Posted on by Nancy Atkinson

Ejected Black Holes Drag Clusters of Stars With Them

This artist’s conception shows a rogue black hole that has been kicked out from the center of two merging galaxies. The black hole is surrounded by a cluster of stars that were ripped from the galaxies. Credit: Space Telescope Science Institute

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The tight cluster of stars surrounding a supermassive black hole after it has been violently kicked out of a galaxy represents a new kind of astronomical object which may provide telltale clues to how the ejection event occurred. “Hypercompact stellar systems” result when a supermassive black hole is violently ejected from a galaxy, following a merger with another supermassive black hole. The evicted black hole rips stars from the galaxy as it is thrown out. The stars closest to the black hole move in tandem with the massive object and become a permanent record of the velocity at which the kick occurred.

“You can measure how big the kick was by measuring how fast the stars are moving around the black hole,” said David Merritt, professor of physics at the Rochester Institute of Technolyg. “Only stars orbiting faster than the kick velocity remain attached to the black hole after the kick. These stars carry with them a kind of fossil record of the kick, even after the black hole has slowed down. In principle, you can reconstruct the properties of the kick, which is nice because there would be no other way to do it.”

In a paper published in the July 10 issue of The Astrophysical Journal, Merritt and his colleagues discusses the theoretical properties of these objects and suggests that hundreds of these faint star clusters might be detected at optical wavelengths in our immediate cosmic environment. Some of these objects may already have been picked up in astronomical surveys. .

“Finding these objects would be like discovering DNA from a long-extinct species,” said team member Stefanie Komossa, from the Max-Planck-Institut for Extraterrestrial Physics in Germany.

The astronomers say the best place to find hypercompact stellar systems is in cluster of galaxies like the nearby Coma and Virgo clusters. These dense regions of space contain thousands of galaxies that have been merging for a long time. Merging galaxies result in merging black holes, which is a prerequisite for the kicks.

“Even if the black hole gets kicked out of one galaxy, it’s still going to be gravitationally bound to the whole cluster of galaxies,” Merritt says. “The total gravity of all the galaxies is acting on that black hole. If it was ever produced, it’s still going to be there somewhere in that cluster.”

Merritt and his co-authors think that scientists may have already seen hypercompact stellar systems and not realized it. These objects would be easy to mistake for common star systems like globular clusters. The key signature making hypercompact stellar systems unique is a high internal velocity. This is detectable only by measuring the velocities of stars moving around the black hole, a difficult measurement that would require a long time exposure on a large telescope.

From time to time, a hypercompact stellar system will make its presence known in a much more dramatic way, when one of the stars is tidally disrupted by the supermassive black hole. In this case, gravity stretches the star and sucks it into the black hole. The star is torn apart, causing a beacon-like flare that signals a black hole. The possibility of detecting one of these “recoil flares” was first discussed in an August 2008 paper by co-authors Merritt and Komossa.

“The only contact of these floating black holes with the rest of the universe is through their armada of stars,” Merritt says, “with an occasional display of stellar fireworks to signal ‘here we are.’”

Source: Rochester Institute of Technology

Posted on by Anne Minard

Messier 87 Shows Off for Hundreds of Earth-bound Astronomers

Artists's Conception of M87's inner core: Black hole, accretion disk, and inner jets. Credit: Bill Saxton, NRAO/AUI/NSF

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When the giant radio galaxy Messier 87 (M 87) unleashed a torrent of gamma radiation and radio flux, an international collaboration of 390 scientists happened to be watching. They’re reporting the discovery in this week’s issue of Science Express.

Large-scale VLA image of M87: White circle indicates the area within which the gamma-ray telescopes could tell the very energetic gamma rays were being emitted. To narrow down the location further required the VLBA. CREDIT: NRAO/AUI/NSF
Large-scale VLA image of M87: White circle indicates the area within which the gamma-ray telescopes could tell the very energetic gamma rays were being emitted. To narrow down the location further required the VLBA. CREDIT: NRAO/AUI/NSF

The results give first experimental evidence that particles are accelerated to extremely high energies in the immediate vicinity of a supermassive black hole and then emit the observed gamma rays. The gamma rays have energies a trillion times higher than the energy of visible light.

Matthias Beilicke and Henric Krawczynski, both physicists at Washington University in St. Louis, coordinated the project using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration. The effort involved three arrays of 12-meter (39-foot) to 17-meter (56-foot) telescopes, which detect very high-energy gamma rays, and the Very Long Baseline Array (VLBA) that detects radio waves with high spatial precision.

