Giant black hole could upset galaxy evolution models

Nov. 27, 2012 — A group of astronomers led by Remco van den Bosch from the Max Planck Institute for Astronomy (MPIA) have discovered a black hole that could shake the foundations of current models of galaxy evolution. At 17 billion times the mass of the Sun, its mass is much greater than current models predict -- in particular since the surrounding galaxy is comparatively small. This could be the most massive black hole found to date.

To the best of our astronomical knowledge, almost every galaxy should contain in its central region what is called a supermassive black hole: a black hole with a mass between that of hundreds of thousands and billions of Suns. The best-studied super-massive black hole sits in the center of our home galaxy, the Milky Way, with a mass of about four million Suns.

For the masses of galaxies and their central black holes, an intriguing trend has emerged: a direct relationship between the mass of a galaxy's black hole and that of the galaxy's stars.

Typically, the black hole mass is a tiny fraction of the galaxy's total mass. But now a search led by Remco van den Bosch (MPIA) has discovered a massive black hole that could upset the accepted relationship between black hole mass and galaxy mass, which plays a key role in all current theories of galaxy evolution. The observations used the Hobby-Eberly Telescope and existing images from the Hubble Space Telescope.

With a mass 17 billion times that of the Sun, the newly discovered black hole in the center of the disk galaxy NGC 1277 might even be the biggest known black hole of all: the mass of the current record holder is estimated to lie between 6 and 37 billion solar masses (McConnell et al. 2011); if the true value lies towards the lower end of that range, NGC 1277 breaks the record. At the least, NGC 1277 harbors the second-biggest known black hole.

The big surprise is that the black hole mass for NGC 1277 amounts to 14% of the total galaxy mass, instead of usual values around 0,1%. This beats the old record by more than a factor 10. Astronomers would have expected a black hole of this size inside blob-like ("elliptical") galaxies ten times larger. Instead, this black hole sits inside a fairly small disk galaxy.

Is this surprisingly massive black hole a freak accident? Preliminary analysis of additional data suggests otherwise -- so far, the search has uncovered five additional galaxies that are comparatively small, yet, going by first estimates, seemed to harbor unusually large black holes too. More definite conclusions have to await detailed images of these galaxies.

If the additional candidates are confirmed, and there are indeed more black holes like this, astronomers will need to rethink fundamentally their models of galaxy evolution. In particular, they will need to look at the early universe: The galaxy hosting the new black hole appears to have formed more than 8 billion years ago, and does not appear to have changed much since then. Whatever created this giant black hole must have happened a long time ago.

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Journal Reference:

Remco C. E. van den Bosch, Karl Gebhardt, Kayhan Gültekin, Glenn van de Ven, Arjen van der Wel, Jonelle L. Walsh. An over-massive black hole in the compact lenticular galaxy NGC?1277. Nature, 2012; 491 (7426): 729 DOI: 10.1038/nature11592

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How black holes change gear

June 7, 2012 — Black holes are extremely powerful and efficient engines that not only swallow up matter, but also return a lot of energy to the Universe in exchange for the mass they eat. When black holes attract mass they also trigger the release of intense X-ray radiation and power strong jets. But not all black holes do this the same way. This has long baffled astronomers. By studying two active black holes researchers at the SRON Netherlands Institute for Space Research have now gathered evidence that suggests that each black hole can change between two different regimes, like changing the gears of an engine.

The team's findings will be published in two papers in the journal Monthly Notices of the Royal Astronomical Society.

Black hole jets -- lighthouse-like beams of material that race outwards at close to the speed of light -- can have a major impact on the evolution of their environment. For example, jets from the super-massive black holes found at the centre of galaxies can blow huge bubbles in and heat the gas found in clusters of galaxies.

Another stunning example of what black hole jets can do is known as Hanny's Voorwerp, a cloud of gas where stars started forming after it was hit by the jet-beam of a black hole in a neighbouring galaxy. These phenomena demonstrate the importance of research into the way black holes produce and distribute energy, but until recently, much of this has remained uncertain.

