Posted on by Anne Minard

Famous Binary Cygnus-X1 Displays First-Ever Polarized Emissions

Artist's impression of the Cygnus-X1 binary. Credit: NASA / Honeywell Max-Q Digital Group / Dana Berry

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Using the IBIS telescope onboard the European Space Agency’s INTEGRAL satellite, researchers have reported the first measurements of polarization from a black hole binary system, which comprises a black hole and a normal star orbiting around a common center of mass.

The new observations reveal that the chaotic region is threaded by magnetic fields, and represent the first time magnetic fields have been identified so close to a black hole. Most importantly, Integral shows they are highly structured magnetic fields that are forming an escape tunnel for hot matter that would otherwise plunge into the black hole within milliseconds.

Credit: ESA, courtesy of Philippe Laurent

Philippe Laurent is a researcher with the Institute for Research into the Fundamental Laws of the Universe (IRFU), of the CEA in France. He is lead author on the paper, which appears today in Science Express.

Laurent and his colleagues detected polarized gamma-ray photons coming from Cygnus X-1 (19h 58m 21.6756s +35° 12′ 05.775″), a well-known black hole X-ray binary system in the constellation Cygnus. They suggest the polarized emission is originating from a jet of relativistic particles in close proximity to the black hole.

The graph above refers to the team’s results: “whereas the low energy photons seem not to be polarized (the inset line at the left is merely flat), the higher energy ones are strongly polarized (the inset line in the right seems to be sinusoidal), and thus should related to the jet,” Laurent wrote in an email.

The authors reveal more detail through the paper: “Spectral modeling of the data reveals two emission mechanisms: The 250-400 keV data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized,” they write. “The second spectral component seen in the 400keV-2MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.”

Their evidence points to the black hole’s magnetic field being strong enough to tear away particles from the black hole’s gravitational clutches and funnel them outwards, creating jets of matter that shoot into space, according to an ESA press release. The particles in the jets are being drawn into spiral trajectories as they climb the magnetic field to freedom and this is affecting a property of their gamma-ray light known as polarization.

A gamma ray, like ordinary light, is a kind of wave, and the orientation of the wave is known as its polarization. When a fast particle spirals in a magnetic field it produces a kind of light, known as synchrotron emission, which displays a characteristic pattern of polarization. It is this polarization that the team have found in the gamma rays. It was a difficult observation to make.

“We had to use almost every observation Integral has ever made of Cygnus X-1 to make this detection,” says Laurent.

Amassed over seven years, these repeated observations of the black hole now total over five million seconds of observing time, the equivalent of taking a single image with an exposure time of more than two months. Laurent’s team added them all together to create just such an exposure.

“We still do not know exactly how the infalling matter is turned into the jets. There is a big debate among theoreticians; these observations will help them decide,” says Laurent.

Jets around black holes have been seen before by radio telescopes but such observations cannot see the black hole in sufficient detail to know exactly how close to the black hole the jets originate. That makes these new observations invaluable. Such polarization measurements can provide direct insights into the nature of many astrophysical processes and the researchers say that, in the future, their discovery could further our understanding of the emission mechanisms of Cygnus X-1, a model for other black-hole binaries in the universe.

Source: Science. The paper appears today, at the Science Express website.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Black Hole Entropy

Black holes - throw something in them and that's the end of the story, right? Well, some physicists can't leave it at that.

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An easy way to think about the entropy of black holes is to consider that entropy represents the loss of free energy – that is, energy that is available to do work – from a system. Needless to say, anything you throw into a black hole is no longer available to do any work in the wider universe.

An easy way to think about the second law of thermodynamics (which is the one about entropy) is to consider that heat can’t flow from a colder location to a hotter location – it only flows the other way. As a result, any isolated system should eventually achieve a state of thermal equilibrium. Or if you like, the entropy of an isolated system will tend to increase over time – achieving a maximum value when that system achieves thermal equilibrium.

