Archive for ‘Active galaxies’

February 8, 2012

How did some early black holes get so big so fast?

The supermassive black holes (SMBHs) found in the centers of large galaxies can be astonishingly large. The closest example to us is in the giant elliptical galaxy M87, and it’s estimated to be 6.6 billion solar masses (M). More distant examples can be even larger, more than 10 billion M (at distances ~300 million light-years).

Those are extremes. 1 or 2 billion M SMBHs are a little more common in our neighborhood, though still rare. Rather surprisingly, however, SMBHs that large can be found even in the very early universe. The largest yet discovered is about 2 billion M, and it’s 12.9 billion light-years away, at a redshift z=7.085. That SMBH reached its observed size only 765 million years after the big bang, i. e. perhaps 500 million years after the very first stars formed. It’s been a difficult problem to understand how SMBHs that large could have formed so quickly. A recently announced computer simulation of a large part of the very early universe may have come up with a good answer.

It is only barely possible to detect very bright objects (such as quasars or large galaxies) at redshifts z~7 with the best telescope technology today, and impossible to detect less bright objects (even the brightest stars) or objects at higher redshifts. So direct observation of the earliest stars – which may have begun to form as early as z~30, 100 million years after the big bang – is currently impossible, and computer simulations must be used to understand their properties and the process in which they formed.

January 7, 2012

A hyperactive young galaxy

Active galaxies contain a supermassive black hole (SMBH) that causes vigorous radiation of electromagnetic energy as a result of rapid accretion of gas and dust. While almost all galaxies except dwarfs contain an SMBH in the center, active galaxies are rare – fewer that 1% of galaxies in the present universe. A very few active galaxies contain two active SMBHs. Even fewer have three. Before the latest discovery, only two examples had ever been documented in the literature.

The most recent example was so obscure it doesn’t even have a name. This example is unusual in other respects as well. It is quite distant, having a redshift of z=1.35. That means its light has taken 8.92 billion years to reach us. We see it as it looked about 4.75 billion years after the big bang. Most galaxies at that time were fairly mature. Not this one. It seems to be quite young and irregular in form, with 4 separate components. Three of those appear to contain active SMBHs.

Some of the details about this galaxy are not known very precisely. It’s (barely) possible that one or more of the apparent active SMBHs are actually bulges hosting very active star formation. But the spectroscopic evidence is heavily against that.

The three SMBHs are not especially large as such things go. The masses could be as much as 3.1×106 M, 1.0×107 M, and 1.2×107 M. However, these are upper bounds, and the actual masses could be only 20% as large. For comparison, the Milky Way’s SMBH is 4.2×106 M, but many SMBHs are in the 109 to 1010 M range.

There are several intriguing questions about this object. The first: how did it form? Is it a 3-way merger of smaller galaxies of similar size, each of which had its own SMBH already? In the modern universe, such mergers are very rare. The evidence is that they were also rare in the era of 5 billion years after the big bang. It’s a lot more likely that this galaxy is still rather young and in an early stage of formation.

If that is the case, then there are very interesting questions about how the object can have three active nuclei.

November 11, 2011

Galaxy interactions accelerate the growth of supermassive black holes

It’s now well-known that there’s a rough correlation between a galaxy’s size and the size of its central supermassive black hole (SMBH). The correlation is even better (for spiral galaxies) between the black hole size and the size of the central bulge of the galaxy. It’s been found that the mass of a SMBH is generally close to 1/1000th of the mass of the central bulge. So in some way or other, it seems that galaxies and their black holes grow together, although it’s still unclear whether the galaxy or the black hole takes the lead.

The question can be asked in a different way. Which factors contribute to galaxy growth and which to black hole growth? The problem is that black holes usually can’t be observed, or even detected, directly. About the only way to find SMBHs is when they are part of an “active galaxy”, in the form of an active galactic nucleus (AGN). AGNs represent a stage in which large amounts of gas and dust are being swept into the vicinity of a black hole, before actually falling into the black hole. This is a fairly unusual state, which occupies perhaps only about 1% of a galaxy’s life time, if it occurs at all. Nevertheless, AGN are commonly used as indicators of a growing SMBH. It’s certainly true that SMBHs grow while its galaxy is “active”.

