Monday, May 22, 2017

First radio detection of lonely planet disk shows similarities between stars and planet-like objects

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. Image: 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.

Similarities with 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!”

Background information

The work described here has been published as A. Bayo et al., "First millimeter detection of the disk around a young, isolated, planetary-mass object" in the May 18, 2017 edition of the Astrophysical Journal Letters.

Link to the article

The MPIA researchers involved are:
Viki Joergens, Yao Liu (also Purple Mountain Observatory, Nanjing, China), Johan Oloffson (also Universidad de Valparaíso), Thomas Henning, and Henrik Beuther in collaboration with Amelia Bayo (first author; Universidad de Valparaíso [UV]), Robert Brauer (University of Kiel), Javier Arancibia (UV), Paola Pinilla (University of Arizona), Sebastian Wolf, Jan Philipp Ruge (both University of Kiel), Antonella Natta (Dublin Institute for Advanced Studies and INAF-Osservatorio Astrofisico di Arcetri), Katharine G. Johnson (University of Leeds), Mickael Bonnefoy (IPAG Grenoble), and Gael Chauvin (IPAG Grenoble and Unidad Mixta Internacional Franco-Chilena de Astronomía, Santiago).




In-depth description: First radio detection of lonely planet disk shows similarities between stars and planet-like objects

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.

From collapsing clouds to stars

Stars are formed when part of a giant cloud of gas collapses under its own gravity. But not every such collapse results in a star. The key criterion is one of mass: If the resulting object has sufficient mass, its gravity is strong enough to compress the central regions to such high densities, and heat them to such high temperatures, that nuclear fusion sets in, turning hydrogen nuclei (protons) into helium. 

The result is, by definition, a star: an object bound by its own gravity, with nuclear fusion in its core region, shining brightly as the energy liberated during the fusion processes is transported outwards.
Initially, the newly born star is surrounded by the remnants of the collapsed cloud. But in the natural course of collapsing, both the star and the cloud have begun to rotate at an appreciable rate. The rotation serves to flatten the material surrounding the young star, forming what is known as a protoplanetary disk of gas and dust. True to its name, this is where planets begin to form: The dust clumps to larger and larger grains and pebbles, increasing in size until, finally, the resulting objects are large enough to join together under the influence of its own gravity, forming solid planets thousands or even tens of thousands kilometers in diameter like our Earth, or collecting appreciable amounts of the surrounding gas to form gas giants, like Jupiter in our solar system.

If the object resulting from the collapse of the initial cloud has between 0.072 and 0.012 times the mass of the Sun – which corresponds to between 75 and 13 times the mass of Jupiter – what emerges is called a brown dwarf: a failed star, with some intermittent fusion reactions of deuterium (heavy hydrogen, consisting of one proton and one neutron) in the core regions, but no sustained, long-lasting phase of hydrogen fusion.

The strange case of OTS44

Can collapse produce even lighter objects, with similar masses as that of planets? A thorough analysis of the object OTS44, published in 2013 by a group of astronomers led by Viki Joergens from the Max Planck Institute for Astronomy (MPIA), presented strong evidence that this is indeed the case. OTS44 is a mere two million years old – in terms of stellar or planetary time-scales a newborn baby. The object has an estimated 12 Jupiter masses and is floating through space without a close companion. It is part of the Chamaeleon star forming region in the Southern constellation Chamaeleon, a little over 500 light-years from Earth, where numerous new stars are in the process of being born from collapsing clouds of gas and dust.

Just like a young star, OTS44 is surrounded by a disk of gas and dust, one of only four known low-mass objects (with about a dozen Jupiter masses or less) known to harbour a disk. Most conspicuously, OTS44 is still in the process of growing – that is, drawing material from the disk onto itself at a substantial rate. The disk itself is quite substantial; both this disk and the infalling material (accretion) are telltale signs of the standard mode of star formation – an indication that there is no fundamental difference between the formation of low-mass objects such as OTS44 and the formation of ordinary stars. OTS44 probably has the lowest mass of all objects where both a disk and infalling material have been detected.

Brown dwarf vs. planet-like object

We have so far avoided calling OTS44 either a brown dwarf or something else. In fact, nomenclature varies: Some astronomers call every object that has formed by direct collapse and is not a star a brown dwarf; by this criterion, only objects that form in disks around a central object can be planets. There is an alternative definition that hinges on the fact that an object like OTS44 does not have sufficient mass for a significant episode of deuterium fusion, and does not qualify as a brown dwarf on that account. We will compromise by referring to OTS44 as a planet-like object.

While the case of OTS44 shows that even planet-like objects can form by collapse, the details are anything but clear. For the formation of comparatively low-mass objects, be they very light stars, or brown dwarfs, or lonely planets, there are two main possibilities – but both are problematic in the case of OTS44. The first possibility is a direct collapse by a small isolated cloud. But going by our current knowledge, such a direct collapse should not be able to form such a planetary-mass object directly.