“We had scheduled gamma-ray observations of M 87 in a close cooperative effort with the three major gamma-ray observatories VERITAS, H.E.S.S. and MAGIC, and we were lucky that an extraordinary gamma-ray flare happened just when the source was observed with the VLBA and its impressive spatial resolving power,” Beilicke said.

“Only combining the high-resolution radio observations with the VHE gamma-ray observations allowed us to locate the site of the gamma-ray production,” added R. Craig Walker, a staff scientist at the National Radio Astronomy Observatory in Socorro, New Mexico.

Peering Deeper Into the Core of M87: At top left, a VLA image of the galaxy shows the radio-emitting jets at a scale of about 200,000 light-years. Subsequent zooms progress closer into the galaxy's core, where the supermassive black hole resides. In the artist's conception (background). the black hole illustrated at the center is about twice the size of our Solar System, a tiny fraction of the size of the galaxy, but holding some six billion times the mass of the Sun. Credit: Bill Saxton, NRAO/AUI/NSF
Peering Deeper Into the Core of M87: At top left, a VLA image of the galaxy shows the radio-emitting jets at a scale of about 200,000 light-years. Subsequent zooms progress closer into the galaxy's core, where the supermassive black hole resides. In the artist's conception (background). the black hole illustrated at the center is about twice the size of our Solar System, a tiny fraction of the size of the galaxy, but holding some six billion times the mass of the Sun. Credit: Bill Saxton, NRAO/AUI/NSF

M 87 is located at a distance of 50 million light years from Earth in the Virgo cluster of galaxies. The black hole in the center of M 87 is six billion times more massive than the Sun.

The size of a non-rotating black hole is given by the Schwarzschild radius. Everything — matter or radiation — that comes within one Schwarzschild radius of the center of the black hole will be swallowed by it. The Schwarzschild radius of the supermassive black hole in M 87 is comparable to the radius of our Solar System.

In the case of some supermassive black holes — as in M 87 — matter orbiting and approaching the black hole powers highly relativistic outflows, called jets. The matter in the jets travels away from the black hole, escaping its deadly gravitational force. The jets are some of the largest objects in the Universe, and they can reach out many thousands of light years from the vicinity of the black hole into the intergalactic medium.

Very high-energy gamma-ray emission from M 87 was first discovered in 1998 with the HEGRA Cherenkov telescopes. “But even today, M 87 is one of only about 25 sources outside our galaxy known to emit [very high energy] gamma rays,” says Beilicke.

The new observations now show that the particle acceleration, and the subsequent emission of gamma rays, can happen in the very “inner jet,” less than about 100 Schwarzschild radii away from the black hole, which is an extremely narrow space as compared with the total extent of the jet or the galaxy.

In addition to VERITAS and the VLBA, the High Energy Stereoscopic System (H.E.S.S.) and the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) gamma-ray observatories were involved in these observations.

Lead image caption: Artists’s Conception of M87’s inner core: Black hole, accretion disk, and inner jets. Credit: Bill Saxton, NRAO/AUI/NSF

Second image: Large-scale VLA image of M87: White circle indicates the area within which the gamma-ray telescopes could tell the very energetic gamma rays were being emitted. To narrow down the location further required the VLBA. CREDIT: NRAO/AUI/NSF

Collage: At top left, a VLA image of the galaxy shows the radio-emitting jets at a scale of about 200,000 light-years. Subsequent zooms progress closer into the galaxy’s core, where the supermassive black hole resides. In the artist’s conception (background). the black hole illustrated at the center is about twice the size of our Solar System, a tiny fraction of the size of the galaxy, but holding some six billion times the mass of the Sun. Credit: Bill Saxton, NRAO/AUI/NSF

Sources: Science and the National Radio Astronomy Observatory, via Eurekalert.

Posted on by Anne Minard

Astronomers Discover Medium-Sized Class of Black Holes

HLX-1 in the periphery of the edge-on spiral galaxy ESO 243-49. Credit: Heidi Sagerud.

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It’s the Goldilocks variety of black holes: not too big and not too small.

The new source HLX-1,  the light blue object to the top left of the galactic bulge, is the ambassador for a new class of black holes, more than 500 times the mass of the Sun. It lies on the periphery of the edge-on spiral galaxy ESO 243-49, about 290 million light years from Earth.