One obvious mechanism for the simultaneous growth of galaxies and their SMBHs is merger between two (or more) galaxies. This has been the favored theory for some time, and a number of studies provide some evidence for its occurrence. Recent examples: here, here, here, here, here, here.

However, more recently, other studies suggest that mergers may not be the most common cause of AGNs: here, here.

Cleary, this is still an issue that’s as yet very much unresolved. The latest research to appear adds more evidence that galaxy mergers – and probably even close encounters – do contribute to the growth of SMBHs and stimulate flare-ups of AGNs.

November 8, 2011

Hubble directly observes the disk around a black hole

Quasars are the brightest persistent objects in the universe. They represent the brightest examples of a somewhat more common object: an active galactic nucleus (AGN). As the name implies, an AGN exists at the center of some galaxies. Most quasars, and even most AGN, are not found in the nearby universe, since they are especially luminous for a relatively brief time compared to the age of the universe, perhaps 50 or 100 million years, and also because the conditions under which they develop were more common in the early universe.

Because most AGN are quite distant, it’s very difficult to observe them in detail. However, persistent efforts of astrophysicists – based on separate observation of many objects, some of which are relatively close to us – have resulted in a reliable theoretical model of the structure of an AGN. At the center of an AGN is a supermassive black hole that may be as large as several billion solar masses. When there is a sufficient amount of galactic gas and dust available, it will be sucked into the vicinity of the black hole and form a relatively thin accretion disk that is heated hot enough to radiate vigorously at wavelengths far into the ultraviolet.

Clouds of somewhat cooler dust form in a toroidal ring outside of the disk. These clouds present an additional factor that makes it difficult to observe the accretion disk itself. Only if our line of sight is almost perpendicular to the disk is it possible to get an unobstructed view, and even then the large distance of most AGNs prevents seeing much detail.

But there are certain very rare instances when we get lucky and can see more – when a quasar is magnified by a gravitational lens formed by another galaxy that is directly between us and the quasar. In research just published, the Hubble Space Telescope was used to study six different quasars that were lensed by intervening galaxies. One objective of the study was to use light from the quasars as a way to probe the amount of dust present in the lensing galaxy.

However, in one case conditions were just right to allow astrophysicists to determine some properties of the quasar itself. The quasar in question is designated HE1104−1805. The image above shows two copies of HE1104−1805 that resulted from the lensing effect of the somewhat dimmer lens galaxy ([WKK93] G) itself (between the two images of HE1104−1805). The redshift of the quasar is z=2.32 and that of the lensing galaxy is z=0.73. These correspond to light travel time distances of 10.8 and 6.5 billion light-years, respectively.

The double image is caused by lensing due to the mass of the entire galaxy. However, individual stars within the galaxy can also cause lensing (“microlensing”) if they lie precisely between us and the quasar. Although that is not visible in the image, the effect can be detected from very subtle variations in the light due to motion of the lensing stars.

A careful analysis of the data yields results never before obtained – direct measurements of both the size of the accretion disk of a quasar and the temperature of some locations on the disk.

Hubble directly observes the disc around a black hole

[T]here were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar’s light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar’s light.

By recording the variation in color, the team were able to reconstruct the color profile across the accretion disc. This is important because the temperature of an accretion disc increases the closer it is to the black hole, and the colors emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disc of hot matter, and plot how hot it is at different distances from the center.

They found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometers). While this measurement shows large uncertainties, it is still a remarkably accurate measurement for a small object at such a great distance, and the method holds great potential for increased accuracy in the future.

Muñoz, J., Mediavilla, E., Kochanek, C., Falco, E., & Mosquera, A. (2011). A STUDY OF GRAVITATIONAL LENS CHROMATICITY WITH THE HUBBLE SPACE TELESCOPE
The Astrophysical Journal, 742 (2) DOI: 10.1088/0004-637X/742/2/67

Further reading:

Gravitational lens lets Hubble zoom in on matter swirling into black hole

Hubble Spots Disk Around Distant Black Hole

Supercharging the Hubble


October 14, 2011

Suspects in the quenching of star formation exonerated

Active galactic nuclei (AGN) – produced by matter swept violently into the vortex around a supermassive black hole that may have billions of times as much mass as our Sun – can put on some of the most spectacular fireworks in the universe, over periods as long as 100 million years.

Astrophysicists have sometimes speculated that AGN may be so energetically active that they diminish or even extinguish formation of new stars in the host galaxy.