If you express entropy mathematically – it is a calculable value and one that tends to increase over time. In the seventies, Jacob Bekenstein expressed black hole entropy as a problem for physics. No doubt he could explain it much better than I could, but I think the idea is that if you suddenly transfer a system with a known entropy value past the event horizon of a black hole, it becomes immeasurable – as though its entropy vanishes. This represents a violation of the second law of thermodynamics – since the entropy of a system should at best stay constant – or more often increase – it can’t suddenly plummet like that.

So the best way to handle that is to acknowledge that whatever entropy a system possesses is transferred to the black hole when the system goes into it. This is another reason why black holes can be considered to have a very high entropy.

Then we come to the issue of information. The sentence The quick brown fox jumped over the lazy dog is a highly engineered system with a low level of entropy – while drawing out 26 tiles from a scrabble set and laying them down however they come delivers an randomly ordered object with a high level of entropy and uncertainty (to the extent that it could be any of a billion possible variations).

Throw your scrabble tiles into a black hole – they will carry with them whatever entropy value they began with – which is likely to increase further within the black hole. Indeed it’s likely that the tiles will not only become more disorganized but actually crushed to bits within the black hole.

Now there is fundamental principle in quantum mechanics which requires that information cannot be destroyed or lost. It’s more about wave functions than about scrabble tiles – but let’s stick with the analogy.

You won’t violate the conservation of information principle by filling a black hole with scrabble tiles. Their information is just transfered to the black hole rather than being lost – and even if the tiles are crushed to bits, the information is still there in some form. This is OK.

But, there is a problem if in a googol or so years, the black hole evaporates via Hawking radiation, which arises from quantum fluctuations at the event horizon and has no apparent causal connection with the contents of the black hole.

The Hawking radiation story. A quantum fluctuation proximal to a black hole's event horizon produces a particle and an antiparticle. The antiparticle enters the black hole and annihilates when it collides with a particle in there. The remaining particle is free to join the rest of the universe outside the event horizon. To an external observer, the black hole appears to have lost mass and radiated a particle. Over time this process would result in the black hole evaporating. To date - good story, evidence nil, but watch this space. Credit: NAU.

A currently favored solution to this problem is the holographic principle – which suggests that whatever enters the black hole leaves an imprint on its event horizon – such that information about the entire contents of the black hole can be derived from just the event horizon ‘surface’ – and any subsequent Hawking radiation is influenced at a quantum level by that information – such that Hawking radiation does succeed in carrying information out of the black hole as the black hole evaporates.

Zhang et al offer another approach of suggesting that Hawking radiation, via quantum tunneling, carries entropy out of the black hole – and since reduced entropy means reduced uncertainty – this represents a nett gain of information drawn out from the black hole. So Hawking radiation carries not only entropy, but also information, out of the black hole.
But is this more or less convincing than the hologram idea? Well, that’s uncertain…

Further reading: Zhang et al. An interpretation for the entropy of a black hole.

Posted on by Anne Minard

Halt, Black Hole! Gemini Captures Explosions That Deprive Black Holes of Mass

Artist’s rendering of the environment around the supermassive black hole at the center of Mrk 231. The broad outflow seen in the Gemini data is shown as the fan-shaped wedge at the top of the accretion disk around the black hole, in side view. A similar outflow is probably present under the disk as well. The total amount of material entrained in the broad flow is at least 400 times the mass of the sun per year. Credit: Gemini Observatory/AURA, artwork by Lynette Cook

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Astronomers have long suspected that something must stymie actively growing black holes, because most galaxies in the local universe don’t have them. Now, the Gemini Observatory has captured a galactic check-and-balance — a large-scale quasar outflow in the galaxy Markarian 231 that appears to be depriving a supermassive black hole its diet of gas and dust.

The work is a collaboration between David Rupke of Rhodes College in Tennessee and the University of Maryland’s Sylvain Veilleux. The results are to be published in the March 10 issue of The Astrophysical Journal Letters.