The discovery, led by Sean Farrell at Britain’s University of Leicester, appears today in the journal Nature.

Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the center of galaxies, or about the size of a typical star (between three and 20 solar masses).

The new discovery is the first solid evidence of a new class of medium-sized black holes and was made using the European Space Agency’s XMM-Newton X-ray space telescope. At the time of the discovery, Farrell and his team were working at the Centre d’Etude Spatiale des Rayonnements in France.

A black hole is a remnant of a collapsed star with such a powerful gravitational field that it absorbs all the light that passes near it and reflects nothing.

“While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed,” Farrell said.

It had been long believed by astrophysicists that there might be a third, intermediate class of black holes, with masses between a hundred and several hundred thousand times that of the Sun. However, such black holes had not been reliably detected until now.

One theory suggests that super-massive black holes may be formed by the merger of a number of intermediate mass black holes, Farrell said.

“To ratify such a theory, however, you must first prove the existence of intermediate black holes. This is the best detection to date of such long sought after intermediate mass black holes. ”

Using XMM-Newton observations carried out in 2004 and 2008, the team showed that HLX-1 displayed a variation in its X-ray signature. This indicated that it must be a single object and not a group of many fainter sources. The huge radiance observed can only be explained if HLX-1 contains a black hole more than 500 times the mass of the Sun. The authors say that no other physical explanation can account for the data.

Lead image caption: Artist’s impression of HLX-1 in the periphery of the edge-on spiral galaxy ESO 243-49. Credit: Heidi Sagerud.

Sources: Nature and the University of Leicester

Posted on by Anne Minard

Mysterious “Blobs” Are Windows Into Galaxy Formation

Credit: Left panel: X-ray (NASA/CXC/Durham Univ./D.Alexander et al.); Optical (NASA/ESA/STScI/IoA/S.Chapman et al.); Lyman-alpha Optical (NAOJ/Subaru/Tohoku Univ./T.Hayashino et al.); Infrared (NASA/JPL-Caltech/Durham Univ./J.Geach et al.); Right, Illustration: NASA/CXC/M.Weiss

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Astronomers say they’ve discovered the “coming of age” of galaxies and black holes, thanks to new data from NASA’s Chandra X-ray Observatory and other telescopes. The new discovery helps resolve the true nature of gigantic blobs of gas observed around very young galaxies, and sheds light on the formation of galaxies and black holes.

The findings, led by Jim Geach of Durham University in the UK, will appear in the July 10 issue of The Astrophysical Journal.

About a decade ago, astronomers discovered immense reservoirs of hydrogen gas — which they named “blobs” – while conducting surveys of young distant galaxies.  The blobs are glowing brightly in optical light, but the source of immense energy required to power this glow and the nature of these objects were unclear.

Based on the new data and theoretical arguments, Geach and his colleagues show that heating of gas by growing supermassive black holes and bursts of star formation, rather than cooling of gas, most likely powers the blobs. The implication is that blobs represent a stage when the galaxies and black holes are just starting to switch off their rapid growth because of these heating processes.  This is a crucial stage of the evolution of galaxies and black holes – known as “feedback” – and one that astronomers have long been trying to understand.

“We’re seeing signs that the galaxies and black holes inside these blobs are coming of age and are now pushing back on the infalling gas to prevent further growth,” said coauthor Bret Lehmer, also of Durham.  “Massive galaxies must go through a stage like this or they would form too many stars and so end up ridiculously large by the present day.”

Chandra and a collection of other telescopes including Spitzer have observed 29 blobs in one large field in the sky dubbed “SSA22.” These blobs, which are several hundred thousand light years across, are seen when the Universe is only about two billion years old, or roughly 15 percent of its current age.

In five of these blobs, the Chandra data revealed the telltale signature of growing supermassive black holes – a point-like source with luminous X-ray emission. These giant black holes are thought to reside at the centers of most galaxies today, including our own.  Another three of the blobs in this field show possible evidence for such black holes.  Based on further observations, including Spitzer data, the research team was able to determine that several of these galaxies are also dominated by remarkable levels of star formation.

The radiation and powerful outflows from these black holes and bursts of star formation are, according to calculations, powerful enough to light up the hydrogen gas in the blobs they inhabit. In the cases where the signatures of these black holes were not detected, the blobs are generally fainter. The authors show that black holes bright enough to power these blobs would be too dim to be detected given the length of the Chandra observations.