Much more likely is the alternative, namely that OTS44 could have formed as part of a larger collapsing cloud, when the collapsing regions fragmented, producing several objects, including OTS44, instead of a single larger body. But this does not mesh well with the observations. OTS44 is not now part of any multiple system. And even if we assume it was somehow ejected from such a system, OTS44 is still very young, and could not have moved far from its birth system – and that birth system would not have had time to dissolve completely into separate stars and/or brown dwarfs. But there is only a single object within 10,000 astronomical units (10,000 times the average Sun-Earth-distance) of OTS44, where the siblings of OTS44 could reasonably be expected, and there are no signs that this object was part of a collapsing, fragmenting cloud.

Tracking dust with ALMA

Clearly, there is more to be learned. That is what motivated a group of researchers led by Amelia Bayo (University of Valparaiso, Chile) to find out more about OTS44. The group includes a number of researchers from the Max Planck Institute for Astronomy (MPIA), as well as several former MPIA astronomers. Amelia Bayo was herself a postdoctoral researcher at MPIA before moving on to the University of Valparaiso, and in science, the international stations of an astronomer’s career often result in collaboration networks – in this case, a strategic collaboration between astronomers at the Universidad de Valparaiso in Chile and the MPIA's Planet and Star Formation Department led by Thomas Henning. The two institutions have an additional link: the Universidad de Valparaiso hosts an astronomical Max Planck Tandem Group, which commenced work in early 2017. With such tandem groups, the Max Planck Society fosters international cooperation with specific excellent research institutions.

In this particular case, the group gathered by Bayo for observing OTS44 included several members with the necessary skills and experience to make full use of the ALMA observatory: a constellation of 50 radio antennae for detecting millimeter and submillimeter radiation, operated by an international consortium and located in the Atacama desert in Chile.

The astronomers applied for ALMA time to observe the disk of OTS44 at millimeter wavelengths. Millimeter wavelengths are particularly suited to detect dust grains, which are present in protoplanetary disks (and account for one percent or more of the disk mass; these mass estimates are a subject of ongoing research). At least in the disks around more massive objects, these dust grains are the seeds of planet formation.

Dust mass and a surprisingly universal relation

For millimeter waves, the disk is optically thin, in other words: observations show the millimeter radiation from all the dust in the disk. (In an optically thick disk, we would only see radiation from the surface layers; the lower layers would be obscured by the upper layers.) This allowed the astronomers to estimate the total amount of dust in the disk – although the result still depends on the disk temperature. Temperature estimates for such disks, given the measured overall luminosity, give values between 5.5 Kelvin and 20 Kelvin for the OTS44 disk. This leads to estimates for the dust mass between 0.07 times the mass of the Earth (for the highest temperature estimate) and 0.64 Earth masses (for the lowest temperature).

These mass estimates confirm the similarity between stars and lower-mass objects: Systematic studies had shown earlier that for young stars and brown dwarfs, there is an approximate relationship between the mass of the central object and the mass of the dust in the surrounding disk. Inserting the data points for OTS44, the lonely planet-like object fits very well into the overall picture – indicating that the same overall mechanism is involved in all these cases, putting all central objects from about a hundredth to a few solar masses onto the same footing.

Dust grains of unusual size

Another interesting consequence stems from the fact that the disk is emitting significant amounts of millimeter radiation in the first place. This indicates the presence of certain amounts of grains of dust that are about a millimeter in size. Going by the current theories of planet formation, this is surprising: such larger dust grains should not have been able to form in a disk around such a low-mass object. In such a disk, the dust grains orbit the central mass like so many microscopic planets, following the laws first found by Johannes Kepler in the early 17th century. The gas of the disk, on the other hand, has internal pressure, which makes it rotation somewhat slower. The “head wind” felt by dust grains as they move through the slower gas should slow down the smaller grains, making them drift inwards before they finally fall onto the central object. There are arguments that these detrimental effects are particularly strong in lower-mass objects. From these calculations, it follows that the dust grains in the disk should have vanished when they were somewhat smaller – and should not have had the time to clump to form the observed millimetre-size grains.

Once the millimetre-size grains are there, the situation becomes less problematic – with their larger size, these grains do not feel the head wind as acutely as their smaller kin. But the presence of these larger grains poses a puzzle – and hints at the intriguing possibility that lonely planets might even be able to grow even larger dust grains, and may be even go as far as forming downright miniature moons, in their surrounding disks.

Similarities with young stars

All, in all, the new results make OTS44 look more and more similar to a young star, surrounded as it is by a disk, given the earlier evidence that it is still growing by incorporating material from that disk, and now with the new evidence that the ratio of the dust mass to the mass of the central object follows the same relation as for brown dwarfs and stars.

Evidently, the current models that preclude low-mass objects from forming in this particular way, via the collapse of a cloud of gas, are missing something. Observations like these new ones for OTS44 can be hoped to point us in the right direction for what that missing something might be, and thus towards a better understanding of the formation of low-mass objects in the universe.




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Sunday, May 21, 2017

SKA precursor telescope MeerKAT releases new 1.5 images using its current 32 antennas

Large scale bubbles and arcs seen with MeerKAT show stellar nurseries (where stars are born) in the Milky Way. For comparison, the previous best image of this star-forming region is shown at the bottom, obtained with the Australia Telescope Compact Array (ATCA). Credit: SKA SA


Friday 19 May 2017, Cape Town –  The South African SKA precursor telescope MeerKAT has just released its recent AR1.5 results, images achieved by using various configurations of the 32 antennas currently operational in the Karoo.