Markarian 231 (12h56’14.23″ +56d52’25.24″) is located about 600 million light-years away in the direction of the constellation of Ursa Major. Although its mass is uncertain, some estimates indicate that Mrk 231 has a mass in stars about three times that of the Milky Way, and its central black hole is estimated to have a mass of at least 10 million solar masses or also about three times that of the supermassive black hole in the Milky Way.

Theoretical modeling specifically points to quasar outflows as the counterbalance to black hole growth. In this negative feedback loop, while the black hole is actively acquiring mass as a quasar, the outflows carry away energy and material, suppressing further growth. Small-scale outflows had been observed before, but none sufficiently powerful to account for this predicted and fundamental aspect of galaxy evolution. The Gemini observations provide the first clear evidence for outflows powerful enough to support the process necessary to starve the galactic black hole and quench star formation by limiting the availability of new material.

This extraction from the data cube shows the large-scale, fast outflow of neutral sodium at the center of the quasar Markarian 231. We are looking down onto the material that moves toward us relative to the galaxy, so the measured velocities are negative. The large black circle marks the location of the black hole, and red lines show the location of a radio jet. In addition to the quasar outflow, the jet pushes the material at the top right, resulting in even greater speeds. Part of the starburst is located at the position of the box at the lower left, and it is likely responsible for the gas motion in this region.

Study author Veilleux says Mrk 231 is an ideal laboratory for studying outflows caused by feedback from supermassive black holes: “This object is arguably the closest and best example that we know of a big galaxy in the final stages of a violent merger and in the process of shedding its cocoon and revealing a very energetic central quasar. This is really a last gasp of this galaxy; the black hole is belching its next meals into oblivion!” As extreme as Mrk 231’s eating habits appear, Veilleux adds that they are probably not unique: “When we look deep into space and back in time, quasars like this one are seen in large numbers, and all of them may have gone through shedding events like the one we are witnessing in Mrk 231.”

Although Mrk 231 is extremely well studied, and known for its collimated jets, the Gemini observations exposed a broad outflow extending in all directions for at least 8,000 light-years around the galaxy’s core. The resulting data reveal gas (characterized by sodium, which absorbs yellow light) streaming away from the galaxy center at speeds of over 1,000 kilometers per second. At this speed, the gas could go from New York to Los Angeles in about 4 seconds. This outflow is removing gas from the nucleus at a prodigious rate — more than 2.5 times the star formation rate. The speeds observed eliminate stars as the possible “engine” fueling the outflow. This leaves the black hole itself as the most likely culprit, and it can easily account for the tremendous energy required.

The energy involved is sufficient to sweep away matter from the galaxy. However, “when we say the galaxy is being blown apart, we are only referring to the gas and dust in the galaxy,” notes Rupke. “The galaxy is mostly stars at this stage in its life, and the outflow has no effect on them. The crucial thing is that the fireworks of new star formation and black hole feeding are coming to an end, most likely as a result of this outflow.”

Source: Gemini press release. The paper appears here. See also some galactic merger animations, courtesy of the Harvard-Smithsonian Center for Astrophysics.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Black Holes: The Early Years

High Mass Xray binaries were probably commonly in the early universe and the black hole partner may have shaped the destiny of the later universe. Credit: ESO.

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There’s a growing view that black holes in the early universe may have been the seeds around which most of today’s big galaxies (now with supermassive black holes within) first grew. And taking a step further back, it might also be the case that black holes were key to reionizing the early interstellar medium – which then influenced the large scale structure of today’s universe.

To recap those early years… First was the Big Bang – and for about three minutes everything was very compact and hence very hot – but after three minutes the first protons and electrons formed and for the next 17 minutes a proportion of those protons interacted to form helium nuclei – until at 20 minutes after the Big Bang, the expanding universe became too cool to maintain nucleosynthesis. From there, the protons and the helium nuclei and the electrons just bounced around for the next 380,000 years as a very hot plasma.

