation of low-mass astronomical objects!" Explore further: Brown dwarfs, stars share formation process, new study indicates More information: Amelia Bayo et al. First Millimeter Detection of the Disk around a Young, Isolated, Planetary-mass Object, The Astrophysical Journal (2017). DOI: 10.3847/2041-8213/aa7046


News Article | May 18, 2017
Site: www.rdmag.com

An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) 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. 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 also 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, Mass., 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, 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 were able to 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. The new ALMA observations are also the first to definitively show "apocenter glow," a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology in Cambridge, who is also a 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. 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 our 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."


News Article | May 18, 2017
Site: www.eurekalert.org

An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) 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. 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 also 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, Mass., 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, 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 were able to 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. The new ALMA observations are also the first to definitively show "apocenter glow," a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology in Cambridge, who is also a 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. 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 our 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." The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.


News Article | May 18, 2017
Site: www.sciencedaily.com

An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) 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. 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 also 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, Mass., 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, 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 were able to 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. The new ALMA observations are also the first to definitively show "apocenter glow," a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology in Cambridge, who is also a 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. 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 Earth and other planets were routinely struck by swarms of asteroids and comets left over from the formation of our 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."


News Article | May 18, 2017
Site: www.cnet.com

A real-life "Eye of Sauron" like the one from "The Lord of the Rings" series isn't too far away, cosmically speaking. An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have captured the most detailed and slightly intimidating images yet of the Fomalhaut star system, which is surrounded by rings of rubble, dust and gas that give it an eerie, eye-like look. The image is a bit of a cheat, though. If we were able to do a sightseeing cruise by Fomalhaut, which is about 25 light-years away, it probably wouldn't look quite so dramatic, or at least not in the same way. It's a very bright star, and the empty, dark ring around the center of the "eye" is actually created by a coronagraphic mask, a tool that filters out much of its overwhelming brightness. The orange dot in the center is the deliberately dimmed star, the blue section is what NASA's Hubble Space Telescope picked up when it observed Fomalhaut directly, and the orange elliptical is the ring of debris ALMA observed, superimposed on the Hubble image to create this creepy composite. The debris ring is fascinating for astronomers who study how planets and solar systems form. Fomalhaut is only one tenth the age of our sun, and rings of debris are common around younger stars. This particular one seems to be well-defined just beyond the edge of the planetary system, similar to our own Kuiper Belt, but in a much more active and chaotic phase. That, or it's actually the watchful eye of the dark lord Morgoth's right-hand man. If that's the case, it's a good thing it's 25 light-years away.


News Article | May 18, 2017
Site: astrobiology.com

We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 μJy/beam. These observations provide the first millimeter map of the continuum dust emission from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt a MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of 136.3±0.9 AU and width of 13.5±1.8 AU. We determine a best-fit eccentricity of 0.12±0.01. Assuming a size distribution power law index of q=3.46±0.09, we constrain the dust absorptivity power law index β to be 0.9 Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with HST, SCUBA, and ALMA. However, we cannot rule out structures ≤10 AU in size or which only affect smaller grains. The central star is clearly detected with a flux density of 0.75±0.02 mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths. 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 (Submitted on 16 May 2017) Comments: 15 pages, 8 figures Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR) Cite as: arXiv:1705.05867 [astro-ph.EP] (or arXiv:1705.05867v1 [astro-ph.EP] for this version) Submission history From: Meredith MacGregor [v1] Tue, 16 May 2017 18:05:00 GMT (5158kb,D) https://arxiv.org/abs/1705.05867 Astrobiology