The Minister of Science and Technology, Naledi Pandor, preceding the delivery of the Budget Vote of the Department of Science and Technology to the South African parliament, embarked on a tour of the Iziko Museum, where SKA South Africa joined other exhibitors for a showcase. During this exhibition, SKA SA Chief Scientist Dr Fernando Camilo and SKA SA Head of Science Commissioning Dr Sharmila Goedhart presented the AR1.5 results to the minister.

This milestone of the integration of 32 antennas with single polarisation correlator was achieved on schedule by the end of March 2017. The 32 antennas are part of the eventual 64 antennas which are being built at the Losberg site in the Northern Cape.

In her Budget Vote, the Minister announced that the Research Development and Support Programme will transfer R693 million to the National Research Foundation to ensure the completion of MeerKAT, as a key priority for 2017/18.

MeerKAT is one of the precursor telescopes to the SKA and will eventually be integrated into SKA1-mid after being a world-class instrument in its own right for several years.



Saturday, May 20, 2017

Hubble Spots Moon Around Third Largest Dwarf Planet

 Dwarf Planet 2007 OR10
Credits: NASA, ESA, C. Kiss (Konkoly Observatory), and J. Stansberry (STScI)


The combined power of three space observatories, including NASA's Hubble Space Telescope, has helped astronomers uncover a moon orbiting the third largest dwarf planet, catalogued as 2007 OR10. The pair resides in the frigid outskirts of our solar system called the Kuiper Belt, a realm of icy debris left over from our solar system's formation 4.6 billion years ago.

With this discovery, most of the known dwarf planets in the Kuiper Belt larger than 600 miles across have companions. These bodies provide insight into how moons formed in the young solar system.

"The discovery of satellites around all of the known large dwarf planets — except for Sedna — means that at the time these bodies formed billions of years ago, collisions must have been more frequent, and that's a constraint on the formation models," said Csaba Kiss of the Konkoly Observatory in Budapest, Hungary. He is the lead author of the science paper announcing the moon's discovery. "If there were frequent collisions, then it was quite easy to form these satellites."

The objects most likely slammed into each other more often because they inhabited a crowded region. "There must have been a fairly high density of objects, and some of them were massive bodies that were perturbing the orbits of smaller bodies," said team member John Stansberry of the Space Telescope Science Institute in Baltimore, Maryland. "This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions."

But the speed of the colliding objects could not have been too fast or too slow, according to the astronomers. If the impact velocity was too fast, the smash-up would have created lots of debris that could have escaped from the system; too slow and the collision would have produced only an impact crater.

Collisions in the asteroid belt, for example, are destructive because objects are traveling fast when they smash together. The asteroid belt is a region of rocky debris between the orbits of Mars and the gas giant Jupiter. Jupiter's powerful gravity speeds up the orbits of asteroids, generating violent impacts.

The team uncovered the moon in archival images of 2007 OR10 taken by Hubble's Wide Field Camera 3. Observations taken of the dwarf planet by NASA's Kepler Space Telescope first tipped off the astronomers of the possibility of a moon circling it. Kepler revealed that 2007 OR10 has a slow rotation period of 45 hours. "Typical rotation periods for Kuiper Belt Objects are under 24 hours," Kiss said. "We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon. The initial investigator missed the moon in the Hubble images because it is very faint."

The astronomers spotted the moon in two separate Hubble observations spaced a year apart. The images show that the moon is gravitationally bound to 2007 OR10 because it moves with the dwarf planet, as seen against a background of stars. However, the two observations did not provide enough information for the astronomers to determine an orbit.

"Ironically, because we don't know the orbit, the link between the satellite and the slow rotation rate is unclear," Stansberry said.

The astronomers calculated the diameters of both objects based on observations in far-infrared light by the Herschel Space Observatory, which measured the thermal emission of the distant worlds. The dwarf planet is about 950 miles across, and the moon is estimated to be 150 miles to 250 miles in diameter. 2007 OR10, like Pluto, follows an eccentric orbit, but it is currently three times farther than Pluto is from the sun.

2007 OR10 is a member of an exclusive club of nine dwarf planets. Of those bodies, only Pluto and Eris are larger than 2007 OR10. It was discovered in 2007 by astronomers Meg Schwamb, Mike Brown, and David Rabinowitz as part of a survey to search for distant solar system bodies using the Samuel Oschin Telescope at the Palomar Observatory in California.

The team's results appeared in The Astrophysical Journal Letters.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.


 
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Contacts


Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514

dweaver@stsci.edu / villard@stsci.edu

Csaba Kiss
Konkoly Observatory, Budapest, Hungary
011-36-1-391-9341

kiss.csaba@csfk.mta.hu

John Stansberry
Space Telescope Science Institute, Baltimore, Maryland
410-338-2442

jstans@stsci.edu

Source: HubbleSite

Friday, May 19, 2017

ALMA Eyes Icy Ring Around Young Planetary System

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. Credit: ALMA (ESO/NAOJ/NRAO), M. MacGregor; NASA/ESA Hubble, P. Kalas; B. Saxton (NRAO/AUI/NSF) | Download image

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. Credit: ALMA (ESO/NAOJ/NRAO); M. MacGregor | Download image


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. Observations Suggest Chemical Kinship to Comets in Our Own Solar System.