There were photons too, but there was little chance for these photons to do anything much except be formed and then immediately reabsorbed by an adjacent particle in that broiling hot plasma. But at 380,000 years, the expanding universe cooled enough for the protons and the helium nuclei to combine with electrons to form the first atoms – and suddenly the photons were left with empty space in which to shoot off as the first light rays – which today we can still detect as the cosmic microwave background.

What followed was the so-called dark ages until around half a billion years after the Big Bang, the first stars began to form. It’s likely that these stars were big, like really big, since the cool, stable hydrogen (and helium) atoms available readily aggregated and accreted. Some of these early stars may have been so big that they quickly blew themselves to pieces as pair-instability supernovae. Others were just very big and collapsed into black holes – many of them having too much self-gravity to permit a supernova explosion to blow any material out from the star.

And it’s about here that the reionization story starts. The cool, stable hydrogen atoms of the early interstellar medium didn’t stay cool and stable for very long. In a smaller universe full of densely-packed massive stars, these atoms were quickly reheated, causing their electrons to dissociate and their nuclei to become free ions again. This created a low density plasma – still very hot, but too diffuse to be opaque to light any more.

Well, really from ions to atoms to ions again - hence the term reionization. The only difference is that at half a billion years since the Big Bang, the reionized plasma of the interstellar medium was so diffuse that it remained - and still remains - transparent to radiation. Credit: New Scientist.

It’s likely that this reionization step then limited the size to which new stars could grow – as well as limiting opportunities for new galaxies to grow – since hot, excited ions are less likely to aggregate and accrete than cool, stable atoms. Reionization may have contributed to the current ‘clumpy’ distribution of matter – which is organized into generally large, discrete galaxies rather than an even spread of stars everywhere.

And it’s been suggested that early black holes – actually black holes in high mass X-ray binaries – may have made a significant contribution to the reionization of the early universe. Computer modelling suggests that the early universe, with a tendency towards very massive stars, would be much more likely to have black holes as stellar remnants, rather than neutron stars or white dwarfs. Also, those black holes would more often be in binaries than in isolation (since massive stars more often form multiple systems than do small stars).

So with a massive binary where one component is a black hole – the black hole will quickly begin to accumulate a large accretion disk composed of matter drawn from the other star. Then that accretion disk will begin to radiate high energy photons, particularly at X-ray energy levels.

While the number of ionizing photons emitted by an accreting black hole is probably similar to that of its bright, luminous progenitor star, it would be expected to emit a much higher proportion of high energy X-ray photons – with each of those photons potentially heating and ionizing multiple atoms in its path, while a luminous star’s photon’s might only reionize one or two atoms.

So there you go. Black holes… is there anything they can’t do?

Further reading: Mirabel et al Stellar black holes at the dawn of the universe.

Posted on by Anne Minard

Message in a Wobble: Black Holes Send Memos in Light

Where is the Nearest Black Hole
Artist concept of matter swirling around a black hole. (NASA/Dana Berry/SkyWorks Digital)

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Imagine a spinning black hole so colossal and so powerful that it kicks photons, the basic units of light, and sends them careening thousands of light years through space. Some of the photons make it to Earth. Scientists are announcing in the journal Nature Physics today that those well-traveled photons still carry the signature of that colossal jolt, as a distortion in the way they move. The disruption is like a long-distance missive from the black hole itself, containing information about its size and the speed of its spin.

The researchers say the jostled photons are key to unraveling the theory that predicts black holes in the first place.

“It is rare in general-relativity research that a new phenomenon is discovered that allows us to test the theory further,” says Martin Bojowald, a Penn State physics professor and author of a News & Views article that accompanies the study.

Black holes are so gravitationally powerful that they distort nearby matter and even space and time. Called framedragging, the phenomenon can be detected by sensitive gyroscopes on satellites, Bojowald notes.

Lead study author Fabrizio Tamburini, an astronomer at the University of Padova (Padua) in Italy, and his colleagues have calculated that rotating spacetime can impart to light an intrinsic form of orbital angular momentum distinct from its spin. The authors suggest visualizing this as non-planar wavefronts of this twisted light like a cylindrical spiral staircase, centered around the light beam.