Besides explaining the power source of the blobs, these results help explain their future. Under the heating scenario, the gas in the blobs will not cool down to form stars but will add to the hot gas found between galaxies. SSA22 itself could evolve into a massive galaxy cluster.

“In the beginning the blobs would have fed their galaxies, but what we see now are more like leftovers,” said Geach.  “This means we’ll have to look even further back in time to catch galaxies and black holes in the act of forming from blobs.”

Sources/more information: the Chandra sites at Harvard and NASA.

Posted on by Nancy Atkinson

Super-Size Me: Black Hole Bigger Than Previously Thought

The illustration shows the relationship between the mass of a galaxy’s central black hole and the mass of its central bulge. Credit: Tim Jones/UT-Austin after K. Cordes & S. Brown (STScI)

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Using a new computer model, astronomers have determined that the black hole in the center of the M87 galaxy is at least twice as big as previously thought. Weighing in at 6.4 billion times the Sun’s mass, it is the most massive black hole yet measured, and this new model suggest that the accepted black hole masses in other large nearby galaxies may be off by similar amounts. This has consequences for theories of how galaxies form and grow, and might even solve a long-standing astronomical paradox.

Astronomers Karl Gebhardt from the University of Texas at Austin and Jens Thomas from the Max Planck Institute for Extraterrestrial Physics detailed their findings Monday at the American Astronomical Society conference in Pasadena, California.

To try to understand how galaxies form and grow, astronomers start with basic information about the galaxies today, such as what they are made of, how big they are and how much they weigh. Astronomers measure this last category, galaxy mass, by clocking the speed of stars orbiting within the galaxy.

Studies of the total mass are important, Thomas said, but “the crucial point is to determine whether the mass is in the black hole, the stars, or the dark halo. You have to run a sophisticated model to be able to discover which is which. The more components you have, the more complicated the model is.”

To model M87, Gebhardt and Thomas used one of the world’s most powerful supercomputers, the Lonestar system at The University of Texas at Austin’s Texas Advanced Computing Center. Lonestar is a Dell Linux cluster with 5,840 processing cores and can perform 62 trillion floating-point operations per second. (Today’s top-of-the-line laptop computer has two cores and can perform up to 10 billion floating-point operations per second.)

Gebhardt and Jens’ model of M87 was more complicated than previous models of the galaxy, because in addition to modeling its stars and black hole, it takes into account the galaxy’s “dark halo,” a spherical region surrounding a galaxy that extends beyond its main visible structure, containing the galaxy’s mysterious “dark matter.”

“In the past, we have always considered the dark halo to be significant, but we did not have the computing resources to explore it as well,” Gebhardt said. “We were only able to use stars and black holes before. Toss in the dark halo, it becomes too computationally expensive, you have to go to supercomputers.”

The Lonestar result was a mass for M87’s black hole several times what previous models have found. “We did not expect it at all,” Gebhardt said. He and Jens simply wanted to test their model on “the most important galaxy out there,” he said.

Extremely massive and conveniently nearby (in astronomical terms), M87 was one of the first galaxies suggested to harbor a central black hole nearly three decades ago. It also has an active jet shooting light out the galaxy’s core as matter swirls closer to the black hole, allowing astronomers to study the process by which black holes attract matter. All of these factors make M87 the “the anchor for supermassive black hole studies,” Gebhardt said.

These new results for M87, together with hints from other recent studies and his own recent telescope observations (publications in preparation), lead him to suspect that all black hole masses for the most massive galaxies are underestimated.

That conclusion “is important for how black holes relate to galaxies,” Thomas said. “If you change the mass of the black hole, you change how the black hole relates to the galaxy.” There is a tight relation between the galaxy and its black hole which had allowed researchers to probe the physics of how galaxies grow over cosmic time. Increasing the black hole masses in the most massive galaxies will cause this relation to be re-evaluated.

Higher masses for black holes in nearby galaxies also could solve a paradox concerning the masses of quasars — active black holes at the centers of extremely distant galaxies, seen at a much earlier cosmic epoch. Quasars shine brightly as the material spiraling in, giving off copious radiation before crossing the event horizon (the region beyond which nothing — not even light — can escape).