News Article | May 24, 2017
Site: www.nature.com

Quasars are the brightest non-transient emitters of light in the Universe. These remarkable objects have been discovered to exist as early as 750 million years after the Big Bang1, 2 and are thought to be powered by black holes that are at least a billion times more massive than the Sun3, 4. Since the discovery of such quasars in the early Universe, major questions have persisted regarding how these extreme objects formed in such a relatively short time. On page 457, Decarli et al.5 report the discovery of four massive star-forming galaxies around distant quasars. The results improve our understanding of the formation of quasars, and provide astronomers with a potentially powerful approach to studying the most massive galaxies in the early Universe. The black holes that power quasars probably started their lives in miniature and grew exponentially by accretion6 — whereby matter close to a black hole cannot escape the strong gravitational field and is ultimately pulled into the black hole. To have assembled such a huge mass so quickly, the bright quasars discovered in the early Universe1, 2 are thought to have resided in regions that had a particularly high density of matter6. Such an environment not only would have fuelled the rapid growth of the black holes powering these quasars, but also would have spurred the growth of galaxies in the quasars' immediate vicinity. There have been many attempts to test this general model for early-quasar growth by looking for galaxies around luminous quasars in the early Universe. However, such efforts have been either limited by the poor spatial (line-of-sight) resolution of standard search methods for distant galaxies7, 8, 9, 10, 11, or restricted by techniques that are sensitive to only low-mass galaxies around quasars12, 13. These studies have provided support for the idea that luminous quasars formed in high-density environments of the Universe, but the evidence has not been overwhelming14. The Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile has provided astronomers with a powerful tool with which to search for spectral-line emission from massive star-forming galaxies in the immediate vicinity of bright quasars. ALMA can achieve this feat because of its immense radiation-collecting area and design as an interferometer — an array of antennas that are linked to combine astronomical observations. The latter allows ALMA to obtain an emission spectrum at every position in space over its 25-arcsecond field of view (about 1.5% of the full Moon's angular diameter). Decarli et al. used the unique capabilities of ALMA to observe 25 luminous quasars that existed less than 900 million years after the Big Bang — at redshifts larger than 6 (the higher the redshift of a cosmological object, the younger the Universe was when the object emitted its light). They then carried out a blind search for additional sources in the same volume of space as a quasar, with a separation of less than 70 kiloparsecs in the plane of the sky and 2 megaparsecs along Earth's line of sight. The authors targeted an emission line associated with singly ionized carbon that is particularly prominent in spectra of the interstellar medium of galaxies in the present-day Universe and of massive star-forming galaxies in the early Universe15, 16. In addition to finding strong emission from the studied quasars, Decarli et al. discovered four bright line-emitting sources in the same observations (Fig. 1). The authors justifiably interpreted these sources as massive star-forming galaxies. Decarli and colleagues' identification of bright galaxies in the vicinity of distant quasars is important for at least two reasons. First, it provides definitive and convincing evidence that luminous high-redshift quasars formed in particularly dense environments, and gives us a concrete and salient answer to how rich these environments are in massive galaxies. By probing luminous quasars at redshifts greater than 6, Decarli et al. have substantially extended the results from previous studies that presented related findings at redshifts of 4.8 (ref. 17) and 5.3 (ref. 18). Second, on the basis of a blind search around bright quasars, the authors have detected what are likely to be three of the most massive star-forming galaxies discovered so far at redshifts larger than 6. Remarkably, their efforts seem to have outstripped dedicated searches for such galaxies that have much larger fields of view (several times the full Moon's angular diameter)16, 19. The authors' galaxies have high luminosities and lack contaminating light from their neighbouring quasars, potentially allowing a detailed characterization of galaxies in the early Universe to be made. The discovered sources seem certain to be targets for ALMA, the James Webb Space Telescope and other facilities in the immediate future. Such follow-up observations are needed to make sense of the full scope of Decarli and colleagues' findings, because our understanding of the sources' properties is limited by the current data. Nevertheless, the authors have presented an extremely valuable pathfinding study that could bring about a fundamental change not only in our probing of the regions surrounding bright high-redshift quasars, but also in how astronomers look for the most massive galaxies in the early Universe.