“The intensity pattern of twisted light transverse to the beam shows a dark spot in the middle — where no one would walk on the staircase — surrounded by concentric circles,” they write. “The twisting of a pure [orbital angular momentum] mode can be seen in interference patterns.” They say researchers need between 10,000 and 100,000 photons to piece a black hole’s story together.

And telescopes need some kind of 3D (or holographic) vision in order to see the corkscrews in the light waves they receive, Bojowald said: “If a telescope can zoom in sufficiently closely, one can be sure that all 10,000-100,000 photons come from the accretion disk rather than from other stars farther away. So the magnification of the telescope will be a crucial factor.”

He believes, based on a rough calculation, that “a star like the sun as far away as the center of the Milky Way would have to be observed for less than a year. So it is not going to be a direct image, but one would not have to wait very long.”

Study co-author Bo Thidé, a professor and program director at the Swedish Institute of Space Physics, said a year may be conservative, even in the case of a small rotation and a need for up to 100,000 photons.

“But who knows,” he said. “We will know more after we have made further detailed modelling – and observations, of course.  At this time we emphasize the discovery of a
new general relativity phenomenon that allows us to make observations, leaving precise quantitative predictions aside.”

Links: Nature Physics

Posted on by Nancy Atkinson

Astronomy Cast Ep. 213: Supermassive Black Holes

Supermassive Black Hole

It’s now believed that there’s a supermassive black hole lurking at the heart of every galaxy in the Universe. These monstrous black holes can contain hundreds of millions of times the mass of our own Sun, with event horizons better than the Solar System. They’re the source of the most energetic particles in the Universe, the brightest objects in the Universe, and the place where the laws of physics go to get mangled.

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

Supermassive Black Holes shownotes and transcript.

Posted on by Tammy Plotner

Black Holes and Dark Matter: Tag! You’re It…

NGC 6503, another example of a bulge-less galaxy with a massive halo and a small black hole.

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We only know they’re there because we can feel them in the dark… Feel their gravity, that is. Like a hide-and-go-seek game played on a moonless summer’s night, we only know that black holes and dark matter exist because we can feel the mass tagging us from beyond what our eyes can see. Are there monsters out there? Massive black holes have been found in galaxies with massive dark matter halos – but it doesn’t necessarily mean they’re in league with each other. Bring your bulge on over here to the dark side…

According to a press release from the Max-Planck-Institut; galaxies, such as our own Milky Way, consist of billions of stars, as well as great amounts of gas and dust. Most of this can be observed at different wavelengths, from radio and infrared for cooler objects up to optical and X-rays for parts that have been heated to high temperatures. However, there are also two important components that do not emit any light and can only be inferred from their gravitational pull. All galaxies are embedded in halos of so-called Dark Matter, which extends beyond the visible edge of the galaxy and dominates its total mass. This component cannot be observed directly, but can be measured through its effect on the motion of stars, gas and dust. The nature of this Dark Matter is still unknown, but scientists believe that it is made up of exotic particles unlike the normal (baryonic) matter, which we, the Earth, Sun and stars are made of.

The other invisible component in a galaxy is the supermassive black hole at its center. Our own Milky Way harbors a black hole, which is some four million times heavier than our Sun. Such gravity monsters, or even larger ones, have been found in all luminous galaxies with central bulges where a direct search is feasible; most and possibly all bulgy galaxies are believed to contain a central black hole. However, also this component cannot be observed directly, the mass of the black hole can only be inferred from the motion of stars around it. In 2002, it was speculated that there may exist a tight correlation between the mass of the Black Hole and the outer rotation velocities of galaxy disks, which is dominated by the Dark Matter halo, suggesting that the unknown physics of exotic Dark Matter somehow controls the growth of black holes. On the other hand, it had already been shown a few years earlier that black hole mass is well correlated with bulge mass or luminosity. Since larger galaxies in general also contain larger bulges, it remained unclear which of the correlations is the primary one driving the growth of black holes.