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, 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.

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 the 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.

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.”



Additional information

This research is presented in a paper titled “A complete ALMA map of the Fomalhaut debris disk,” M. MacGregor, et al., appearing in the Astrophysical Journal, and “Detection of exocometary CO within the 440MYR-old Fomalhaut belt: A similar CO+CO2 ice abundance in exocomets and solar system comets,” L. Matrà et al., appearing in the Astrophysical Journal.

This work benefited from: NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate, NASA grants NNX15AC89G, NNX15AD95G, NSF grant AST-1518332, NSF Graduate Research Fellowship DGE1144152, and from NRAO Student Observing Support. This work has also been possible thanks to an STFC postgraduate studentship and the European Union through ERC grant number 279973.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the 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.


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Thursday, May 18, 2017

Punching Above Its Weight, a Brown Dwarf Launches a Parsec-Scale Jet

The image shows the jet HH 1165 (in green) launched by the brown dwarf Mayrit 1701117. The image is composed of observations of Hα (from ionised hydrogen), of [SII] (ionised sulfur) and in the R-band (red optical wavelengths) obtained with the SOAR telescope of the Cerro Tololo Inter-American Observatory. The jet extends over a distance of 0.7 light years (equivalent to 0.2 parsecs) northwest of the brown dwarf.


Astronomers led by the Max Planck Institute for Extraterrestrial Physics and using the Cerro Tololo Inter-American Observatory report the discovery of a spectacular extended jet from a young brown dwarf. With masses too low to sustain hydrogen fusion in their interiors, brown dwarfs occupy the mass range between stars and giant planets. While young stars are commonly found to launch jets that extend over a light year or more, this is the first jet with a similar extent detected from a brown dwarf. The result lends new insight into how sub-stellar objects form.

Intrinsically faint, brown dwarfs have been more elusive and difficult to study than stars. While not quite massive enough to sustain nuclear fusion at their core – the main energy source for brightly shining stars, they are substantially more massive than giant planets, with about ten times the mass of Jupiter or more. Brown dwarfs are actually quite numerous; there are many more brown dwarfs in our Galaxy than stars like the Sun. Nevertheless, observational information about brown dwarfs in scarce and there is an ongoing debate between astronomers if they form rather like planets or rather like stars do.

“We were looking for very young brown dwarfs, and picked this particular object because it showed a wealth of prominent emission lines associated with strong outflow activity from previous ESO VLT spectral observations, which indicated that there is a shock front close to the source,” explains Basmah Riaz, from the Max Planck Institute for Extraterrestrial Physics, who led the study. The image obtained over a total of three nights with the SOAR telescope of the Cerro Tololo Inter-American Observatory shows the newly detected jet, HH 1165, launched by the brown dwarf Mayrit 1701117, which is located in the outer periphery of the 3 million year old sigma Ori cluster. As described by co-author Cesar Briceno from the Cerro Tololo Inter-American Observatory: “We could see surprisingly extended jet emission after the first 30 minutes of integration. It was a real ‘Wow’ moment!”

The jet extends over a distanced of 0.7 light years (equivalent to 0.2 parsecs) northwest of the brown dwarf. The emission knots along the jet reveal that the mass loss is time variable, probably a result of episodic accretion onto the brown dwarf. While outflows have been detected previously from young brown dwarfs, the earlier detections were of "microjets" 10-100 times smaller in extent. “This discovery shows that, like young stars, brown dwarfs can launch powerful parsec-scale jets, and that they build up their mass through an unsteady, episodic process,” explains Basmah Riaz.


The surroundings of the HH 1165 jet is shown in this WISE infrared image. Its host brown dwarf Mayrit 1701117 is located in the outer periphery of the 3 million year old sigma Ori cluster, whose core is marked to the west. The star HR 1950 is located very close to the brown dwarf and several other B-type stars are marked as well, which trace the ionisation front (bright yellow region) to the east of Mayrit 1701117. © MPE, NOAO


“The HH 1165 jet shows all the familiar hallmarks of outflows from stars: emission knots, a cavity with reflection nebulosity, and bow shocks at the ends of the flow. It checks all the boxes quite convincingly,” commented co-author Emma Whelan from the National University of Ireland.

Brown dwarfs are known to be surrounded by disks at birth and to build up their masses by accretion from molecular cloud cores. While it may seem counterintuitive that mass loss (in a jet) is an integral part of how an object grows or gains mass, this situation may arise because of excess angular momentum. When spinning skaters pull in their arms, they spin faster as a result of conservation of angular momentum. Similarly, when large, slowly rotating molecular cloud cores collapse, they may spin up too fast to squeeze down to the much smaller sizes of stars or sub-stellar objects like brown dwarfs.

Riaz speculates that indeed “Molecular cloud cores have much more angular momentum than can be contained by stars or brown dwarfs. So the system needs to lose angular momentum for the object to grow in mass. By removing angular momentum from the system, jets help solve the `angular momentum problem’ faced by stars as well as brown dwarfs.”

