richardmitnick
4:23 pm on September 22, 2017 Permalink
| Reply Tags: A mini-halo is a faint diffuse region of radio emission that surrounds a cluster of galaxies, CfA ( 91 ), Karl V Jansky NRAO VLA ( 4 ), Nature of Galaxy Cluster Mini-Halos, Supermassive Black Holes
A galaxy cluster mini-halo as seen around the galaxy NGC 1275 in the radio, with its main structures labeled: the northern extension, the two eastern spurs, the concave edge to the south, the south-western edge and a plume of emission to the south-south-west. Astronomers used radio and X-ray data to conclude that mini-halos, rather than being simple structures resulting from turbulence, are actually the result of multiple processes. Gendron-Marsolais et al.
A mini-halo is a faint, diffuse region of radio emission that surrounds a cluster of galaxies. So far about thirty of these cluster mini-halos have been detected via their X-ray and radio emission, the result of radiation from electrons in the ionized gas, including one mini-halo in the nearby Perseus cluster of galaxies. These electrons are thought to arise from activity around a supermassive black hole at a galactic nucleus, which injects steams of particles into the intracluster medium and which also produces turbulence and shocks. One issue puzzling astronomers is that such electrons should rapidly lose their energy, faster than the time it takes for them to reach the mini-halo regions. Suggested solutions include processes in which turbulence reaccelerates the electrons, and in which cosmic rays generate new ones.
CfA astronomer Reinout van Weeren and his colleagues used the radio Karl G. Jansky Very Large Array (JVLA) to obtain the first detailed study of the structure of the mini-halo in Perseus, and to compare it with Chandra X-Ray images.
NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
NASA/Chandra Telescope
They find that the radio emission comes primarily from gas behind a cold front as would be expected if the gas is sloshing around within the cluster as particles are re-accelerated. They also detect unexpected, filamentary structures that seem to be associated with edges of X-ray features. The scientists conclude that mini-halos are not simply diffuse structures produced by a single process, but reflect a variety of structures and processes including turbulent re-acceleration of electrons, relativistic activity from the black hole jets, and also some magnetic field effects. Not least, the results demonstrate the sensitivity of the new JVLA and the need to obtain such sensitive images to understand the mini-halo phenomenon.
The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.
Discarded gas from black holes spreads across galaxies and can even influence the formation of stars. Image credit – Flickr/ NASA Goddard Space Flight Center
It’s easy to picture a black hole as a kind of all-powerful cosmic drain, a sinkhole of super-strong gravity that snags and swallows passing nebulae or stars. While it is true we can’t observe matter once it crosses a black hole’s event horizon, scientists are zeroing in on what happens in the margins, where molecular clouds release vast amounts of energy as it circles the plughole.
EU scientists are honing in on just what happens to gas discarded by a black hole’s ferocious velocity, and how this can influence star formation in galaxies like ours, and even interstellar space.
Astronomer Dr Bjorn Emonts, from the National Radio Astronomy Observatory in the US, has been using some of the world’s biggest radio telescopes to look into what happens to such jets of gases as part of the EU-funded BLACK HOLES AND JWST project.
‘We wanted to see how black holes can affect the evolution of galaxies as a whole,’ he said.
Using advanced radio telescopes in the Atacama Desert of northern Chile, located 5 000 metres above sea-level, Dr Emonts can detect the characteristic spectral signatures of gas molecules as they are driven outward by the black hole.
ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres
‘If you have a rotating black hole with an accretion disk (particles orbiting the black hole), it can actually act like a kind of dynamo. It can trigger magnetic fields on either side of the accretion disk and these magnetic fields can trap charged particles,’ he said.
‘What you get is two jets … that can really propagate out very far away from the black hole – they can cross the entire galaxy and even influence the surroundings.’
Almost every galaxy is likely to have a rotating supermassive black hole at its centre. Dr Emonts found that the Dragonfly Galaxy, an ancient system from the early universe made up of merging galaxies, had tornado-like jets of particles coming off its black hole which could, in fact, kick-start its star formation.
‘We actually saw the amount of gas being displaced is the same rate as which stars are being formed,’ Dr Emonts said.
By scanning radio waves to detect carbon monoxide in another star system, the Spiderweb Galaxy, he was also able to show that molecular gas could exist and form stars outside of galaxies, and that jets of particles could even help the process by triggering cooling.