By studying galaxies embedded in massive dark halos with high rotation velocities but small or no bulges, John Kormendy and Ralf Bender tried to answer this question. They indeed found that galaxies without a bulge – even if they are embedded in massive dark matter halos – can at best contain very low mass black holes. Thus, they could show that black hole growth is mostly connected to bulge formation and not to dark matter. “It is hard to conceive how the low-density, widely distributed non-baryonic Dark Matter could influence the growth of a black hole in a very tiny volume deep inside a galaxy,” says Ralf Bender from the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich. John Kormendy, from the University of Texas, adds: “It seems much more plausible that black holes grow from the gas in their vicinity, primarily when the galaxies were forming.” In the accepted scenario of structure formation, galaxy mergers occur frequently, which scramble disks, allow gas to fall into the centre and thus trigger starbursts and feed black holes. The observations carried out by Kormendy and Bender indicate that this must indeed be the dominant process of black hole formation and growth.

So watch out next time you decide to play games in the dark… You might just get eaten instead of… ahem… tagged.

Original Story Source: Max-Planck-Institut / Image: wikisky.org. We thank you so much!

Posted on by Tammy Plotner

Hide And Go Seek…. Supermassive Black Hole Peeks From Behind The Skirt Of A Dwarf Galaxy

Composite image of the dwarf galaxy Henize 2-10. Hubble Space Telescope data is colored red, green and blue, Very Large Array data is yellow and the Chandra X-Ray Observatory data is purple. Cross marks presumed location of the supermassive black hole in the galaxy.

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It’s a bird… It’s a plane… It’s a million times more massive than our Sun! Just how big do you have to be to hide something really big? Well, in the case of a supermassive black hole all you have to be is a small galaxy.

According to the American Astronomical Society Press Release the surprising discovery of a supermassive black hole in a small nearby galaxy has given astronomers a tantalizing look at how black holes and galaxies may have grown in the early history of the Universe. Finding a black hole a million times more massive than the Sun in a star-forming dwarf galaxy isn’t exactly child’s play – but it is a strong indication that supermassive black holes formed before the buildup of galaxies.

So what’s its name? The big little galaxy is called Henize 2-10. Located 30 million light-years from Earth, it’s not unknown to astonomers and is noted for rapid star formation. This irregular player is roughly 3% the size of the Milky Way and scientists think it may greatly resemble some of the first galaxies for form in the early Universe. “This galaxy gives us important clues about a very early phase of galaxy evolution that has not been observed before,” said Amy Reines, a Ph.D. candidate at the University of Virginia.

We’ve been aware for some time that supermassive black holes are present in the cores of all “full-sized” galaxies – however, we’re a bit more used to balancing the scale. In the nearby Universe, there is a direct relationship — a constant ratio — between the masses of the black holes and that of the central “bulges” of the galaxies, leading them to conclude that the black holes and bulges affected each others’ growth.

Two years ago, an international team of astronomers found that black holes in young galaxies in the early Universe were more massive than this ratio would indicate…

The dwarf galaxy Henize 2-10, seen in visible light by the Hubble Space Telescope. The central, light-pink region shows an area of radio emission, seen with the Very Large Array. This area indicates the presence of a supermassive black hole drawing in material from its surroundings.

“Now, we have found a dwarf galaxy with no bulge at all, yet it has a supermassive black hole. This greatly strengthens the case for the black holes developing first, before the galaxy’s bulge is formed,” Reines said. She, along with Gregory Sivakoff and Kelsey Johnson of the University of Virginia and the National Radio Astronomy Observatory (NRAO), and Crystal Brogan of the NRAO, observed Henize 2-10 with the National Science Foundation’s Very Large Array radio telescope and with the inquisitive eye of the Hubble Space Telescope. What did they find hiding behind the neighbor’s hedges? How about a region near the center of the galaxy that strongly emits radio waves with characteristics of those emitted by super-fast “jets” of material spewed outward from areas close to a black hole. A concept we’ve come quite familiar with in recent years!