“There is a long-standing problem in that quasar black hole masses were very large — 10 billion solar masses,” Gebhardt said. “But in local galaxies, we never saw black holes that massive, not nearly. The suspicion was before that the quasar masses were wrong,” he said. But “if we increase the mass of M87 two or three times, the problem almost goes away.”

Today’s conclusions are model-based, but Gebhardt also has made new telescope observations of M87 and other galaxies using new powerful instruments on the Gemini North Telescope and the European Southern Observatory’s Very Large Telescope. He said these data, which will be submitted for publication soon, support the current model-based conclusions about black hole mass.

For future telescope observations of galactic dark haloes, Gebhardt notes that a relatively new instrument at The University of Texas at Austin’s McDonald Observatory is perfect. “If you need to study the halo to get the black hole mass, there’s no better instrument than VIRUS-P,” he said. The instrument is a spectrograph. It separates the light from astronomical objects into its component wavelengths, creating a signature that can be read to find out an object’s distance, speed, motion, temperature, and more.

VIRUS-P is good for halo studies because it can take spectra over a very large area of sky, allowing astronomers to reach the very low light levels at large distances from the galaxy center where the dark halo is dominant. It is a prototype, built to test technology going into the larger VIRUS spectrograph for the forthcoming Hobby-Eberly Telescope Dark Energy Experiment (HETDEX).

Read the team’s paper.

Sources: AAS, McDonald Observatory

Posted on by Nancy Atkinson

No Nature VS. Nurture for Stars

The Arches Cluster, with young, massive stars, taken by the NACO on ESO’s Very Large Telescope.

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Stars don’t seem to mind where they grow up. Either in a nice quiet neighborhood or in the hellish environment near a supermassive black hole, astronomers were surprised to find the same proportions of low- and high-mass young stars in different types of star forming regions. Using the Very Large Telescope, astronomers snapped one of the sharpest views ever of the Arches Cluster — an extraordinary dense cluster of young stars near the supermassive black hole at the center of the Milky Way. “With the extreme conditions in the Arches Cluster, one might indeed imagine that stars won’t form in the same way as in our quiet solar neighbourhood,” says Pablo Espinoza, the lead author of the paper reporting the new results. “However, our new observations showed that the masses of stars in this cluster actually do follow the same universal law”.

The massive Arches Cluster is located 25 000 light-years away towards the constellation of Sagittarius. It contains about a thousand young, massive stars, less than 2.5 million years old. Astronomers say this region is an ideal laboratory to study how massive stars are born in extreme conditions, as the stars in the cluster experience huge opposing forces from all the activity going on near the supermassive black hole. The Arches Cluster is also ten times heavier than typical young star clusters scattered throughout our Milky Way and is enriched with chemical elements heavier than helium.

The Arches Cluster is located in the centre of the image, but its stars are hidden behind large amount of dust. The bright star at the top of the image is 3 Sagittarii, while the cluster of stars seen at the bottom left is NGC 6451. Credit: Digitized Sky Survey
The Arches Cluster is located in the centre of the image, but its stars are hidden behind large amount of dust. The bright star at the top of the image is 3 Sagittarii, while the cluster of stars seen at the bottom left is NGC 6451. Credit: Digitized Sky Survey

Using the NACO adaptive optics on the VLT, astronomers were able to take the clearest images yet of the Arches Cluster. Observing the Arches Cluster is very challenging because of the huge quantities of light-absorbing dust between Earth and the Galactic Centre, which visible light cannot penetrate. This is why NACO was used to observe the region in near-infrared light.

The new study confirms the Arches Cluster to be the densest cluster of massive young stars known. It is about three light-years across with more than a thousand stars packed into each cubic light-year — an extreme density a million times greater than in the Sun’s neighborhood.
Astronomers studying clusters of stars have found that higher mass stars are rarer than their less massive brethren, and their relative numbers are the same everywhere, following a universal law.

The astronomers were also able to study the brightest stars in the cluster. “The most massive star we found has a mass of about 120 times that of the Sun,” says co-author Fernando Selman. “We conclude from this that if stars more massive than 130 solar masses exist, they must live for less than 2.5 million years and end their lives without exploding as supernovae, as massive stars usually do.”

The total mass of the cluster seems to be about 30,000 times that of the Sun, much more than was previously thought. “That we can see so much more is due to the exquisite NACO images,” says co-author Jorge Melnick.

Read the team’s paper.