To test this hypothesis, the team is on the hunt for more extended jets from brown dwarfs, to understand how commonly they occur. Such jets may be rare due to a lack of the environmental conditions needed to allow the jet to propagate to, and still be visible at, large distances from the source. We would expect that low-luminosity sources are more likely to be found in low-density environments, after disruption and fragmentation of very-low-mass cloud cores. So the problem is more likely to be a lack of dense material to shock against than the difficulty of propagation, which should be easier in a lower density medium.



Contact


Riaz, Basmah
Marie Sklodowska-Curie EU Research Fellow
Phone: +49 (0)89 30000-3007
Fax: +49 (0)89 30000-3950
Email: hanneh@mpe.mpg.de

Hämmerle, Hannelore
press officer
Phone: +49 (0)89 30000-3980
Fax: +49 (0)89 30000-3569
Email: hanneh@mpe.mpg.de



Original Publication

Basmah Riaz et al.
First Large Scale Herbig-Haro Jet Driven by a Proto-Brown Dwarf
Astrophysical Journal, in press

Source



More Information



Wednesday, May 17, 2017

Feeding a Baby Star with a Dusty Hamburger

Credit: ALMA (ESO/NAOJ/NRAO)/ Lee et al.


This intriguing image may look like a collection of coloured blobs, but it is actually a high-resolution snapshot of a newborn star enshrouded in dust. Just 1300 light-years away in the Orion Nebula, the star, named HH 212, is remarkably young. The average lifespan of such a low-mass star is around 100 billion years, but this star is only 40 000 years old — truly an infant in stellar terms.

In the cores of the vast molecular clouds in star formation regions, an ongoing battle rages; gravity versus the pressure of gas and dust. If gravity wins, it forces the gas and dust to collapse into a hot dense core that eventually ignites — forming a protostar. All the leftover gas and dust form a spinning disc around this baby star, and in many star systems they eventually coalesce to make planets. Such very young protostellar discs have been hard to image because of their relatively small size, but now the exceedingly high resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) makes it possible to understand the intricate details of star and planet formation.

A closer look at HH 212 reveals a prominent, cool, dark dust lane running through the disc, sandwiched between two brighter regions that are heated by the protostar. The result resembles a cosmic “hamburger”. This is the very first time astronomers have spotted such a dust lane in the earliest phases of star formation, and so it may provide clues as to how planetary systems are born.


Source: ESO/Potw

Tuesday, May 16, 2017

Gemini Tracks Distant Star Cluster with Adaptive Optics

Figure 1: Gemini Multi-Object Spectrograph (GMOS-South) of the Pyxis field (left image), with the center of the cluster marked with a red star. A zoom of the pseudo color image of Pyxis observed with the Gemini South Adaptive Optics Imager (GSAOI) used with the Gemini Multi-conjugate adaptive optics System (GeMS) is shown at right. The field of view of GMOS is 5 x 5 arcminutes, 85 x 85 arcseconds for GeMS.

Figure 2: Absolute velocity of Pyxis. Each blue dot stands for a velocity derived from a single background galaxy. The red box shows the weighted average velocity derived from all galaxies. Only galaxies with small errors are shown for clarity.


Researchers combine images from Gemini South’s wide-field adaptive optics system (GeMS/GSAOI) with data from the Hubble Space Telescope (HST) to determine the proper motion of a distant cluster of stars. The observations, the first to use ground-based adaptive optics to precisely measure the motion of a cluster at such a large distance, allowed astronomers to set a lower limit for the mass of our Milky Way while providing clues about the cluster’s origin. 
 
A study of the proper motion (apparent motion in the sky due to an object's motion around our galaxy) of several substructures across the Milky Way’s halo is underway at Gemini South. As part of this study the team used Adaptive Optics (AO) at Gemini South, along with data from HST, to focus on a distant cluster called Pyxis. The work allowed the team to set a lower limit for the Milky Way’s mass of 950 million solar masses. This value is consistent with most, but not all, previous determinations.

The wide-field Gemini Multi-conjugate adaptive optics System (GeMS) combined with the Gemini South Adaptive Optics Imager (GSAOI) provided the Gemini data. “We used GeMS/GSAOI to estimate the proper motion for halo objects because normal (seeing limited) ground-based telescopes need a time baseline of more than 15 years for this measurement,” says Tobias Fritz (University of Virginia) who leads the research team. “GeMS/GSAOI with its better spatial resolution can make that measurement in five years, the same types of baselines required from space-based proper motions (like HST),” continues Fritz. The team was able to measure absolute proper motions of Pyxis using GeMS/GSAOI, which provided a resolution of 0.08 arcsecond and combined that with archival HST images, with a resolution of ~ 0.1 arcsecond. Fritz adds, “The study of motions for halo objects, like Pyxis, can constrain the mass distribution of our Galaxy at large distances and thus the mass of the Milky Way.”

Pyxis, a densely packed collection of ancient stars, is one of the most distant examples of a globular clusters, dense clusters of stars which orbit our galaxy. The cluster is located some 130,000 light years away and is thought to be about 2 billion years younger than other globular clusters with the same ratio of heavier elements (metallicity). Together, these characteristics imply Pyxis did not form with other Milky Way clusters. Instead, it is likely that Pyxis was formed in a massive dwarf galaxy that was then accreted by the Milky Way. Thus, Pyxis has an extragalactic origin. However, the orbits of the known massive dwarf galaxies are inconsistent with the orbit of Pyxis, which is derived from the new proper motion measurements.