In 2003 it became clear from astronomical observations that there is a connection between the X-ray emission from a black hole and its jet outflow. This connection needs to be explained if we want to understand how the black hole engine works. In the first years after this connection was discovered, it seemed that it was the same for all feeding black holes, but soon oddballs were found. These unusual examples still have a clear connection between the energy released in the X-ray emission and that put in the jet ejection. But the proportion differs from that in the "standard" black holes. As the number of oddballs grew, it started to appear that there were two groups of black hole engines working in a slightly different way, as if one were running on petrol and the other on diesel.

For years astronomers struggled to justify this difference based on the properties of the two groups of black holes, but to no avail. Recently a step forward was made: a team of astronomers led by Michael Coriat (now at University of Southampton) found a black hole that seemed to switch between the two flavours of X-ray/jet connection, depending on its brightness changed. This suggested that black holes do not necessarily come with two different engines, but that each black hole can run in two different regimes, like two gears of the same engine.

Now Peter Jonker and PhD-student Eva Ratti, two researchers from the SRON Netherlands Institute for Space Research -- have taken an important step forward in the attempts to solve this puzzle. Using X-ray observations from the Chandra X-ray Observatory and radio observations from the Expanded Very Large Array in New Mexico they watched two black hole systems until their feeding frenzies ended.

Eva Ratti comments: "We found that these two black holes could also 'change gear', demonstrating that this is not an exceptional property of one peculiar black hole. Our work suggests that changing gear might be common among black holes. We also found that the switch between gears happens at a similar X-ray luminosity for all the three black holes."

These discoveries provide a new and important input to theoretical models that aim to explain both the functioning of the black hole engine itself and its impact on the surrounding environment.

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Journal References:

P. G. Jonker, J. C. A. Miller-Jones, J. Homan, J. Tomsick, R. P. Fender, P. Kaaret, S. Markoff, E. Gallo. The black hole candidate MAXI J1659-152 in and towards quiescence in X-ray and radio. Monthly Notices of the Royal Astronomical Society, 2012; DOI: 10.1111/j.1365-2966.2012.21116.xE. M. Ratti, P. G. Jonker, J. C. A. Miller-Jones, M. A. P. Torres, J. Homan, S. Markoff, J. A. Tomsick, P. Kaaret, R. Wijnands, E. Gallo, F. Özel, D. T. H. Steeghs, R. P. Fender. The black hole candidate XTE J1752-223 towards and in quiescence: optical and simultaneous X-ray-radio observations. Monthly Notices of the Royal Astronomical Society, 2012; DOI: 10.1111/j.1365-2966.2012.21071.x

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How to build a middleweight black hole: New model for intermediate black hole formation parallels growth of giant planets

July 19, 2012 — A new model shows how an elusive type of black hole can be formed in the gas surrounding their supermassive counterparts.

In research published in the Monthly Notices of the Royal Astronomical Society, scientists from the American Museum of Natural History, the City University of New York, the Jet Propulsion Laboratory of the California Institute of Technology, and the Harvard-Smithsonian Center for Astrophysics propose that intermediate-mass black holes -- light-swallowing celestial objects with masses ranging from hundreds to many thousands of times the mass of the Sun -- can grow in the gas disks around supermassive black holes in the centers of galaxies. The physical mechanism parallels the model astrophysicists use to describe the growth of giant planets in the gas disks surrounding stars.

"We know about small black holes, which tend to be close to us and have masses a few to 10 times that of our Sun, and we know about supermassive black holes, which are found in the centers of galaxies and have a mass that's millions to billions of times the mass of the sun," said coauthor Saavik Ford, who is a research associate in the Museum's Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, City University of New York (CUNY) and a faculty member at CUNY's Graduate Center. "But we have no evidence for the middle stage. Intermediate-mass black holes are much harder to find."

The birth of an intermediate black hole starts with the death of a star that forms a stellar or low-mass black hole. In order for this "seed" to grow, it must collide with and consume other dead and living stars. But even though there are many billions of stars in large galaxies, there's an even greater proportion of empty space, making collisions a very rare occurrence.

The researchers' new model suggests that previous searches for middleweight black holes might have been focused on the wrong birthing ground.

"The recent focus had been on star clusters, but objects there move very quickly and there's no gas, which makes the chances of a collision very slim," said Barry McKernan, a research associate in the Museum's Department of Astrophysics who is a professor at CUNY's Borough of Manhattan Community College and a faculty member at CUNY's Graduate Center.