News Article | May 24, 2017
Site: www.nature.com

We used the Atacama Large Millimeter Array (ALMA) to survey the fine-structure line of singly ionized carbon ([C ii] at 158 μm) and its underlying continuum emission in high-redshift quasars in the southern sky (declination of less than 15°). The [C ii] line, a strong coolant of the interstellar medium, is the brightest far-infrared emission line at these frequencies9, 15, 16. It arises ubiquitously in galaxies and is therefore an ideal tracer of gas morphology and dynamics in quasar hosts. The far-infrared continuum emission is associated with the light from young stars that has been reprocessed by dust and is therefore a measure of the dust mass and puts constraints on the star-formation rate of the host galaxies. The parent sample includes 35 luminous (rest-frame 1,450-Å absolute magnitude of less than −25.25 mag) quasars at z > 5.95 (for which the redshifted [C ii] line would fall in ALMA band 6), most of which were selected from the Pan-STARRS1 survey17; of these, 25 targets were observed with ALMA, all in single pointings with similar depth (0.6–0.9 mJy per beam per 30 km s−1 channel). The survey resulted in a very high detection rate (>90%) in both the continuum and the line emission from the host galaxies of the quasars. We searched the data cubes (in projected sky position and frequency or redshift) for additional sources in the quasar fields. The field of view of ALMA at these frequencies is about 25″, or 140 physical kiloparsecs at the mean redshift of the quasars (assuming a Lambda cold dark matter cosmology with Hubble constant H  = 70 km s−1 Mpc−1, mass density Ω  = 0.3 and vacuum density Ω  = 0.7). The detection algorithm and strategy follows previous work with ALMA data18. We imposed a conservative significance threshold of 7σ (corresponding to a [C ii] luminosity of L  ≈ 109L , where L is the luminosity of the Sun), which excludes any contamination from noise peaks. This search resulted in the discovery of four bright line-emitting sources around four of the targeted quasars (Fig. 1). The modest frequency differences with respect to the nearby quasars, the brightness of the lines compared to the underlying continua, and the lack of optical and near-infrared counterparts (which suggests that the companion sources reside at high redshift; see Fig. 1) imply that the detected lines are also [C ii]. Furthermore, chance alignments of low-redshift CO emitters are expected to be more than 20 times rarer at these fluxes18. These newly detected galaxies are also seen (at various degrees of significance) in their dust continuum emission. The line and continuum fluxes are comparable to, and in some cases even brighter than, those of the quasars (see Table 1), although the companion sources are not detected in near-infrared images (which sample the rest-frame ultraviolet emission). Any potential accreting supermassive black holes in these companions would therefore be at least one order of magnitude fainter than the quasars, or strongly obscured (see Fig. 1). Two quasars (J0842+1218 and J2100−1715) have a companion source at about 50 kpc in projected separation, with line-of-sight velocity differences of 440 km s−1 and 40 km s−1, respectively. This result suggests that the respective quasar–companion pairs lie within a common physical structure, and might even be at an early stage of interaction. The [C ii] lines in these quasar companions have luminosities of about 2 × 109L . The marginally resolved, beam-deconvolved size of the [C ii]-emitting region is about 7 kpc and 5 kpc in these two galaxies. A Gaussian fit of the line profile yields linewidths of 370 km s−1 and 690 km s−1, comparable to those of submillimetre galaxies at lower redshift9, 19. The implied dynamical masses of the companions within the [C ii]-emitting regions are in the range (1–3) × 1011M (where M is the mass of the Sun; see Table 1). The dust continuum is only marginally detected in the companion source of J0842+1218, whereas it is clearly seen in the companion source of J2100−1715. The other two quasars, PSO J231.6576−20.8335 and PSO J308.0416−21.2339 (hereafter, PJ231−20 and PJ308−21), have [C ii]-bright companions at much smaller projected separation, about 10 kpc. The companion source of PJ231−20 has very bright [C ii] emission and far-infrared continuum emission, whereas that of PJ308−21 is fainter in the [C ii] line and is only marginally detected in the continuum. Most remarkably, the [C ii] emission in the companion of PJ308−21 stretches over about 25 kpc (4.5″) and about 1,000 km s−1 towards and beyond the quasar