Dr Emonts hopes these findings will lay the groundwork for using the next generation of space telescope, the James Webb telescope, which can see molecular gas near black holes in unprecedented detail. This will lead to an even deeper understanding of the important role that black holes play in the evolution of galaxies.
X-ray collisions
Another way to detect energy given off from black hole accretion disks is through the X-ray spectrum. Dr Gabriele Ponti, who led the EU-funded HIGH-Z & MULTI-λ project at Max Planck Institute in Germany, said: ‘Most of the emissions from material that is falling into the black holes is in X-rays.’
His goal was to look for evidence that X-ray flares are caused when clouds of gas cross over supermassive black holes in the centre of galaxies.
For the first time ever he was able to observe an X-ray flare while gas clouds were being sucked into the black hole at the centre of our galaxy, called Sagittarius A*.
SGR A* NASA’s Chandra X-Ray Observatory
Nevertheless, it’s still too soon to say for sure if that may be the only reason for increased X-rays.
‘The X-rays are very bright. If you take a nuclear reaction, you have only a small fraction of energy from matter that is released – black hole accretion is many times more efficient,’ Dr Ponti said.
Better observations of emissions from black hole accretion disks can also lead to increased understanding of the size of black holes, as well as how exactly they help seed star formation.
‘We observed a sample of nearby supermassive black holes and we measured their variability, and we saw that it’s extremely well correlated with the black hole mass,’ Dr Ponti said.
That correlation can be used to determine distance, because they can correlate the intensity of emissions with the object’s mass and distance.
Star formation
‘If the earth was size of the galaxy, a (super massive) black hole would only be as big as your finger nail. Yet that object can influence the physics of something the size of the earth,’ Dr Ponti said.
To better understand the particle wind that comes off supermassive black holes, Dr Ponti looked at stellar mass black holes, millions of times smaller than those in galactic cores, and more manageable.
The surprising thing they observed was that they only saw the winds occasionally, depending on the orientation of the accretion disk to earth. That meant that such winds were flowing off on the same plane as the disk.
‘When the accretion disk is face on, our line of sight is not crossing through the wind and so we don’t observe it through absorption,’ Dr Ponti said.
Such particle winds, carrying gas that can form stars, are probably a feature of most black holes, and some studies have speculated they may even cast off more material than some black holes absorb. This adds to the growing evidence that black holes aren’t just an intergalactic destructive force, but rather a key player in the formation of galaxies.
There’s a supermassive black hole at the center of almost every galaxy in the Universe. How did they get there? What’s the relationship between these monster black holes and the galaxies that surround them?
Every time astronomers look farther out in the Universe, they discover new mysteries. These mysteries require all new tools and techniques to understand. These mysteries lead to more mysteries. What I’m saying is that it’s mystery turtles all the way down.
One of the most fascinating is the discovery of quasars, understanding what they are, and the unveiling of an even deeper mystery, where do they come from?
As always, I’m getting ahead of myself, so first, let’s go back and talk about the discovery of quasars.
Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a class of bizarre objects in the distant Universe. They were very bright, and incredibly far away; hundreds of millions or even billion of light-years away. The first ones were discovered in the radio spectrum, but over time, astronomers found even more blazing in the visible spectrum.
In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation. It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the “excited atoms” perspective.
26 October 2016 [Just found this. Don’t know how I missed it.]
Elena Borisova
ETH Zurich
Switzerland
Tel: +41 44 633 77 09
Email: borisova@phys.ethz.ch
Mathias Jäger
Public Information Officer
Garching bei München, Germany
Tel: +49 176 62397500
Email: mjaeger@partner.eso.org
An international team of astronomers has discovered glowing gas clouds surrounding distant quasars. This new survey by the MUSE instrument on ESO’s Very Large Telescope indicates that halos around quasars are far more common than expected. The properties of the halos in this surprising find are also in striking disagreement with currently accepted theories of galaxy formation in the early Universe.
An international collaboration of astronomers, led by a group at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, has used the unrivalled observing power of MUSE on the Very Large Telescope (VLT) at ESO’s Paranal Observatory to study gas around distant active galaxies, less than two billion years after the Big Bang.
ESO MUSE on the VLT
These active galaxies, called quasars, contain supermassive black holes in their centres, which consume stars, gas, and other material at an extremely high rate. This, in turn, causes the galaxy centre to emit huge amounts of radiation, making quasars the most luminous and active objects in the Universe.