Source: ESO

Posted on by Nancy Atkinson

Could Ghost-Like Object Found by Chandra Be Another ‘Voorwerp’?

The ghost of HDF 130. Credit: X-ray (NASA/CXC/IoA/A.Fabian et al.); Optical (SDSS), Radio (STFC/JBO/MERLIN)

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The Chandra X-ray Observatory has found a cosmic “ghost” lurking around a distant supermassive black hole. Astronomers think this high-energy apparition is evidence of a huge eruption produced by the black hole. But this blue blob looks eerily similar to another cosmic blob of gas found by Galaxy Zoo member Hanny Van Arkel, the famous object called Hanny’s Voorwerp. Could the two objects be similar?

Astronomers say the “ghost” found by Chandra is the remains of a diffuse X-ray source, lingering after other radiation from the black hole’s outburst died away. The object, HDF 130 is over 10 billion light years away and existed at a time 3 billion years after the Big Bang, when galaxies and black holes were forming at a high rate.

Hanny's Voorwerp. Credit: Galaxy Zoo
Hanny's Voorwerp. Credit: Galaxy Zoo

Hanny’s Voorwerp has been a mystery ever since it was found in 2007 as part of the Galaxy Zoo project. Recent research on the object reveals that the Voorwerp is also likely to be a remnant from a black hole outburst. In the original Sloan Digital Sky Survey images of Hanny’s Voorwerp, the object showed up as blue, however further spectral analysis showed it is actually green. The Voorwerp was studied by the Swift gamma-ray satellite, which also can pick up ultraviolet and X-ray emissions, but the satellite didn’t come up with anything conclusive. However, the Westerbork Synthesis Radio Telescope (WSRT) took a look at Hanny’s Voorwerp and determined that indeed, black hole jets were allowing beams of intense optical and ultraviolet emissions from the black hole to heat and illuminate a small part of a large gas cloud that partially surrounds the nearby galaxy, IC 2497.

But Galaxy Zoo astronomers suspect X-rays might play a role in the Voorwerp, too. It was recently imaged by the Suzaku X-ray telescopes to see if is visible in that part of the spectrum, as well as to probe the current activity of the supermassive black hole. The results of that observation are still being analyzed. Yale astronomer Kevin Schawinski recently wrote in the Galaxy Zoo Blog that detecting hard X-ray photons would provide evidence of an active supermassive black hole in IC 2497, which would be illuminating the Voorwerp. “If on the other hand we don’t pick up anything, then we can be sure that the black hole has stopped feeding, i.e. it has genuinely shut down,” Schawinski wrote.

So are the two objects, the “ghost” of HDF 130 and Hanny’s Voorwerp similar? Yes – and no – said Chandra scientist Dr. Peter Edmonds.

“There are indeed some basic similarities between these two objects, in that both were generated by eruptions from a supermassive black hole, either in the form of bright radiation or jets, Edmonds told Universe Today.”Also, in both cases the eruption from the black hole seems to have died down.”

The details of the two objects, however, are very different, Edmonds said. “Hanny’s Voorwerp involves a light echo while the X-ray ghost was thought to form by an interaction between the comic background radiation and particles in a jet. They’re obviously seen at very different wavelengths. Also, the ghost is found in the early Universe at much greater distances than Hanny’s Voorwerp and is physically much larger.”

Additionally, the Chandra team suspects a very powerful and large eruption was responsible for the formation of the ghost, much more powerful than the one for Hanny’s Voorwerp.

Andy Fabian of the Cambridge University in the United Kingdom, lead author on the paper on the ghost of HDF 130, thinks the object’s X-ray glow is evidence of an outburst equivalent to about a billion supernovas, which blasted particles at almost the speed of light. When the eruption was ongoing, it produced prodigious amounts of radio and X-radiation, but after several million years, the radio signal faded from view as the electrons radiated away their energy.

This is the first X-ray ghost ever seen after the demise of radio-bright jets. Astronomers have observed extensive X-ray emission with a similar origin, but only from galaxies with radio emission on large scales, signifying continued eruptions. In HDF 130, only a point source is detected in radio images, coinciding with the massive elliptical galaxy seen in its optical image.

This radio source indicates that HDF 130’s supermassive black hole may be growing.

With Hanny’s Voorwerp, however, astronomers are still searching for any sign of activity from the black hole.