host, suggesting that the companion is undergoing a tidal disruption due to interaction or merger with the quasar host (see Fig. 2). This extent is twice as large as the interacting groups around the submillimetre galaxy AzTEC-3 and the nearby ultraviolet-selected galaxy LBG-1, at z = 5.3 (ref. 12). Figure 2 is therefore a map of the earliest known merger of massive galaxies, 820 Myr after the Big Bang. Modelling the dust emission as a modified black body with a dust opacity index of β = 1.6 and dust temperature of T  = 47 K (ref. 20), we find that the far-infrared luminosities (corrected for the effects of the cosmic microwave background) of the quasars and their companions are in the range (4–100) × 1011L , with corresponding far-infrared-derived star-formation rates between 80M yr−1 (for the companion of PJ308−21) and about 2,000M yr−1 (for the quasar PJ231−20; see Table 1). The dust mass21 is M  ≈ (108–109)M , or higher if the dust is not optically thin at 158 μm or if its temperature is lower than assumed. For typical gas-to-dust ratios of about 100 (ref. 22), this dust mass yields gas masses of (1010–1011)M . In Fig. 3a we show the [C ii]-to-far-infrared luminosity ratio as a function of the far-infrared luminosity. This key diagnostic shows the contribution of the [C ii] line to the cooling of the interstellar medium: in local spiral galaxies, [C ii] is responsible for approximately 0.3% of the entire luminosity of the galaxy; in ultra-luminous infrared galaxies and high-redshift starburst galaxies, its contribution can be a factor of 10 lower9, 15, 23. The quasars and their continuum-bright companions in our sample have low [C ii]-to-far-infrared luminosity ratios (about 0.1% or less), whereas the companions of J0842+1218 and PJ308−21 have higher ratios (at least 0.15%), closer to the parameter space occupied by normal star-forming galaxies in the local Universe24. In Fig. 3b we show the average number of [C ii]-bright galaxies that were observed within a given distance from a quasar in our survey. The detection of four such galaxies in 25 targeted fields exceeds the expected count rates from the (coarse) constraints (approximately 2 × 10−4 co-moving Mpc−3 at L  > 109L ) that are currently available on the [C ii] luminosity function at z > 6 (refs 25, 26) by orders of magnitudes (the survey volume within ±1,000 km s−1 from the quasars is only about 400 co-moving Mpc−3). However, the high number of companion sources might be reconciled with the [C ii] luminosity function constraints if large-scale clustering of galaxies and quasars is accounted for (such as in the quasar–Lyman-break-galaxy correlation function at z ≈ 4 (ref. 27) shown in Fig. 3b). Bright, high-redshift quasars therefore represent ideal beacons of the earliest dark matter overdensities (local peaks in the number of galaxies per unit volume compared to the average field). Together with the host galaxies of the quasars, the newly discovered objects (the four companion galaxies) are the observational manifestation of rapid, very early star formation in massive halos. If representative of the bright end of the [C ii] luminosity function, then they are sufficiently common to explain the abundance of massive galaxies (approximately 1.8 × 10−5 co-moving Mpc−3) that already existed by z ≈ 4 (ref. 1). These galaxies cannot be accounted for by the much more numerous, but an order of magnitude less star-forming, z > 6 galaxies that are typically found in deep Hubble Space Telescope images4, for which sensitive observations have ruled out strong dust-reprocessed emission28, 29. If an accreting supermassive black hole is present in any of these sources, then it is either much fainter than the nearby quasars, or heavily reddened. This property makes these companion galaxies unique objects for studying the build-up of the most massive systems in the first billion years of the Universe: from an observational perspective, the absence of a blinding central light source enables in-depth characterization of these massive star-forming objects. Moreover, their interstellar medium, far-infrared luminosities and implied star-formation rates are less affected by any feedback processes from the central supermassive black hole. Future observations of these companion galaxies with the James Webb Space Telescope have the promise to accurately constrain their stellar masses, a key physical parameter given the young age of the Universe. Such a measurement is very difficult in the host galaxies of quasars, owing to their compact emission and the enormous brightness of their central accreting supermassive black holes.