The study involved 19 quasars, selected from among the brightest that are observable with MUSE. Previous studies have shown that around 10% of all quasars examined were surrounded by halos, made from gas known as the intergalactic medium. These halos extend up to 300 000 light-years away from the centres of the quasars. This new study, however, has thrown up a surprise, with the detection of large halos around all 19 quasars observed — far more than the two halos that were expected statistically. The team suspects this is due to the vast increase in the observing power of MUSE over previous similar instruments, but further observations are needed to determine whether this is the case.
“It is still too early to say if this is due to our new observational technique or if there is something peculiar about the quasars in our sample. So there is still a lot to learn; we are just at the beginning of a new era of discoveries”, says lead author Elena Borisova, from the ETH Zurich.
The original goal of the study was to analyse the gaseous components of the Universe on the largest scales; a structure sometimes referred to as the cosmic web, in which quasars form bright nodes [1].
Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
The gaseous components of this web are normally extremely difficult to detect, so the illuminated halos of gas surrounding the quasars deliver an almost unique opportunity to study the gas within this large-scale cosmic structure.
The 19 newly-detected halos also revealed another surprise: they consist of relatively cold intergalactic gas — approximately 10 000 degrees Celsius. This revelation is in strong disagreement with currently accepted models of the structure and formation of galaxies, which suggest that gas in such close proximity to galaxies should have temperatures upwards of a million degrees.
The discovery shows the potential of MUSE for observing this type of object [2]. Co-author Sebastiano Cantalupo is very excited about the new instrument and the opportunities it provides: “We have exploited the unique capabilities of MUSE in this study, which will pave the way for future surveys. Combined with a new generation of theoretical and numerical models, this approach will continue to provide a new window on cosmic structure formation and galaxy evolution.”
Notes
[1] The cosmic web is the structure of the Universe at the largest scale. It is comprised of spindly filaments of primordial material (mostly hydrogen and helium gas) and dark matter which connect galaxies and span the chasms between them. The material in this web can feed along the filaments into galaxies and drive their growth and evolution.
[2] MUSE is an integral field spectrograph and combines spectrographic and imaging capabilities. It can observe large astronomical objects in their entirety in one go, and for each pixel measure the intensity of the light as a function of its colour, or wavelength.
The team is composed of Elena Borisova, Sebastiano Cantalupo, Simon J. Lilly, Raffaella A. Marino and Sofia G. Gallego (Institute for Astronomy, ETH Zurich, Switzerland), Roland Bacon and Jeremy Blaizot (University of Lyon, Centre de Recherche Astrophysique de Lyon, Saint-Genis-Laval, France), Nicolas Bouché (Institut de Recherche en Astrophysique et Planétologie, Toulouse, France), Jarle Brinchmann (Leiden Observatory, Leiden, The Netherlands; Instituto de Astrofísica e Ciências do Espaço, Porto, Portugal), C Marcella Carollo (Institute for Astronomy, ETH Zurich, Switzerland), Joseph Caruana (Department of Physics, University of Malta, Msida, Malta; Institute of Space Sciences & Astronomy, University of Malta, Malta), Hayley Finley (Institut de Recherche en Astrophysique et Planétologie, Toulouse, France), Edmund C. Herenz (Leibniz-Institut für Astrophysik Potsdam, Potsdam, Germany), Johan Richard (Univ Lyon, Centre de Recherche Astrophysique de Lyon, Saint-Genis-Laval, France), Joop Schaye and Lorrie A. Straka (Leiden Observatory, Leiden, The Netherlands), Monica L. Turner (MIT-Kavli Center for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA), Tanya Urrutia (Leibniz-Institut für Astrophysik Potsdam, Potsdam, Germany), Anne Verhamme (University of Lyon, Centre de Recherche Astrophysique de Lyon, Saint-Genis-Laval, France), Lutz Wisotzki (Leibniz-Institut für Astrophysik Potsdam, Potsdam, Germany).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.
VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.
ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.
ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.
VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level.
ALMA on the Chajnantor plateau at 5,000 metres.
ESO/E-ELT to be built at Cerro Armazones at 3,060 m.
APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert.
Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)
Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)
4 August 2017 [I do not know how I missed this one.]
Susanna Kohler
Still from a simulation of stars orbiting the supermassive black hole at the center of a galaxy. Stars like these can sometimes be perturbed onto close circular orbits where they very slowly lose mass to the black hole as they spiral inward. [ESO/ S. Gillessen, R. Genzel]
When a normally dormant supermassive black hole burps out a brief flare, it’s assumed that a star was torn apart and fell into the black hole. But a new study suggests that some of these flares might have a slightly different cause.