Recently published research, making use of the PRIMUS faint galaxy spectroscopic redshift survey, suggests that the speculations are wrong, and that AGN can be found even in galaxies in which very active star formation is occurring. The research gives some answers to the larger question of what special characteristics, if any, the host galaxies of AGN may have.

Suspects in the quenching of star formation exonerated

Because astronomers had seen these objects primarily in the oldest, most massive galaxies that glow with the red light of aging stars, many thought active galactic nuclei might help to bring an end to the formation of new stars, though the evidence was always circumstantial.

That idea has now been overturned by a new survey of the sky that found active galactic nuclei in all kinds and sizes of galaxies, including young, blue, star-making factories.

“The misconception was simply due to observational biases in the data,” said Alison Coil, assistant professor of physics at the University of California, San Diego and an author of the new report, which will be published in The Astrophysical Journal.

“Before this study, people found active galactic nuclei predominantly at the centers of the most massive galaxies, which are also the oldest and are making no new stars,” said James Aird, a postdoc at the University of California, San Diego’s Center for Astrophysics and Space Sciences, who led the study.

October 3, 2011

Space Telescopes Reveal Secrets of Turbulent Black Hole

An active galactic nucleus (AGN) is among the most dramatic and complex astrophysical objects in the universe. Extensive new research of a particular AGN in the galaxy Markarian 509 has provided a wealth of new details on the complex internal structure and dynamics of a medium size AGN.

A supermassive black hole at the center of a galaxy drives all the activity of an AGN. The mass of the black hole may be as much as 10 billion solar masses, or more. Quasars represent the most spectacular and active AGNs. They are visible at distances of over 13 billion light-years. Very recent research has connected the intrinsic brightness of an AGN with more detailed physical characteristics, making possible the use of AGNs as “standard candles” for measuring the universe out to very large distances. The largest AGN near Earth is in the giant elliptical galaxy M87, at a distance of about 53 million light-years. The mass of its black hole is about 6.5 billion solar masses.

Somewhat less spectacular AGNs were initially detected as galaxies with unusual properties, such as very bright central cores and broad emission lines in their spectra. These are termed Seyfert galaxies, after the astronomer who identified them. Markarian 509, located about 500 million light-years from Earth, is in the not precisely defined space between Seyfert galaxies and quasars. The mass of its central black hole is estimated at 300 million solar masses. Markarian 509 was selected for detailed study, in part, because earlier observations gave indications of considerable internal turbulence in the gas, plasma, and dust, in the vicinity of the black hole, that comprise the AGN.

All of the seven papers that make up the initial set of analyses of the results are open access, for anyone really curious about the details.

ESA spacecraft reveal new anatomy around a black hole

The black hole was monitored for 100 days. “XMM-Newton really led these observations because it has such a wide X-ray coverage, as well as an optical monitoring camera,” says Jelle Kaastra, SRON Netherlands Institute for Space Research, who coordinated an international team of 26 astronomers from 21 institutes on four continents to make these observations.

During the campaign, the galaxy surpassed itself; instead of the usual 25 percent fluctuations in its brightness, it leapt up in the soft X-ray band by 60 percent, indicating that a major disturbance occurred in the gas flow to the black hole’s deadly clutches.

The resulting observations have shown that the outflow consists of giant bullets propelled at millions of kilometres per hour. The bullets are stripped away from a dusty reservoir of matter waiting to fall into the black hole. The surprise is that the reservoir is situated more than 15 light years away from the black hole. This is further than some astronomers thought was possible for the wind to originate.

Further reading:

Space telescopes reveal secrets of turbulent black hole

Dissecting a Black Hole’s Surroundings: Major study includes observations with NASA/ESA Hubble Space Telescope

Space Telescopes Reveal Secrets of Turbulent Black Hole

Astronomers reveal new findings from ‘monster’ black hole

The environment of a supermassive black hole revealed

Scientists zero in on black hole

Astronomy Cmarchesin: Space Telescopes Reveal Secrets of Turbulent Black Hole

Space telescopes reveal secrets of turbulent black hole (has links to the technical papers – more details here)

Correlated X-Ray and Optical Variability in Markarian 509 (earlier 2008 paper on Markarian 509)

September 28, 2011

Astronomers Discover New Standard Candle

Just a few days ago there was a story about a new type of “standard candle” for use in measuring very large astronomical distances. Such standard candles have been lacking when the distances to be measured are a bit more than halfway back to the big bang – around 7 or 8 billion light-years. Consequently, it’s very difficult to obtain reasonably precise data about many things that happened in the early universe, such as the rate at which the universe was expanding then. This is a major problem since phenomena such as dark energy are difficult to theorize about without good data.