News Article | May 25, 2017
Site: phys.org

Scientists made the methanol discovery around TW Hydrae, a star about 80 percent of our sun's mass and roughly 5 million to 10 million years old. It represents a younger version of what our solar system may have looked like during its formation more than 4 billion years ago. At about 170 light-years away, TW Hydrae has the closest protoplanetary disk to Earth. The methanol appears to be located in a ring peaking 30 astronomical units from the star. (An astronomical unit, or AU, is the average distance between Earth and the sun, or about 93 million miles.) This methanol gas likely came from methanol ice located slightly further away from the star. The scientists detailed their findings in the paper, "First detection of gas-phase methanol in a protoplanetary disk," published the journal Astrophysical Journal Letters. "Methanol is an important molecule because it has been shown in laboratory ice experiments to be a feedstock of larger and more complex molecules," said study lead author Catherine Walsh, an astrochemist at the University of Leeds in England. "The successful detection of methanol in a protoplanetary disk provides compelling evidence that larger molecules are also present." To analyze TW Hydrae, scientists used the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, the most powerful observatory to date for analyzing the chemistry of nearby protoplanetary disks. ALMA can also map where cold dust and gas is located in these disks with unprecedented resolution. For example, it recently detected gaps in these disks potentially carved by nascent planets. Methanol in protoplanetary disks is thought to begin first as ice, forming through chemical reactions on the surfaces of dust grains. Methanol formation is a process that releases heat, and the dust grains help absorb this excess energy to stabilize newly-made methanol molecules, Walsh said. Dust grain surfaces can also catalyze methanol formation, reducing the amount of energy these molecules need to form. If methanol ice in a protoplanetary disk spirals closer to its star, the volatile molecule can get excited by solar radiation and become a gas. This gas is what researchers have now detected. The researchers suggest that dust grains up to a millimeter large that are laden with methanol ice reside within 50 AU from TW Hydrae. As dust grains grow bigger, they reach a size where they experience drag from surrounding gas. This drag slows the dust grains down, and they move inward toward the star, Walsh said. Once methanol ice gets closer to TW Hydrae, it becomes methanol gas, but the researchers do not think this happens because the ice is heating up, as previous research suggested. Instead, they suggest that other mechanisms are responsible, such as ultraviolet rays from the star. This finding may alter how scientists model protoplanetary disk evolution in the future. Methanol is one of the largest complex organic compounds detected in protoplanetary disks to date. Moreover, it is the first protoplanetary disk organic molecule with an unambiguous origin as ice. As Walsh noted, methanol can serve as the building block of larger organic molecules. Measuring levels of methanol could, in principle, shed light on the amounts of other organic compounds that may exist within the icy, comet-forming material orbiting stars, Walsh said. These complex organic molecules may have helped life emerge on Earth. "It has been suggested that comets contributed some, if not all, of the organic feedstock to the young Earth needed to initiate or drive life," Walsh said. "The presence of organic-rich comets in other disks suggests that the basic ingredients for initiating or driving life are also presence in these disks." There are a number of mysteries that remain unsolved in these new findings. For instance, the observed methanol gas levels are, unexpectedly, as little as 100 times less than previously expected from recent models of protoplanetary disk chemistry. One possibility is that the models overestimate the rate at which methanol ice releases methanol gas, Walsh said. Another possibility is that these models underestimate how much stellar radiation and other factors destroy methanol molecules, she said. The methanol gas "is also in a different region of the disk than predicted in earlier models, which remains a puzzle," Walsh said. "We are working hard trying to understand this puzzle, and new data from ALMA, which will be in hand in 2017, will help us to do this." Future research will also hunt for methanol in other nearby protoplanetary disks, and for larger organic molecules in all these disks, Walsh said. Explore further: First detection of methyl alcohol in a planet-forming disc

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