Not a Disruption?
Artist’s impression of a tidal disruption event, in which a star has been pulled apart and its gas feeds the supermassive black hole. [NASA/JPL-Caltech].
When a star swings a little too close by a supermassive black hole, the black hole’s gravity can pull the star apart, completely disrupting it. The resulting gas can then accrete onto the black hole, feeding it and causing it to flare. The predicted frequency of these tidal disruption events and their expected light curves don’t perfectly match all our observations of flaring black holes, however.
This discrepancy has led two scientists from the Columbia Astrophysics Laboratory, Brian Metzger and Nicholas Stone, to wonder if we can explain flares from supermassive black holes in another way. Could a different event masquerade as a tidal disruption?
Evolution of a star’s semimajor axis (top panel) and radius (bottom panel) as a function of time since Roche-lobe overflow began onto a million-solar-mass black hole. Curves show stars of different masses. [Metzger & Stone 2017]
Inspirals and Outspirals
In the dense nuclear star cluster surrounding a supermassive black hole, various interactions can send stars on new paths that take them close to the black hole. In many of these interactions, the stars will end up on plunging orbits, often resulting in tidal disruption. But sometimes stars can approach the black hole on tightly bound orbits with lower eccentricities.
A main-sequence star on such a path, in what is known as an “extreme mass ratio inspiral (EMRI)”, slowly approaches the black hole over a period of millions of years, eventually overflowing its Roche lobe and losing mass. The radius of the star inflates, driving more mass loss and halting the star’s inward progress. The star then reverses course and migrates outward again as a brown dwarf.
Metzger and Stone demonstrate that the timescale for this process is shorter than the time delay expected between successive EMRIs. The likelihood is high, they show, that two consecutive EMRIs would collide while one is inspiraling and the other is outspiraling.
Results of a Collision
Schematic diagram (not to scale) showing how two circular EMRI orbits can intersect as the main-sequence star migrates inward (blue) and the brown dwarf very slowly migrates outward (red). [Metzger & Stone 2017]
Because both stars are deep in the black hole’s gravitational well, they collide with enormous relative velocities (~10% the speed of light!). If this collision is head-on, one or both stars will be completely destroyed. The resulting gas then accretes onto the black hole, producing a flare very similar to a classical tidal disruption event.
If the stars instead meet on a grazing collision, Metzger and Stone show that this liberates gas from at least one of the stars. The gas forms an accretion disk around the black hole, causing a transient flare similar to some of the harder-to-explain flares we’ve observed that don’t quite fit our models for tidal disruption events.
In this latter scenario, the stars survive to encounter each other again, decades to millennia later. These grazing collisions between the pair can continue to produce quasi-periodic flares for thousands of years or longer.
Metzger and Stone argue that EMRI collisions have the potential to explain some of the flares from supermassive black holes that we had previously attributed to tidal disruption events. More detailed modeling will allow us to explore this idea further in the future.
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
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richardmitnick
1:03 pm on August 16, 2017 Permalink
| Reply Tags: ESO ( 28 ), ESO MUSE ( 7 ), GASP-GAs Stripping Phenomena in galaxies with MUSE, Jellyfish galaxies, Ram pressure stripping, Supermassive Black Holes, To date just over 400 candidate jellyfish galaxies have been found
16 August 2017
Bianca Poggianti
INAF-Astronomical Observatory of Padova
Padova, Italy
+39 340 7448663 bianca.poggianti@oapd.inaf.it
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591 rhook@eso.org
Observations of “Jellyfish galaxies” with ESO’s Very Large Telescope have revealed a previously unknown way to fuel supermassive black holes. It seems the mechanism that produces the tentacles of gas and newborn stars that give these galaxies their nickname also makes it possible for the gas to reach the central regions of the galaxies, feeding the black hole that lurks in each of them and causing it to shine brilliantly. The results appeared today in the journal Nature.
This picture of one of the galaxies, nicknamed JW100, from the MUSE instrument on ESO’s Very Large Telescope in Chile, shows clearly how material is streaming out of the galaxy in long tendrils. Red shows the glow from ionised hydrogen gas and the whiter regions are where most of the stars in the galaxy are located. Credit:
ESO/GASP collaboration
This visualisation shows a jellyfish galaxy in the three-dimensional view of the MUSE instrument on ESO’s Very Large Telescope. This combines the normal two-dimensional view with the third dimension of wavelength. This galaxy has undergone ram pressure stripping as it move rapidly into the hot gas in a galaxy cluster, and streamers of gas and young stars are trailing behind it. These show up as the tentacles extending to the right in this picture as they have different velocities to the main disc of the galaxy, shown at the left. Credit: ESO.