The paper, titled: The Proper Motion of Pyxis: The First Use of Adaptive Optics in Tandem with HST on a Faint Halo Object is published in The Astrophysical Journal. The work is part of a Large and Long program at Gemini that is also targeting other clusters, dwarf galaxies, and individual stars in stellar streams.

Abstract

We present a proper motion measurement for the halo globular cluster Pyxis, using HST/ACS data as the first epoch, and GeMS/GSAOI Adaptive Optics data as the second, separated by a baseline of ∼ 5 years. This is both the first measurement of the proper motion of Pyxis and the first calibration and use of Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a faint, distant halo object. Consequently, we present our analysis of the Adaptive Optics data in detail. We obtain a proper motion of µα cos(δ) =1.09±0.31 mas yr−1 and µδ =0.68±0.29 mas yr−1. From the proper motion and the line-of-sight velocity we find the orbit of Pyxis is rather eccentric with its apocenter at more than 100 kpc and its pericenter at about 30 kpc. We also investigate two literature-proposed associations for Pyxis with the recently discovered ATLAS stream and the Magellanic system. Combining our measurements with dynamical modeling and cosmological numerical simulations we find it unlikely Pyxis is associated with either system. We examine other Milky Way satellites for possible association using the orbit, eccentricity, metallicity, and age as constraints and find no likely matches in satellites down to the mass of Leo II. We propose that Pyxis probably originated in an unknown galaxy, which today is fully disrupted. Assuming that Pyxis is bound and not on a first approach, we derive a 68% lower limit on the mass of the Milky Way of 0.95×1012 M⊙.


Monday, May 15, 2017

CXO J101527.2+625911: Astronomers Pursue Renegade Supermassive Black Hole

 CXO J101527.2+625911
 Credit  X-ray: NASA/CXC/NRAO/D.-C.Kim; Optical: NASA/STScI; 
Illustration: NASA/CXC/M.Weiss

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 Tour of CXO J101527.2+625911

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Supermassive holes are generally stationary objects, sitting at the centers of most galaxies. However, using data from NASA's Chandra X-ray Observatory and other telescopes, astronomers recently hunted down what could be a supermassive black hole that may be on the move.

This possible renegade black hole, which contains about 160 million times the mass of our Sun, is located in an elliptical galaxy about 3.9 billion light years from Earth. Astronomers are interested in these moving supermassive black holes because they may reveal more about the properties of these enigmatic objects.

This black hole may have "recoiled," in the terminology used by scientists, when two smaller supermassive black holes collided and merged to form an even larger one. At the same time, this collision would have generated gravitational waves that emitted more strongly in one direction than others. This newly formed black hole could have received a kick in the opposite direction of those stronger gravitational waves. This kick would have pushed the black hole out of the galaxy's center, as depicted in the artist's illustration.

The strength of the kick depends on the rate and direction of spin of the two smaller black holes before they merge. Therefore, information about these important but elusive properties can be obtained by studying the speed of recoiling black holes.

Astronomers found this recoiling black hole candidate by sifting through X-ray and optical data for thousands of galaxies. First, they used Chandra observations to select galaxies that contain a bright X-ray source and were observed as part of the Sloan Digital Sky Survey (SDSS). Bright X-ray emission is a common feature of supermassive black holes that are rapidly growing.

Next, the researchers looked to see if Hubble Space Telescope observations of these X-ray bright galaxies revealed two peaks near their center in the optical image. These two peaks might show that a pair of supermassive black holes is present or that a recoiling black hole has moved away from the cluster of stars in the center of the galaxy.

If those criteria were met, then the astronomers examined the SDSS spectra, which show how the amount of optical light varies with wavelength. If the researchers found telltale signatures in the spectra indicative of the presence of a supermassive black hole, they followed up with an even closer examination of those galaxies.

After all of this searching, a good candidate for a recoiling black hole was discovered. The left image in the inset is from the Hubble data, which shows two bright points near the middle of the galaxy. One of them is located at the center of the galaxy and the other is located about 3,000 light years away from the center. The latter source shows the properties of a growing supermassive black hole and its position matches that of a bright X-ray source detected with Chandra (right image in inset). Using data from the SDSS and the Keck telescope in Hawaii, the team determined that the growing black hole located near, but visibly offset from, the center of the galaxy has a velocity that is different from the galaxy. These properties suggest that this source may be a recoiling supermassive black hole.

The host galaxy of the possible recoiling black hole also shows some evidence of disturbance in its outer regions, which is an indication that a merger between two galaxies occurred in the relatively recent past. Since supermassive black hole mergers are thought to occur when their host galaxies merge, this information supports the idea of a recoiling black hole in the system.

Moreover, stars are forming at a high rate in the galaxy, at several hundred times the mass of the Sun per year. This agrees with computer simulations, which predict that star formation rates may be enhanced for merging galaxies particularly those containing recoiling black holes.
Another possible explanation for the data is that two supermassive black holes are located in the center of the galaxy but one of them is not producing detectable radiation because it is growing too slowly. The researchers favor the recoiling black hole explanation, but more data are needed to strengthen their case.

A paper describing these results was recently accepted for publication in The Astrophysical Journal and is available online. The first author is Dongchan Kim from the National Radio Astronomy Observatory in Charlottesville, Virginia. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.