Next up, they then searched images from the Chandra X-Ray Observatory that showed this same, radio-bright region to be strongly emitting energetic X-rays. This combination, they said, indicates an active, black-hole-powered, galactic nucleus. “Not many dwarf galaxies are known to have massive black holes,” Sivakoff said.

Of course, there are central black holes of roughly the same mass as the one in Henize 2-10 have been found in other galaxies, those galaxies all have much more regular shapes – the “normal” kids of the hood. Henize 2-10 differs not only in its irregular shape and small size but also in its furious star formation, concentrated in numerous, very dense super star clusters. “This galaxy probably resembles those in the very young Universe, when galaxies were just starting to form and were colliding frequently. All its properties, including the supermassive black hole, are giving us important new clues about how these black holes and galaxies formed at that time,” Johnson said.

Kids… Gotta’ love ’em!

CREDIT: Reines, et al., David Nidever, NRAO/AUI/NSF, NASA

Posted on by Nicholos Wethington

Galactic Mergers Fail to Feed Black Holes

By comparing 140 galaxies that had Active Galactic Nuclei with over 1200 galaxies in a "control group", the likelihood that mergers are the cause of AGN has been brought into doubt. Credit: NASA, ESA, M. Cisternas (Max-Planck Institute for Astronomy)

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The large black holes that reside at the center of galaxies can be hungry beasts. As dust and gas are forced into the vicinity around the black holes, it crowds up and jostles together, emitting lots of heat and light. But what forces that gas and dust the last few light years into the maw of these supermassive black holes?

It has been theorized that mergers between galaxies disturbs the gas and dust in a galaxy, and forces the matter into the immediate neighborhood of the black hole. That is, until a recent study of 140 galaxies hosting Active Galactic Nuclei (AGN) – another name for active black holes at the center of galaxies – provided strong evidence that many of the galaxies containing these AGN show no signs of past mergers.

The study was performed by an international team of astronomers. Mauricio Cisternas of the Max Planck Institute for Astronomy and his team used data from 140 galaxies that were imaged by the XMM-Newton X-ray observatory. The galaxies they sampled had a redshift between z= 0.3 – 1, which means that they are between about 4 and 8 billion light-years away (and thus, the light we see from them is about 4-8 billion years old).

They didn’t just look at the images of the galaxies in question, though; a bias towards classifying those galaxies that show active nuclei to be more distorted from mergers might creep in. Rather, they created a “control group” of galaxies, using images of inactive galaxies from the same redshift as the AGN host galaxies. They took the images from the Cosmic Evolution Survey (COSMOS), a survey of a large region of the sky in multiple wavelengths of light. Since these galaxies were from the same redshift as the ones they wanted to study, they show the same stage in galactic evolution. In all, they had 1264 galaxies in their comparison sample.

The way they designed the study involved a tenet of science that is not normally used in the field of astronomy: the blind study. Cisternas and his team had 9 comparison galaxies – which didn’t contain AGN – of the same redshift for each of their 140 galaxies that showed signs of having an active nucleus.

What they did next was remove any sign of the bright active nucleus in the image. This means that the galaxies in their sample of 140 galaxies with AGN would essentially appear to even a trained eye as a galaxy without the telltale signs of an AGN. They then submitted the control galaxies and the altered AGN images to ten different astronomers, and asked them to classify them all as “distorted”, “moderately distorted”, or “not distorted”.

Since their sample size was pretty manageable, and the distortion in many of the galaxies would be too subtle for a computer to recognize, the pattern-seeking human brain was their image analysis tool of choice. This may sound familiar – something similar is being done with enormous success with people who are amateur galaxy classifiers at Galaxy Zoo.

When a galaxy merges with another galaxy, the merger distorts its shape in ways that are identifiable – it will warp a normally smooth elliptical galaxy out of shape, and if the galaxy is a spiral the arms seem to be a bit “unwound”. If it were the case that galactic mergers are the most likely cause of AGN, then those galaxies with an active nucleus would be more probable to show distortion from this past merger.