Observations of “Jellyfish galaxies” with ESO’s Very Large Telescope have revealed a previously unknown way to fuel supermassive black holes. It seems the mechanism that produces the tentacles of gas and newborn stars that give these galaxies their nickname also makes it possible for the gas to reach the central regions of the galaxies, feeding the black hole that lurks in each of them and causing it to shine brilliantly.
This quick video explains the main points. Credit: ESO.
Directed by: Nico Bartmann.
Editing: Nico Bartmann.
Web and technical support: Mathias André and Raquel Yumi Shida.
Written by: Izumi Hansen and Richard Hook.
Music: tonelabs (http://www.tonelabs.com).
Footage and photos: ESO, A. Tudorica, NASA, ESA, Callum Bellhouse and the GASP collaboration, M. Kornmesser, L. Calçada.
Executive producer: Lars Lindberg Christensen.
An Italian-led team of astronomers used the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile to study how gas can be stripped from galaxies.
ESO MUSE on the VLT
They focused on extreme examples of jellyfish galaxies in nearby galaxy clusters, named after the remarkable long “tentacles” of material that extend for tens of thousands of light-years beyond their galactic discs [1][2].
The tentacles of jellyfish galaxies are produced in galaxy clusters by a process called ram pressure stripping. Their mutual gravitational attraction causes galaxies to fall at high speed into galaxy clusters, where they encounter a hot, dense gas which acts like a powerful wind, forcing tails of gas out of the galaxy’s disc and triggering starbursts within it.
Six out of the seven jellyfish galaxies in the study were found to host a supermassive black hole at the centre, feeding on the surrounding gas [3]. This fraction is unexpectedly high — among galaxies in general the fraction is less than one in ten.
“This strong link between ram pressure stripping and active black holes was not predicted and has never been reported before,” said team leader Bianca Poggianti from the INAF-Astronomical Observatory of Padova in Italy. “It seems that the central black hole is being fed because some of the gas, rather than being removed, reaches the galaxy centre.” [4]
A long-standing question is why only a small fraction of supermassive black holes at the centres of galaxies are active. Supermassive black holes are present in almost all galaxies, so why are only a few accreting matter and shining brightly? These results reveal a previously unknown mechanism by which the black holes can be fed.
Yara Jaffé, an ESO fellow who contributed to the paper explains the significance: “These MUSE observations suggest a novel mechanism for gas to be funnelled towards the black hole’s neighbourhood. This result is important because it provides a new piece in the puzzle of the poorly understood connections between supermassive black holes and their host galaxies.”
The current observations are part of a much more extensive study of many more jellyfish galaxies that is currently in progress.
“This survey, when completed, will reveal how many, and which, gas-rich galaxies entering clusters go through a period of increased activity at their cores,” concludes Poggianti. “A long-standing puzzle in astronomy has been to understand how galaxies form and change in our expanding and evolving Universe. Jellyfish galaxies are a key to understanding galaxy evolution as they are galaxies caught in the middle of a dramatic transformation.”
Notes
[1] To date, just over 400 candidate jellyfish galaxies have been found.
[2] The results were produced as part of the observational programme known as GASP (GAs Stripping Phenomena in galaxies with MUSE), which is an ESO Large Programme aimed at studying where, how and why gas can be removed from galaxies. GASP is obtaining deep, detailed MUSE data for 114 galaxies in various environments, specifically targeting jellyfish galaxies. Observations are currently in progress.
[3] It is well established that almost every, if not every, galaxy hosts a supermassive black hole at its centre, between a few million and a few billion times as massive as our Sun. When a black hole pulls in matter from its surroundings, it emits electromagnetic energy, giving rise to some of the most energetic of astrophysical phenomena: active galactic nuclei (AGN).
[4] The team also investigated the alternative explanation that the central AGN activity contributes to stripping gas from the galaxies, but considered it less likely. Inside the galaxy cluster, the jellyfish galaxies are located in a zone where the hot, dense gas of the intergalactic medium is particularly likely to create the galaxy’s long tentacles, reducing the possibility that they are created by AGN activity. There is therefore stronger evidence that ram pressure triggers the AGN and not vice versa.