Now another type of standard candle has been identified, and it also makes it possible to gauge large distances – even to objects whose light was emitted about 1.5 billion years after the big bang, at a distance of 12 billion light-years (redshift z~4).

Astronomers Discover New Standard Candle – Technology Review

In theory, distance should be simple to work out. If you know the intrinsic brightness of an object, a simple measure of its apparent brightness will tell you how far away it is (since brightness falls as an inverse square of its distance).

So in astronomy, the problem of distance is intimately linked to the problem of knowing an object’s intrinsic brightness.

But that’s hard. There’s simply no way to tell the intrinsic brightness of most stars and galaxies and so no way to work out their distance.

September 16, 2011

Small distant galaxies host supermassive black holes

There’s been an ongoing debate (of sorts) over the supermassive black holes that seem to exist in most medium to large-size galaxies – did their growth precede, parallel, or follow the growth of the galaxy itself? This arises out of the rough correlation that exists between the size of a galaxy and the size of its black hole. Relatively large black holes have been observed even in nearby dwarf galaxies. This suggests that black hole growth may precede galaxy growth.

The best way to investigate the issue is to observe many galaxies in the early universe, which is difficult because galaxies were smaller and dimmer then. Observations have been reported of very small samples and support the idea that the black holes grow first. There’s a new study out that surveys 28 small galaxies as they were about 3.3 billion years after the big bang. The galaxies are only about one seventh the size of previously studied galaxies of that age. However, by combining the data from all the galaxies, there is evidence for active galactic nuclei in the sample, and hence the presence of supermassive black holes.

Small distant galaxies host supermassive black holes

Using the Hubble Space Telescope to probe the distant universe, astronomers have found supermassive black holes growing in surprisingly small galaxies. The findings suggest that central black holes formed at an early stage in galaxy evolution.

“It’s kind of a chicken or egg problem: Which came first, the supermassive black hole or the massive galaxy? This study shows that even low-mass galaxies have supermassive black holes,” said Jonathan Trump, a postdoctoral researcher at the University of California, Santa Cruz.

Further reading:

Small galaxies with large black holes

A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation

September 14, 2011

Black holes act as galactic thermostats

The supermassive black holes at the centers of most medium to large size galaxies may drive a periodic self-regulating boom-bust cycle. This may be like a pendulum which swings only so far in one direction before it reaches a limit, turns around, and eventually reaches a limit in the other direction. In this case it would be interstellar gas acting like the pendulum. Starting from a very hot state the gas cools by emission of X rays, falls in towards the black hole, and a portion of it forms into an energetic active galactic nucleus (AGN) – which reheats gas further out until the AGN burns itself out and the cycle begins all over again. That is, at least, the scenario explored in more detail in research being reported this week.

Black holes act as galactic thermostats –

The supermassive black hole at the centre of a massive galaxy or galaxy cluster acts as a furnace, pumping heat into its surroundings. But astronomers have struggled to understand how a steady temperature is maintained throughout the whole galaxy when the black hole only appears to interact with nearby gas. Now, researchers in Canada and Australia believe the answer could be a feedback loop in which gravity causes gas to accumulate around the black hole until its density reaches a tipping point. Then, the gas rushes into the black hole, temporarily turning up the heat.

September 12, 2011

Fermi’s Latest Gamma-ray Census Highlights Cosmic Mysteries

NASA’s Fermi Gamma-ray Space Telescope has produced a catalog of 1873 persistent gamma-ray sources detected over the entire sky. The nature of each object has been determined, when possible, and it turns out that a majority of them are very bright active galaxies. However, it wasn’t possible to conclusively identify about 30% of the sources.

Fermi’s Latest Gamma-ray Census Highlights Cosmic Mysteries – NASA

Astronomers delight in the possibility of finding new types of gamma-ray-emitting objects within the “unassociated sources” that constitute roughly a third of the catalog. But Fermi’s LAT is revealing gamma-rays from an increasing — and sometimes, surprising — variety of astronomical objects.