Fast Facts for CXO J101527.2+625911:

Scale: Inset images are 10 arcsec across (about 163,000 light years)
Category: Quasars & Active Galaxies, Black Holes
Coordinates (J2000): RA 10h 15m 27.2s | Dec +62° 59' 11.5"
Constellation: Ursa Major
Observation Date: 1 pointing in Apr 2008
Observation Time: 0 hours 51 minutes 40 seconds
Obs. ID: 9203
Instrument: ACIS
References: Kim, D.-C. et al., 2017, ApJ [in print]; arXiv: 1704.05549v1
Distance Estimate: About 3.9 billion light years (z=0.3504)



Saturday, May 13, 2017

Discovery in the early universe poses black hole growth puzzle




Quasars are luminous objects with supermassive black holes at their centers, visible over vast cosmic distances. Infalling matter increases the black hole mass and is also responsible for a quasar's brightness. Now, using the W.M. Keck observatory in Hawaii, astronomers led by Christina Eilers have discovered extremely young quasars with a puzzling property: these quasars have the mass of about a billion suns, yet have been collecting matter for less than 100,000 years. Conventional wisdom says quasars of that mass should have needed to pull in matter a thousand times longer than that – a cosmic conundrum. The results have been published in the May 2 edition of the Astrophysical Journal.

Within the heart of every massive galaxy lurks a supermassive black hole. How these black holes formed, and how they have grown to be as massive as millions or even billions of suns, is an open question. At least some phases of vigorous growth are highly visible to astronomical observers: Whenever there are substantial amounts of gas swirling into the black hole, matter in the direct vicinity of the black hole emits copious amount of light. The black hole has intermittently turned into a quasar, one of the most luminous objects in the universe.

Now, researchers from the Max Planck Institute for Astronomy (MPIA) have discovered three quasars that challenge conventional wisdom on black hole growth. These quasars are extremely massive, but should not have had sufficient time to collect all that mass. The discovery, which is based on observations at the W.M. Keck observatory in Hawaii, glimpses into ancient cosmic history: Because of their extreme brightness, quasars can be observed out to large distances. The astronomers observed quasars whose light took nearly 13 billion years to reach Earth. In consequence, the observations show these quasars not as they are today, but as they were almost 13 billion years ago, less than a billion years after the big bang.

The quasars in question have about a billion times the mass of the sun. All current theories of black hole growth postulate that, in order to grow that massive, the black holes would have needed to collect infalling matter, and shine brightly as quasars, for at least a hundred million years. But these three quasars proved to be have been active for a much shorter time, less than 100,000 years. “This is a surprising result,” explains Christina Eilers, a doctoral student at MPIA and lead author of the present study. “We don’t understand how these young quasars could have grown the supermassive black holes that power them in such a short time.”

To determine how long these quasars had been active, the astronomers examined how the quasars had influenced their environment – in particular, they examined heated, mostly transparent “proximity zones” around each quasar. “By simulating how the light from quasars ionizes and heats gas around them, we can predict how large the proximity zone of each quasar should be,” explains Frederick Davies, a postdoctoral researcher at MPIA who is an expert in the interaction between quasar light and intergalactic gas. Once the quasar has been “switched on” by infalling matter, these proximity zones grow very quickly. “Within a lifetime of 100,000 years, quasars should already have large proximity zones.”

Surprisingly, three of the quasars had very small proximity zones – indicating that the active quasar phase cannot have set in more than 100,000 years earlier. “No current theoretical models can explain the existence of these objects,” says Professor Joseph Hennawi, who leads the research group at MPIA that made the discovery. “The discovery of these young objects challenges the existing theories of black hole formation and will require new models to better understand how black holes and galaxies formed.“

The astronomers have already planned their next steps. “We would like to find more of these young quasars,“ says Christina Eilers, “While finding these three unusual quasars might have been a fluke, finding additional examples would imply that a significant fraction of the known quasar population is much younger than expected.” The scientists have already applied for telescope time to observe several additional candidates. The results, they hope, will constrain new theoretical models about the formation of the first supermassive black holes in the universe – and, by implication, help astronomers understand the history of the giant supermassive black holes at the center of present-day galaxies like our own Milky Way.




Background information 

The work described here has been published as A. C. Eilers, "Implications of z ∼ 6 Quasar Proximity Zones for the Epoch of Reonisation and Quasar Lifetimes" in the May 2 edition of the Astrophysical Journal.
The MPIA researchers involved are:

Anna-Christina Eilers, Frederick B. Davies, Joseph F. Hennawi, (also University of California at Santa Barbara), and Chiara Mazzuchelli
in collaboration with
J. Xavier Prochaska (University of California, Santa Cruz) and Zarija Lukic (Lawrence Berkeley National Laboratory).
Both A. C. Eilers and C. Mazzuchelli are members of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg.



Science Contact:

Eilers, Anna-Christina
Anna-Christina Eilers
Phone: (+49|0) 6221 528-432

Public Information Office: 

Markus Pössel
Öffentlichkeitsarbeit
Phone:(06221) 528-261
Email: pr@mpia.de


Friday, May 12, 2017

Close encounter

Credit: ESA/Hubble & NASA

This image from the NASA/ESA Hubble Space Telescope shows the unusual galaxy IRAS 06076-2139, found in the constellation Lepus (The Hare). Hubble’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) instruments observed the galaxy from a distance of 500 million light-years.