More information
This research was presented in a paper entitled “Ram Pressure Feeding Supermassive Black Holes” by B. Poggianti et al., to appear in the journal Nature on 17 August 2017.
The team is composed of B. Poggianti (INAF-Astronomical Observatory of Padova, Italy), Y. Jaffé (ESO, Chile), A. Moretti (INAF-Astronomical Observatory of Padova, Italy), M. Gullieuszik (INAF-Astronomical Observatory of Padova, Italy), M. Radovich (INAF-Astronomical Observatory of Padova, Italy), S. Tonnesen (Carnegie Observatory, USA), J. Fritz (Instituto de Radioastronomía y Astrofísica, Mexico), D. Bettoni (INAF-Astronomical Observatory of Padova, Italy), B. Vulcani (University of Melbourne, Australia; INAF-Astronomical Observatory of Padova, Italy), G. Fasano (INAF-Astronomical Observatory of Padova, Italy), C. Bellhouse (University of Birmingham, UK; ESO, Chile), G. Hau (ESO, Chile) and A. Omizzolo (Vatican Observatory, Vatican City State).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.
VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.
ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.
ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.
VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level.
ALMA on the Chajnantor plateau at 5,000 metres.
ESO/E-ELT to be built at Cerro Armazones at 3,060 m.
APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert.
Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)
Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)
Artist’s illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)]
Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.
The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]
A Torus Puzzle
The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.
Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One hypothesis is that the torus structure itself changes with changing AGN luminosity. In this model, the torus recedes as AGN become brighter, causing fewer of these AGN to be obscured from our view.
But a team of scientists led by Silvia Mateos (Institute of Physics of Cantabria, Spain) suggests that we may instead be missing the bigger picture. What if the problem is just that many of the brightest obscured AGN are too well hidden?
Geometry Matters
Type 2 AGN fraction vs. torus covering factor for AGN in the authors’ three luminosity bins. The black line shows the 1-to-1 relation describing the expected Type 2 AGN fraction; the black data points show the observed fraction. The red points show the best-fit model including the “missing” AGN, and the inset shows the covering-factor distribution for the missing sources. [Mateos et al. 2017]
Mateos and collaborators built a sample of nearly 200 X-ray-observed AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). They then determined the intrinsic fraction of these AGN that were obscured (i.e., classified as Type 2) at a given luminosity, for redshifts between 0.05 ≤ z ≤ 1.
ESA/XMM Newton
The team next used clumpy torus models to estimate the distributions of AGN covering factors, the geometric factor that describes the fraction of the sky around the AGN central engine that’s obscured.
The pointing directions for AGN should be randomly distributed, and geometry then dictates that the covering factor distributions combined over the total AGN population should match the intrinsic fraction of AGN classified as Type 2 AGN. Instead, the sample from BUXS reveals a “missing” population of high-covering-factor tori that we have yet to detect in X-rays.
Missing Sources
When they include the missing AGN, Mateos and collaborators find that the total fraction of Type 2 AGN is around 58%. They also show that more of these AGN are missing at higher luminosities. By including the missing ones, the total fraction of obscured AGN therefore has a much weaker dependence on luminosity than we thought — which suggests that the receding torus model isn’t necessary to explain observations.
Mateos and collaborators’ results support the idea that the majority of very bright, rapidly accreting supermassive black holes at redshifts of z ≤ 1 live in nuclear environments that are extremely obscured. These black holes are so well embedded in their environments that they’ve escaped detection in X-ray surveys thus far.
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Light from the first galaxies clears the universe. ESO/L. Calçada
Not long after the Big Bang, all went dark. The hydrogen gas that pervaded the early universe would have snuffed out the light of the universe’s first stars and galaxies. For hundreds of millions of years, even a galaxy’s worth of stars—or unthinkably bright beacons such as those created by supermassive black holes—would have been rendered all but invisible.
Eventually this fog burned off as high-energy ultraviolet light broke the atoms apart in a process called reionization.
Reionization era and first stars, Caltech
But the questions of exactly how this happened—which celestial objects powered the process and how many of them were needed—have consumed astronomers for decades.
Now, in a series of studies, researchers have looked further into the early universe than ever before. They’ve used galaxies and dark matter as a giant cosmic lens to see some of the earliest galaxies known, illuminating how these galaxies could have dissipated the cosmic fog. In addition, an international team of astronomers has found dozens of supermassive black holes—each with the mass of millions of suns—lighting up the early universe. Another team has found evidence that supermassive black holes existed hundreds of millions of years before anyone thought possible. The new discoveries should make clear just how much black holes contributed to the reionization of the universe, even as they’ve opened up questions as to how such supermassive black holes were able to form so early in the universe’s history.