Another argument that the two objects are different is their shape. The linear shape of the HDF 130’s X-ray source is consistent with the shape of radio jets and not with that of a galaxy cluster, which is expected to be circular. The energy distribution of the X-rays is also consistent with the interpretation of an X-ray ghost.

WSRT observations of Hanny's Voorwerp. Credit: ASTRON
WSRT observations of Hanny's Voorwerp. Credit: ASTRON

Hanny’s Voorwerp has all the hallmarks of an interacting system. “The gas probably arises from a tidal interaction between IC 2497 and another galaxy, which occurred several hundred million years ago,” said Dr. Tom Oosterloo, part of the team that studied the Voorwerp with WSRT.

There are more differences between the two objects, primarily that ghosts like the one from HDF 130 may be prevalent in the universe, while the Voorwerp might just be a one-time occurance. “The stream of gas ends three hundred thousand light years westwards of IC2497, and all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident,” said Oosterloo.

Chandra astronomer Caitlin Casey, also of Cambridge said, “This result hints that the X-ray sky should be littered with such ghosts, especially if black hole eruptions are as common as we think they are in the early Universe.”

So now that astronomers know where and now to look for X-ray objects like the one by HDF 130, we’re likely to hear about more cosmic X-ray ghosts in the future. But Hanny’s Voorwerp appears to be unique.

Sources: Chandra, previous UT article, email exchange with Dr. Peter Edmonds, Galaxy Zoo

Posted on by Anne Minard

Distant black hole poses for a close-up

1H0707-495

 

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Astronomers have probed closer than ever to a supermassive black hole lying deep at the core of a distant active galaxy that was once thought to be shrouded in dust — which will greatly advance the look captured in this NASA file image from the mid-1990s. Using new data from ESA’s X-ray Multi-Mirror Mission (XMM)-Newton spaceborne observatory, researchers peered into the innermost depths of the object, which lies at the heart of the galaxy known as 1H0707-495.

“We can now start to map out the region immediately around the black hole,” says Andrew Fabian, at the University of Cambridge, who headed the observations and analysis.

Artist's conception of a black hole. Credit: ESA
Artist's conception of a black hole. Credit: ESA

The galaxy — known as 1H0707-495 — was observed during four 48-hr-long orbits of XMM-Newton around Earth, starting in January 2008. 

X-rays are produced as matter swirls into a supermassive black hole, illuminating and reflecting from the matter before eventually accreting into it. Iron atoms in the flow can be observed in the reflected light, affected by the speed of the orbiting iron atoms, the energy required for the X-rays to escape the black hole’s gravitational field, and the spin of the black hole. All these features indicate that the astronomers are tracking matter to within twice the radius of the black hole itself.  

XMM-Newton detected two bright features of iron emission in the reflected X-rays that had never been seen together in an active galaxy. These bright features are known as the iron L and K lines, and they can be so bright only if there is a high abundance of iron. Seeing both in this galaxy suggests that the core is much richer in iron than the rest of the galaxy. 

Statistical analysis of the data revealed a time lag of 30 seconds between changes in the X-ray light observed directly, and those seen in its reflection from the disc. This delay in the echo enabled the size of the reflecting region to be measured, which leads to an estimate of the mass of the black hole at about 3 to 5 million solar masses.

The observations of the iron lines also show that the black hole is spinning very rapidly and eating matter so quickly that it verges on the theoretical limit of its eating ability, swallowing the equivalent of two Earths per hour.

Source: ESA. The paper appears in Nature.

Posted on by Nancy Atkinson

Is Everything Made of Mini Black Holes?

In 1971 physicist Stephen Hawking suggested that there might be “mini” black holes all around us that were created by the Big Bang. The violence of the rapid expansion following the beginning of the Universe could have squeezed concentrations of matter to form miniscule black holes, so small they can’t even be seen in a regular microscope. But what if these mini black holes were everywhere, and in fact, what if they make up the fabric of the universe? A new paper from two researchers in California proposes this idea.

Black holes are regions of space where gravity is so strong that not even light can escape, and are usually thought of as large areas of space, such as the supermassive black holes at the center of galaxies. No observational evidence of mini-black holes exists but, in principle, they could be present throughout the Universe.

Since black holes have gravity, they also have mass. But with mini black holes, the gravity would be weak. However, many physicists have assumed that even on the tiniest scale, the Planck scale, gravity regains its strength.