The new mechanism turns attention instead to active galactic nuclei, the piping hot and ultra-bright cores of galaxies that feed supermassive black holes. The gas in this system is key, causing the stars to slow down and conform to a circularized orbit.

"You can think of the stars as cars traveling on a 10-lane highway," McKernan said. "If there were no gas, the cars would be going at very different speeds and mostly staying in their lanes, making the odds of collision low. When you add gas, it slows the cars to matching speeds but also moves them into other lanes, making the odds of collision and consumption much higher."

The resulting collisions allow a stellar black hole to swallow stars and grow. The black hole's size and gravitational pull increase as its mass expands, escalating its chance of further collisions. This phenomenon, called "runaway growth," can lead to the creation of an intermediate-mass black hole.

As they increase in size, the black holes start altering the gas disk that controls them. The researchers' model shows that black holes of a certain mass can create a gap in the gas disk, a signature that might give scientists the first glimpse of intermediate black holes.

The model describing this growth is a scaled-up version of the mechanism for the formation of gas giant planets like Jupiter and Saturn. Like intermediate black holes, these planets are thought to have grown in gas disks. The planets, though, developed in disks surrounding newly forming stars. Mordecai-Mark Mac Low, chair of the Department of Astrophysics at the Museum, has modeled that case.

"In some regions, we showed that rocky planets could be moved by the gas into common orbits, where they collide to form objects more than ten times the mass of the Earth, massive enough to attract gas and form gas giant planets," Mac Low said. "The creative work described here applies the same principles to the far more massive disks found at the centers of galaxies, to form black holes rather than giant planets."

Other authors on the paper include Museum research associate Wladimir Lyra from the Jet Propulsion Laboratory at the California Institute of Technology and Hagai Perets from the Harvard-Smithsonian Center for Astrophysics.

This work was supported in part by NASA and CUNY.

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Journal Reference:

B. McKernan, K. E. S. Ford, W. Lyra, H. B. Perets. Intermediate mass black holes in AGN discs - I. Production and growth. Monthly Notices of the Royal Astronomical Society, 2012; DOI: 10.1111/j.1365-2966.2012.21486.x

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Super-massive black hole inflates giant bubble

Oct. 29, 2012 — Like symbiotic species, a galaxy and its central black hole lead intimately connected lives. The details of this relationship still pose many puzzles for astronomers.

Some black holes actively accrete matter. Part of this material do not fall into the black hole but is ejected in a narrow stream of particles, traveling at nearly the speed of light. When the stream slows down, it creates a tenuous bubble that can engulf the entire galaxy. Invisible to optical telescopes, the bubble is very prominent at low radio frequencies. The new International LOFAR Telescope - designed and built by ASTRON in an international collaboration - is ideally suited to detect this low frequency emission.

Astronomers have produced one of the best images ever of such a bubble, using LOFAR to detect frequencies from 20 to 160 MHz. "The result is of great importance", says Francesco de Gasperin, lead author of the study that is being published in the journal Astronomy & Astrophysics. "It shows the enormous potential of LOFAR, and provides compelling evidence of the close ties between black hole, host galaxy, and their surroundings."

The image was made during the test-phase of LOFAR, and targeted the giant elliptical galaxy Messier 87, at the centre of a galaxy cluster in the constellation of Virgo. This galaxy is 2000 times more massive than our Milky Way and hosts in its centre one of the most massive black holes discovered so far, with a mass six billion times that of our Sun. Every few minutes this black hole swallows an amount of matter similar to that of the whole Earth, converting part of it into radiation and a larger part into powerful jets of ultra-fast particles, which are responsible for the observed radio emission.

“This is the first time such high-quality images are possible at these low frequencies", says professor Heino Falcke, chairman of the board of the ILT and co-author of the study. "This was a challenging observation - we did not expect to get such fantastic results so early in the commissioning phase of LOFAR."

To determine the age of the bubble, the authors added radio observations at different frequencies from the Very Large Array in New Mexico (USA), and the Effelsberg 100-meter radio telescope near Bonn (Germany). The team found that this bubble is surprisingly young, just about 40 million years, which is a mere instant on cosmic time scales. The low frequency observation does not reveal any relic emission outside the well-confined bubble boundaries, this means that the bubble is not just a relic of an activity that happened long ago but is constantly refilled with fresh particles ejected by the central black hole.