First Light
In the first years after the Big Bang, the universe was too hot to allow atoms to form. Protons and electrons flew about, scattering any light. Then after about 380,000 years, these protons and electrons cooled enough to form hydrogen atoms, which coalesced into stars and galaxies over the next few hundreds of millions of years.
Starlight from these galaxies would have been bright and energetic, with lots of it falling in the ultraviolet part of the spectrum. As this light flew out into the universe, it ran into more hydrogen gas. These photons of light would break apart the hydrogen gas, contributing to reionization, but as they did so, the gas snuffed out the light.
Lucy Reading-Ikkanda/Quanta Magazine
To find these stars, astronomers have to look for the non-ultraviolet part of their light and extrapolate from there. But this non-ultraviolet light is relatively dim and hard to see without help.
A team led by Rachael Livermore, an astrophysicist at the University of Texas at Austin, found just the help needed in the form of a giant cosmic lens.
Gravitational Lensing NASA/ESA
These so-called gravitational lenses form when a galaxy cluster, filled with massive dark matter, bends space-time to focus and magnify any object on the other side of it. Livermore used this technique with images from the Hubble Space Telescope to spot extremely faint galaxies from as far back as 600 million years after the Big Bang—right in the thick of reionization.
NASA/ESA Hubble Telescope
In a recent paper that appeared in The Astrophysical Journal, Livermore and colleagues also calculated that if you add galaxies like these to the previously known galaxies, then stars should be able to generate enough intense ultraviolet light to reionize the universe.
Yet there’s a catch. Astronomers doing this work have to estimate how much of a star’s ultraviolet light escaped its home galaxy (which is full of light-blocking hydrogen gas) to go out into the wider universe and contribute to reionization writ large. That estimate—called the escape fraction—creates a huge uncertainty that Livermore is quick to acknowledge.
In addition, not everyone believes Livermore’s results. Rychard Bouwens, an astrophysicist at Leiden University in the Netherlands, argues in a paper submitted to The Astrophysical Journal that Livermore didn’t properly subtract the light from the galaxy clusters that make up the gravitational lens.
As a result, he said, the distant galaxies aren’t as faint as Livermore and colleagues claim, and astronomers have not found enough galaxies to conclude that stars ionized the universe.
If stars couldn’t get the job done, perhaps supermassive black holes could. Beastly in size, up to a billion times the mass of the sun, supermassive black holes devour matter. They tug it toward them and heat it up, a process that emits lots of light and creates luminous objects that we call quasars. Because quasars emit way more ionizing radiation than stars do, they could in theory reionize the universe.
The trick is finding enough quasars to do it. In a paper posted to the scientific preprint site arxiv.org last month, astronomers working with the Subaru Telescope announced the discovery of 33 quasars that are about a 10th as bright as ones identified before.
NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA
With such faint quasars, the astronomers should be able to calculate just how much ultraviolet light these supermassive black holes emit, said Michael Strauss, an astrophysicist at Princeton University and a member of the team.
The researchers haven’t done the analysis yet, but they expect to publish the results in the coming months.
The oldest of these quasars dates back to around a billion years after the Big Bang, which seems about how long it would take ordinary black holes to devour enough matter to bulk up to supermassive status.
This is why another recent discovery [ApJ] is so puzzling. A team of researchers led by Richard Ellis, an astronomer at the European Southern Observatory, was observing a bright, star-forming galaxy seen as it was just 600 million years after the Big Bang.
The galaxy’s spectrum—a catalog of light by wavelength—appeared to contain a signature of ionized nitrogen. It’s hard to ionize ordinary hydrogen, and even harder to ionize nitrogen. It requires more higher-energy ultraviolet light than stars emit. So another strong source of ionizing radiation, possibly a supermassive black hole, had to exist at this time, Ellis said.
One supermassive black hole at the center of an early star-forming galaxy might be an outlier. It doesn’t mean there were enough of them around to reionize the universe. So Ellis has started to look at other early galaxies. His team now has tentative evidence that supermassive black holes sat at the centers of other massive, star-forming galaxies in the early universe. Studying these objects could help clarify what reionized the universe and illuminate how supermassive black holes formed at all. “That is a very exciting possibility,” Ellis said.