Experiments at the Large Hadron Collider are aimed at detecting mini black holes, but suffer from not knowing exactly how a reduced-Planck-mass black hole would behave, say Donald Coyne from UC Santa Cruz (now deceased) and D. C. Cheng from the Almaden Research Center near San Jose.

String theory also proposes that gravity plays a stronger role in higher dimensional space, but it is only in our four dimensional space that gravity appears weak.

Since these dimensions become important only on the Planck scale, it’s at that level that gravity re-asserts itself. And if that’s the case, then mini-black holes become a possibility, say the two researchers.

They looked at what properties black holes might have at such a small scale, and determined they could be quite varied.

Black holes lose energy and shrink in size as they do so, eventually vanishing, or evaporating. But this is a very slow process and only the smallest back holes will have had time to significantly evaporate over the enter 14 billion year history of the universe.

The quantization of space on this level means that mini-black holes could turn up at all kinds of energy levels. They predict the existence of huge numbers of black hole particles at different energy levels. And these black holes might be so common that perhaps “All particles may be varying forms of stabilized black holes.”

“At first glance the scenario … seems bizarre, but it is not,” Coyne and Cheng write. “This is exactly what would be expected if an evaporating black hole leaves a remnant consistent with quantum mechanics… This would put a whole new light on the process of evaporation of large black holes, which might then appear no different in principle from the correlated decays of elementary particles.”

They say their research need more experimentation. This may come from the LHC, which could begin to probe the energies at which these kinds of black holes will be produced.

Original paper.

Source: Technology Review

Posted on by Nancy Atkinson

Rogue Black Holes May Wander the Galaxy

Artists concept of a rogue black hole floating near a globular cluster star near the outskirts of the Milky Way. Credit: David A. Aguilar, CfA

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Here’s another “rogue black hole” theory, which hopefully doesn’t set the doomsday crowd off on a new tangent. But new research suggests that hundreds of massive black holes, left over from the early galaxy-building days of the Universe, may wander the Milky Way. Astrophysicists Ryan O’Leary and Avi Loeb say that rogue black holes originally lurked at the centers of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way. But they also predict that Earth should be safe, as the closest rogue black hole should reside thousands of light-years away.

“These black holes are relics of the Milky Way’s past,” said Loeb, from the Harvard Smithsonian Center for Astrophysics. “You could say that we are archaeologists studying those relics to learn about our galaxy’s history and the formation history of black holes in the early universe.”

Astronomers say if these wandering black holes could be located, they could provides clues to the formation of the Milky Way.

The theory predicts that each time two proto-galaxies with central black holes collided, their black holes merged to form a single, “relic” black hole. During the merger, directional emission of gravitational radiation would cause the black hole to recoil. A typical kick would send the black hole speeding outward fast enough to escape its host dwarf galaxy, but not fast enough to leave the galactic neighborhood completely. As a result, such black holes would still be around today in the outer reaches of the Milky Way halo.

This sounds similar to another “rogue black hole” theory released in 2008 from Vanderbilt University, where a supercomputer simulation predicted colliding black holes created in globular clusters would be kicked out of their home and left to wander the galaxy. Astronomers have been looking for them for years, and even after all that searching, they’ve only come up with a couple of tentative candidates.
But Loeb and O’Leary say hundreds of rogue black holes should be traveling the Milky Way’s outskirts, each containing the mass of 1,000 to 100,000 suns. They would be difficult to spot on their own because a black hole is visible only when it is swallowing, or accreting, matter.

There could be on telltale sign, however. A surrounding cluster of stars could be yanked from the dwarf galaxy when the black hole escaped. Only the stars closest to the black hole would be tugged along, so the cluster would be very compact.

But still it would be hard to determine. Due to the cluster’s small size on the sky, appearing to be a single star, astronomers would have to look for more subtle clues to its existence and origin. For example, its spectrum would show that multiple stars were present, together producing broad spectral lines. The stars in the cluster would be moving rapidly, their paths influenced by the gravity of the black hole.
O’Leary and Loeb say now that they know what to look for, astronomers should begin scanning the skies for a population of highly compact star clusters in the Milky Way’s halo.

The number of rogue black holes in our galaxy will depend on how many of the proto-galactic building blocks contained black holes at their cores, and how those proto-galaxies merged to form the Milky Way. Finding and studying them will provide new clues about the history of our galaxy.

Loeb and O’Leary’s journal paper will be published in the Monthly Notices of the Royal Astronomical Society and is available online at arXiv.