"What is particularly fascinating", says Andrea Merloni from the Max-Planck Institute of Extraterrestrial Physics in Garching, who supervised de Gasperin's doctoral work, "is that the results also provide clues on the violent matter-to-energy conversion that occurs very close to the black hole. In this case the black hole is particularly efficient in accelerating the jet, and much less effective in producing visible emission."

Francesco de Gasperin performed the study as part of his PhD work at the Max Planck Institute for Astrophysics and at the Excellence Cluster Universe. De Gasperin is now a postdoctoral researcher at the University of Hamburg.

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Journal Reference:

F. de Gasperin, E. Orrú, M. Murgia, A. Merloni, H. Falcke, R. Beck, R. Beswick, L. Bîrzan, A. Bonafede, M. Brüggen, G. Brunetti, K. Chyzy, J. Conway, J. H. Croston, T. Enßlin, C. Ferrari, G. Heald, S. Heidenreich, N. Jackson, G. Macario, J. McKean, G. Miley, R. Morganti, A. Offringa, R. Pizzo, D. Rafferty, H. Röttgering, A. Shulevski, M. Steinmetz, C. Tasse, S. van der Tol, W. van Driel, R. J. van Weeren, J. E. van Zwieten, A. Alexov, J. Anderson, A. Asgekar, M. Avruch, M. Bell, M. R. Bell, M. Bentum, G. Bernardi, P. Best, F. Breitling, J. W. Broderick, A. Butcher, B. Ciardi, R. J. Dettmar, J. Eisloeffel, W. Frieswijk, H. Gankema, M. Garrett, M. Gerbers, J. M. Griessmeier, A. W. Gunst, T. E. Hassall, J. Hessels, M. Hoeft, A. Horneffer, A. Karastergiou, J. Köhler, Y. Koopman, M. Kuniyoshi, G. Kuper, P. Maat, G. Mann, M. Mevius, D. D. Mulcahy, H. Munk, R. Nijboer, J. Noordam, H. Paas, M. Pandey, V. N. Pandey, A. Polatidis, W. Reich, A. P. Schoenmakers, J. Sluman, O. Smirnov, C. Sobey, B. Stappers, J. Swinbank, M. Tagger, Y. Tang, I. van Bemmel, W. van Cappellen, A. P. van Duin, M. van Haarlem, J. van Leeuwen, R. Vermeulen, C. Vocks, S. White, M. Wise, O. Wucknitz, P. Zarka. M 87 at metre wavelengths: the LOFAR picture. Astronomy & Astrophysics, 2012; 547: A56 DOI: 10.1051/0004-6361/201220209

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Bonanza of black holes, hot DOGs: NASA's WISE survey uncovers millions of black holes

Aug. 29, 2012 — NASA's Wide-field Infrared Survey Explorer (WISE) mission has led to a bonanza of newfound supermassive black holes and extreme galaxies called hot DOGs, or dust-obscured galaxies.

Images from the telescope have revealed millions of dusty black hole candidates across the universe and about 1,000 even dustier objects thought to be among the brightest galaxies ever found. These powerful galaxies, which burn brightly with infrared light, are nicknamed hot DOGs.

"WISE has exposed a menagerie of hidden objects," said Hashima Hasan, WISE program scientist at NASA Headquarters in Washington. "We've found an asteroid dancing ahead of Earth in its orbit, the coldest star-like orbs known and now, supermassive black holes and galaxies hiding behind cloaks of dust."

WISE scanned the whole sky twice in infrared light, completing its survey in early 2011. Like night-vision goggles probing the dark, the telescope captured millions of images of the sky. All the data from the mission have been released publicly, allowing astronomers to dig in and make new discoveries.

The latest findings are helping astronomers better understand how galaxies and the behemoth black holes at their centers grow and evolve together. For example, the giant black hole at the center of our Milky Way galaxy, called Sagittarius A*, has 4 million times the mass of our sun and has gone through periodic feeding frenzies where material falls towards the black hole, heats up and irradiates its surroundings. Bigger central black holes, up to a billion times the mass of our sun, may even shut down star formation in galaxies.