This particular object stands out from the crowd by actually being composed of two separate galaxies rushing past each other at about 2 million kilometres per hour. This speed is most likely too fast for them to merge and form a single galaxy. However, because of their small separation of only about 20 000 light-years, the galaxies will distort one another through the force of gravity while passing each other, changing their structures on a grand scale.

Such galactic interactions are a common sight for Hubble, and have long been a field of study for astronomers. The intriguing behaviours of interacting galaxies take many forms; galactic cannibalism, galaxy harassment and even galaxy collisions. The Milky Way itself will eventually fall victim to the latter, merging with the Andromeda Galaxy in about 4.5 billion years. The fate of our galaxy shouldn’t be alarming though: whilst galaxies are populated by billions of stars, the distances between individual stars are so large that hardly any stellar collisions will occur.


 

Thursday, May 11, 2017

Crab Nebula: Observatories Combine to Crack Open the Crab Nebula

NGC 1952/Crab Nebula
Credit  X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; 
Radio: NSF/NRAO/VLA; Ultraviolet: ESA/XMM-Newton



A Quick Look at the Crab Nebula
 
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Astronomers have produced a highly detailed image of the Crab Nebula, by combining data from telescopes spanning nearly the entire breadth of the electromagnetic spectrum, from radio waves seen by the Karl G. Jansky Very Large Array (VLA) to the powerful X-ray glow as seen by the orbiting Chandra X-ray Observatory. And, in between, the Hubble Space Telescope's crisp visible-light view and the infrared perspective of the Spitzer Space Telescope.

The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, is 6,500 light-years from Earth. At its center is a super-dense neutron star, rotating once every 33 milliseconds, shooting out rotating lighthouse-like beams from radio waves to gamma-ray wavelengths — a pulsar. The nebula's intricate shape is caused by a complex interplay of the pulsar, a fast-moving wind of particles coming from the pulsar, and material originally ejected by the supernova explosion and by the star itself before the explosion.

This image combines data from five different telescopes: The VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.

The new VLA, Hubble, and Chandra observations were largely made at about the same time in November 2012. Chandra has been observing the Crab Nebula since shortly after the telescope was launched into space in 1999 and has repeatedly done so in the years since. X-ray data reveal the distribution and behavior of the high-energy particles being spewed from the pulsar at the center of the Crab, which provides important clues to the workings of this mighty cosmic generator producing energy at the rate of 1,000 suns.

A paper describing the latest multi-wavelength work on the Crab, led by Gloria Dubner (IAFE), appears in The Astrophysical Journal and is available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.



Fast Facts for Crab Nebula:

Scale: Image is about 5 arcmin across (10 light years)
Category: Supernovas & Supernova Remnants, Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 05h 34m 32s | Dec +22° 0.0' 52.00"
Constellation: Taurus
Observation Date: 48 pointings between March 2000 and Nov 2013
Observation Time: 25 hours 28 min. (1 day 1 hour 28 min)
Obs. ID: 769-773, 1994-2001, 4607, 13139, 13146, 13147, 13150-13154, 13204-13210, 13750-13752, 13754-13757, 14416, 14458, 14678-14682, 14685, 16245, 16257, 16357, 16358
Instrument: ACIS
Also Known As: NGC 1952
References: Dubner, G. et al., 2017, ApJ [in print]; arXiv: 1704.02968
Color Code: X-ray (Purple), Ultraviolet (Blue), Optical (Green), Infrared (Yellow-Green), Radio (Red)
Distance Estimate: About 6,500 light years



Wednesday, May 10, 2017

Merging Galaxies Have Enshrouded Black Holes

This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right). Credit: National Astronomical Observatory of Japan.  › Larger view



Black holes get a bad rap in popular culture for swallowing everything in their environments. In reality, stars, gas and dust can orbit black holes for long periods of time, until a major disruption pushes the material in.

A merger of two galaxies is one such disruption. As the galaxies combine and their central black holes approach each other, gas and dust in the vicinity are pushed onto their respective black holes. An enormous amount of high-energy radiation is released as material spirals rapidly toward the hungry black hole, which becomes what astronomers call an active galactic nucleus (AGN).

A study using NASA's NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is enshrouded. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.

"The further along the merger is, the more enshrouded the AGN will be," said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. "Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust."

Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.

The study was published in the Monthly Notices of the Royal Astronomical Society. Researchers compared NuSTAR observations of the galaxies with data from NASA's Swift and Chandra and ESA's XMM-Newton observatories, which look at lower energy components of the X-ray spectrum. If high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, that is a sign that an AGN is heavily obscured.

The study helps confirm the longstanding idea that an AGN's black hole does most of its eating while enshrouded during the late stages of a merger.

"A supermassive black hole grows rapidly during these mergers," Ricci said. "The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy."

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's Jet Propulsion Laboratory for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. JPL is managed by Caltech for NASA.

For more information on NuSTAR, visit:  http://www.nasa.gov/nustar  -  http://www.nustar.caltech.edu


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Elizabeth Landau
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