The team went through this process of blinding the study to eliminate any bias that those looking at the images would have towards classifying AGN as more distorted. By both having a reasonably large sample size of galaxies and removing any bias when analyzing the images, they hoped to definitively show whether the correlation between AGN and mergers exists.

The result? Those galaxies with an Active Galactic Nucleus did not show any more distortion on the whole than those galaxies in the comparison sample. As the authors state in the paper, “Mergers and interactions involving AGN hosts are not dominant, and occur no more frequently than for inactive galaxies.”

This means that astronomers can’t point towards galactic mergers as the main reason for AGN. The study showed that at least 75% of AGN creation – at least between the last 4-8 billion years – must be from sources other than galactic mergers. Likely candidates for these sources include: “galactic harrassment”, those galaxies that don’t collide, but come close enough to gravitationally influence each other; the instability of the central bar in a galaxy; or the collision of giant molecular clouds within the galaxy.

Knowing that AGN aren’t caused in large part by galactic mergers will help astronomers to better understand the formation and evolution of galaxies. The active nuclei in galaxies that host them greatly influence galactic formation. This process is called ‘AGN feedback’, and the mechanisms and effects that result from the interplay between the energy streaming out of the AGN and the surrounding material in the center of a galaxy is still a hot topic of study in astronomy.

Mergers in the more distant past than 8 billion years might yet correlate with AGN – this study only rules out a certain population of these galaxies – and this is a question that the team plans to take on next, pending surveys by the Hubble Space Telescope and the James Webb Space Telescope. Their study will be published in the January 10 issue of the Astrophysical Journal, and a pre-print version is available on Arxiv.

Source: HST news release, Max Planck Institute for Astronomy, Arxiv paper

Posted on by Nancy Atkinson

Oldest Black Holes are Growing the Fastest

Top-down illustration of a black hole
Top-down illustration of a black hole

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Astronomers have determined that the era of first fast growth of the most massive black holes occurred when the universe was much younger than previously thought. A team of researchers from Tel Aviv University found that the epoch of the first fast growth of black holes occurred when the Universe was only about 1.2 billion years old, and not two to four billion years old, as was previously believed. The team also found that these black holes are continuing to grow at a very fast rate.


The supermassive blackholes that most galaxies are thought to have vary in mass from about one million to about 10 billion times the size of our sun. To find them, astronomers look for the enormous amount of radiation emitted by gas which falls into such objects during the times that the black holes are “active,” or accreting matter. This gas infall into massive black holes is believed to be the means by which black holes grow.

Artist concept of a black hole. Credit: Tel Aviv University

Prof. Hagai Hetzer and his research student Benny Trakhtenbrot used data from two different telescopes, Gemini North on top of Mauna Kea in Hawaii, and the Very Large Telescope Array on Cerro Paranal in Chile.

The data show that the black holes that were active when the universe was 1.2 billion years old are about ten times smaller than the most massive black holes that are seen at later times. However, they are growing much faster. The measured rate of growth allowed the researchers to estimate what happened to these objects at much earlier as well as much later times. The team found that the very first black holes, those that started the entire growth process when the universe was only several hundred million years old, had masses of only 100-1000 times the mass of the sun. Such black holes may be related to the very first stars in the universe. They also found that the subsequent growth period of the observed sources, after the first 1.2 billion years, lasted only 100-200 million years.

The team found that the very first black holes ? those that started growing when the universe was only several hundred million years old ? had masses of only 100-1000 times the mass of the sun. Such black holes may be related to the very first stars in the universe. They also found that the subsequent growth period of these black holes, after the first 1.2 billion years, lasted only 100-200 million years.

The new study is the culmination of a seven year-long project at Tel Aviv University designed to follow the evolution of the most massive black holes and compare them with the evolution of the galaxies in which such objects reside.

The results will be reported in the Astrophysical Journal.

Source: American Friends of Tel Aviv University