In one study, astronomers used WISE to identify about 2.5 million actively feeding supermassive black holes across the full sky, stretching back to distances more than 10 billion light-years away. About two-thirds of these objects never had been detected before because dust blocks their visible light. WISE easily sees these monsters because their powerful, accreting black holes warm the dust, causing it to glow in infrared light.

"We've got the black holes cornered," said Daniel Stern of NASA's Jet Propulsion Laboratory, Pasadena, Calif., lead author of the WISE black hole study and project scientist for another NASA black-hole mission, the Nuclear Spectroscopic Telescope Array (NuSTAR). "WISE is finding them across the full sky, while NuSTAR is giving us an entirely new look at their high-energy X-ray light and learning what makes them tick."

In two other WISE papers, researchers report finding what are among the brightest galaxies known, one of the main goals of the mission. So far, they have identified about 1,000 candidates.

These extreme objects can pour out more than 100 trillion times as much light as our sun. They are so dusty, however, that they appear only in the longest wavelengths of infrared light captured by WISE. NASA's Spitzer Space Telescope followed up on the discoveries in more detail and helped show that, in addition to hosting supermassive black holes feverishly snacking on gas and dust, these DOGs are busy churning out new stars.

"These dusty, cataclysmically forming galaxies are so rare WISE had to scan the entire sky to find them," said Peter Eisenhardt, lead author of the paper on the first of these bright, dusty galaxies, and project scientist for WISE at JPL. "We are also seeing evidence that these record setters may have formed their black holes before the bulk of their stars. The 'eggs' may have come before the 'chickens.'"

More than 100 of these objects, located about 10 billion light-years away, have been confirmed using the W.M. Keck Observatory on Mauna Kea, Hawaii, as well as the Gemini Observatory in Chile, Palomar's 200-inch Hale telescope near San Diego, and the Multiple Mirror Telescope Observatory near Tucson, Ariz.

The WISE observations, combined with data at even longer infrared wavelengths from Caltech's Submillimeter Observatory atop Mauna Kea, revealed that these extreme galaxies are more than twice as hot as other infrared-bright galaxies. One theory is their dust is being heated by an extremely powerful burst of activity from the supermassive black hole.

"We may be seeing a new, rare phase in the evolution of galaxies," said Jingwen Wu of JPL, lead author of the study on the submillimeter observations. All three papers are being published in the Astrophysical Journal.

The three technical journal articles, including PDFs, can be found at http://arxiv.org/abs/1205.0811, http://arxiv.org/abs/1208.5517 and http://arxiv.org/abs/1208.5518 .

JPL manages and operates WISE for NASA's Science Mission Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The mission was competitively selected under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing and archiving take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

More information is online at http://www.nasa.gov/wise, http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise .

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X-ray vision can reveal moment of birth of violent supernovae

Dec. 7, 2012 — A team of astronomers led by the University of Leicester has uncovered new evidence that suggests that X-ray detectors in space could be the first to witness new supernovae that signal the death of massive stars.

Astronomers have measured an excess of X-ray radiation in the first few minutes of collapsing massive stars, which may be the signature of the supernova shock wave first escaping from the star.

The findings have come as a surprise to Dr Rhaana Starling, of the University of Leicester Department of Physics and Astronomy whose research is published in the Monthly Notices of the Royal Astronomical Society.

Dr Starling said: "The most massive stars can be tens to a hundred times larger than the Sun. When one of these giants runs out of hydrogen gas it collapses catastrophically and explodes as a supernova, blowing off its outer layers which enrich the Universe. But this is no ordinary supernova; in the explosion narrowly confined streams of material are forced out of the poles of the star at almost the speed of light. These so-called relativistic jets give rise to brief flashes of energetic gamma-radiation called gamma-ray bursts, which are picked up by monitoring instruments in Space, that in turn alert astronomers."