All this work is beginning to converge on a relatively straightforward explanation for what reionized the universe. The first population of young, hot stars probably started the process, then drove it forward for hundreds of millions of years. Over time, these stars died; the stars that replaced them weren’t quite so bright and hot. But by this point in cosmic history, supermassive black holes had enough time to grow and could start to take over. Researchers such as Steve Finkelstein, an astrophysicist at the University of Texas at Austin, are using the latest observational data and simulations of early galactic activity to test out the details of this scenario, such as how much stars and black holes contribute to the process at different times.
His work—and all work involving the universe’s first billion years—will get a boost in the coming years after the 2018 launch of the James Webb Space Telescope, Hubble’s successor, which has been explicitly designed to find the first objects in the universe.
NASA/ESA/CSA Webb Telescope annotated
Its findings will probably provoke many more questions, too.
The orbits of two stars, S0-2 and S0-38 located near the Milky Way’s supermassive black hole will be used to test Einstein’s theory of General Relativity and potentially generate new gravitational models. IMAGE CREDIT: S. SAKAI/A.GHEZ/W. M. KECK OBSERVATORY/ UCLA GALACTIC CENTER GROUP
W. M. Keck Observatory Data Leads To First Of Its Kind Test of Einstein’s Theory of General Relativity.
May 26, 2017
No writer credit found.
A UCLA-led team has discovered a new way of probing the hypothetical fifth force of nature using two decades of observations at W. M. Keck Observatory, the world’s most scientifically productive ground-based telescope.
There are four known forces in the universe: electromagnetic force, strong nuclear force, weak nuclear force, and gravitational force. Physicists know how to make the first three work together, but gravity is the odd one out. For decades, there have been theories that a fifth force ties gravity to the others, but no one has been able to prove it thus far.
“This is really exciting. It’s taken us 20 years to get here, but now our work on studying stars at the center of our galaxy is opening up a new method of looking at how gravity works,” said Andrea Ghez, Director of the UCLA Galactic Center Group and co-author of the study.
Ghez and her co-workers analyzed extremely sharp images of the center of our galaxy taken with Keck Observatory’s adaptive optics (AO). Ghez used this cutting-edge system to track the orbits of stars near the supermassive black hole located at the center of the Milky Way.
Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way
Their stellar path, driven by gravity created from the supermassive black hole, could give clues to the fifth force.
“By watching the stars move over 20 years using very precise measurements taken from Keck Observatory data, you can see and put constraints on how gravity works. If gravitation is driven by something other than Einstein’s theory of General Relativity, you’ll see small variations in the orbital paths of the stars,” said Ghez.
Pictured above: UCLA Professor of Astrophysics and Galactic Center Group Director Andrea Ghez, a Keck Observatory astronomer and recipient of the 2015 Bakerian Medal. IMAGE CREDIT: KYLE ALEXANDER
This is the first time the fifth force theory has been tested in a strong gravitational field such as the one created by the supermassive black hole at the center of the Milky Way. Historically, measurements of our solar system’s gravity created by our sun have been used to try and detect the fifth force, but that has proven difficult because its gravitational field is relatively weak.
“It’s exciting that we can do this because we can ask a very fundamental question – how does gravity work?” said Ghez. “Einstein’s theory describes it beautifully well, but there’s lots of evidence showing the theory has holes. The mere existence of supermassive black holes tells us that our current theories of how the universe works are inadequate to explain what a black hole is.”
Ghez and her team, including lead author Aurelien Hees and co-author Tuan Do, both of UCLA, are looking forward to summer of 2018. That is when the star S0-2 will be at its closest distance to our galaxy’s supermassive black hole. This will allow the team to witness the star being pulled at maximum gravitational strength – a point where any deviations to Einstein’s theory is expected to be the greatest.
About Adaptive Optics
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere.
Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and our current systems now deliver images three to four times sharper than the Hubble Space Telescope. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors.
Mission
To advance the frontiers of astronomy and share our discoveries with the world.
The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometer and world-leading laser guide star adaptive optics systems. Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.
Today Keck Observatory is supported by both public funding sources and private philanthropy. As a 501(c)3, the organization is managed by the California Association for Research in Astronomy (CARA), whose Board of Directors includes representatives from the California Institute of Technology and the University of California, with liaisons to the board from NASA and the Keck Foundation.
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).
SDSS Telescope at Apache Point Observatory, NM, USA
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.
NASA/ESA Hubble Telescope
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.
Keck Observatory, Mauna Kea, Hawaii, USA
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.
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