Gamma-ray bursts are known to arise in stellar deaths because coincident supernovae are seen with ground-based optical telescopes about ten to twenty days after the high energy flash. The true moment of birth of a supernova, when the star's surface reacts to the core collapse, often termed the supernova shock breakout, is missed. Only the most energetic supernovae go hand-in-hand with gamma-ray bursts, but for this sub-class it may be possible to identify X-ray emission signatures of the supernova in its infancy. If the supernova could be detected earlier, by using the X-ray early warning system, astronomers could monitor the event as it happens and pinpoint the drivers behind one of the most violent events in our Universe.

The X-ray detectors being used for this research, built partly in the UK at the University of Leicester, are on the X-Ray Telescope on-board the Swift satellite. Swift is named after the bird because, like its namesake, it is able to swiftly turn around to catch a gamma-ray burst in action. Data from Swift of a number of gamma-ray bursts with visible supernovae have shown an excess in X-rays received compared with expectations. This excess is thermal emission, also known as blackbody radiation.

Dr Starling added: "We were surprised to find thermal X-rays coming from a gamma-ray burst, and even more surprising is that all confirmed cases so far are those with a secure supernova identification from optical data. This phenomenon is only seen during the first thousand seconds of an event, and it is challenging to distinguish it from X-ray emission solely from the gamma-ray burst jet. That is why astronomers have not routinely observed this before, and only a small subset of the 700+ bursts we detect with Swift show it."

"It all hangs on the positive identification of the extra X-ray radiation as directly emerging from the supernova shock front, rather than from the relativistic jets or central black hole. If this radiation turns out to be from the central black-hole-powered engine of the gamma-ray burst instead, it will still be a very illuminating result for gamma-ray burst physics, but the strong association with supernovae is tantalising."

The team, comprising scientists from the UK, Ireland, USA and Denmark, plan to extend their searches, and make more quantitative comparisons with theoretical models both for stellar collapse and the dynamics of fast jet-flows.

Astronomers will continue to view supernovae at their visible-light peak, when they are already tens of days old, but for the most energetic among them it may become possible to routinely witness the very moment they are born, through X-ray eyes.

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Journal References:

R. L. C. Starling, K. L. Page, A. Pe'er, A. P. Beardmore and J. P. Osborne. A search for thermal X-ray signatures in gamma-ray bursts – I. Swift bursts with optical supernovae. Monthly Notices of the Royal Astronomical Society, 28 NOV 2012 DOI: 10.1111/j.1365-2966.2012.22116.xMartin Sparre and Rhaana L. C. Starling. A search for thermal X-ray signatures in gamma-ray bursts – II. The Swift sample. Monthly Notices of the Royal Astronomical Society, 28 NOV 2012 DOI: 10.1111/j.1365-2966.2012.21858.x

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Black hole caught red-handed in a stellar homicide

May 2, 2012 — Astronomers have gathered the most direct evidence yet of a supermassive black hole shredding a star that wandered too close. NASA's Galaxy Evolution Explorer, a space-based observatory, and the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii were among the first to help identify the stellar remains.

Supermassive black holes, weighing millions to billions times more than the Sun, lurk in the centers of most galaxies. These hefty monsters lay quietly until an unsuspecting victim, such as a star, wanders close enough to get ripped apart by their powerful gravitational clutches.

Astronomers have spotted these stellar homicides before, but this is the first time they can identify the victim. Using a slew of ground- and space-based telescopes, a team of astronomers led by Suvi Gezari of The Johns Hopkins University in Baltimore, Md., has identified the victim as a star rich in helium gas. The star resides in a galaxy 2.7 billion light-years away.

Her team's results will appear in May 2 online edition of the journal Nature.

"When the star is ripped apart by the gravitational forces of the black hole, some part of the star's remains falls into the black hole, while the rest is ejected at high speeds. We are seeing the glow from the stellar gas falling into the black hole over time. We're also witnessing the spectral signature of the ejected gas, which we find to be mostly helium. It is like we are gathering evidence from a crime scene. Because there is very little hydrogen and mostly helium in the gas we detect, we know from the carnage that the slaughtered star had to have been the helium-rich core of a stripped star," Gezari explained.

This observation yields insights about the harsh environment around black holes and the types of stars swirling around them.

This is not the first time the unlucky star had a brush with the behemoth black hole. Gezari and her team think the star's hydrogen-filled envelope surrounding its core was lifted off a long time ago by the same black hole. In their scenario, the star may have been near the end of its life. After consuming most of its hydrogen fuel, it had probably ballooned in size, becoming a red giant. The astronomers think the bloated star was looping around the black hole in a highly elliptical orbit, similar to a comet's elongated orbit around the Sun. On one of its close approaches, the star was stripped of its puffed-up atmosphere by the black hole's powerful gravity. Only its core remained intact. The stellar remnant continued its journey around the black hole, until it ventured even closer to the behemoth monster and faced its ultimate demise.

Astronomers have predicted that stripped stars circle the central black hole of our Milky Way galaxy, Gezari pointed out. These close encounters, however, are rare, occurring roughly every 100,000 years. To find this one event, Gezari's team monitored hundreds of thousands of galaxies in ultraviolet light with NASA's Galaxy Evolution Explorer (GALEX), a space-based observatory, and in visible light with the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii. Pan-STARRS, short for Panoramic Survey Telescope and Rapid Response System, scans the entire night sky for all kinds of transient phenomena, including supernovae.

The team was looking for a bright flare in ultraviolet light from the nucleus of a galaxy with a previously dormant black hole. They found one in June 2010, which was spotted with both telescopes. Both telescopes continued to monitor the flare as it reached peak brightness a month later and then slowly began to fade over the next 12 months. The brightening event was similar to that of a supernova, but the rise to the peak was much slower, taking nearly one and a half months.

"The longer the event lasted, the more excited we got, since we realized that this is either a very unusual supernova or an entirely different type of event, such as a star being ripped apart by a black hole," said team member Armin Rest of the Space Telescope Science Institute in Baltimore, Md.

By measuring the increase in brightness, the astronomers calculated the black hole's mass to be several million suns, which is comparable to the size of our Milky Way's black hole.

Spectroscopic observations with the MMT (Multiple Mirror Telescope) Observatory on Mount Hopkins in Arizona showed that the black hole was swallowing lots of helium. Spectroscopy divides light into its rainbow colors, which yields an object's characteristics, such as its temperature and gaseous makeup.

"The glowing helium was a tracer for an extraordinarily hot accretion event," Gezari said. "So that set off an alarm for us. And, the fact that no hydrogen was found set off a big alarm that this was not typical gas. You can't find gas like that lying around near the center of a galaxy. It's processed gas that has to have come from a stellar core. There's nothing about this event that could be easily explained by any other phenomenon."

The observed speed of the gas also linked the material to a black hole's gravitational pull. MMT measurements revealed that the gas was moving at more than 20 million miles an hour (over 32 million kilometers an hour). However, measurements of the speed of gas in the interstellar medium reveal velocities of only about 224,000 miles an hour (360,000 kilometers an hour).

"The place we also see these kinds of velocities are in supernova explosions," Rest said. "But the fact that it is still shining in ultraviolet light is incompatible with any supernova we know."

To completely rule out the possibility of an active nucleus flaring up in the galaxy, the team used NASA's Chandra X-ray Observatory to study the hot gas. Chandra showed that the characteristics of the gas didn't match those from an active galactic nucleus.

"This is the first time where we have so many pieces of evidence, and now we can put them all together to weigh the perpetrator (the black hole) and determine the identity of the unlucky star that fell victim to it," Gezari said. "These observations also give us clues to what evidence to look for in the future to find this type of event."

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The above story is reprinted from materials provided by NASA/ Hubble.

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Journal Reference:

S. Gezari, R. Chornock, A. Rest, M. E. Huber, K. Forster, E. Berger, P. J. Challis, J. D. Neill, D. C. Martin, T. Heckman, A. Lawrence, C. Norman, G. Narayan, R. J. Foley, G. H. Marion, D. Scolnic, L. Chomiuk, A. Soderberg, K. Smith, R. P. Kirshner, A. G. Riess, S. J. Smartt, C. W. Stubbs, J. L. Tonry, W. M. Wood-Vasey, W. S. Burgett, K. C. Chambers, T. Grav, J. N. Heasley, N. Kaiser, R.-P. Kudritzki, E. A. Magnier, J. S. Morgan, P. A. Price. An ultraviolet–optical flare from the tidal disruption of a helium-rich stellar core. Nature, 2012; DOI: 10.1038/nature10990

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