Sunday, January 16, 2011

What activates a supermassive black hole?

There's good evidence that massive black holes exist at the centers of most large galaxies having a central bulge, and even within galaxies that lack a central bulge, are small, or have an irregular form. Such black holes can range in size up to more than 10 billion solar masses (M). Little is known about what the average or typical mass of a central black hole is, although most are probably a lot smaller, such as that of Sagittarius A* in our galaxy, which is only ~4.2×106 M.

Four million solar masses is still pretty hefty, so such objects are usually called supermassive black holes (SMBHs), as opposed to black holes that form as supernova remnants and are only at most a few M. It's not known exactly how SMBHs form and evolve. One clue is that most seem to reside in non-dwarf galaxies with a regular shape and a noticeable central bulge. This suggests that SMBHs form and evolve in tandem with the bulge. However, there are exceptions, such as one discussed here: Supermassive black hole in a dwarf galaxy. Another survey of small (under 1010 M) inactive galaxies in the Virgo cluster found that at least 24% had an X-ray-emitting SMBH.

Since a black hole emits little or no radiation directly, even SMBHs are difficult to detect at distances of millions of light years, unless they are surrounded by a substantial amount of gas and dust that is heated enough in the process of falling into the SMBH that it can strongly emit radiation on its own or produce other detectable effects, like jets. Objects that fall in this category are active galactic nuclei (AGN). In most cases only SMBHs that are active as AGNs are readily detectable, so these are the only specimens we know much at all about.

I've discussed AGN a lot, most recently here, here, here, here, here.

In order to study how SMBHs form and evolve we pretty much have to rely on studies of AGNs, which can provide many clues about this issue. Unfortunately, we don't know much about what causes a relatively quiescent SMBH to become active and turn into an AGN. It's this latter question that's addressed, indirectly, by the research to be discussed here.

But first let's back up to SMBHs in general. There are several interrelated questions concerning their origin and evolution. What accounts for their formation and periods of rapid growth? Do they form before, after, or in parallel with the formation of the galaxies in which they reside? What stimulates their intense outbursts of energy as AGNs or quasars?

The most basic question is: What are the typical ways that SMBHs grow? Possible answers include merger between smaller SMBHs, slow but steady accretion of matter from the surrounding galaxy, or bursts of rapid accretion when substantial amounts of gas and dust are swept up by the SMBH.

Each of these questions, among others, stimulates intense debates among astrophysicists who study such things. These questions are interesting and important not just for their own sake. Since there is a lot of evidence that the evolution of a galaxy and of its central SMBH occur in tandem, understanding the evolution of the SMBH helps us understand that of the whole galaxy.

The research we're concerned with here was designed to study the question by surveying a large number of galaxies that can be examined in some detail because they are not too distant. In this case, that means having a redshift z≤1. That corresponds to a distance (measured in light travel time) of about 7.7 billion light years – a little more than half the size of the visible universe. Since the research needs to examine the visible form of the object, anything farther away is too distant for even the Hubble telescope to resolve in sufficient detail. Also, at z=1 all light from the visible part of the spectrum is shifted to infrared, which Hubble's optics aren't optimized for.

An AGN produces quite energetic radiation across most of the electromagnetic spectrum. So, at least in most cases, it is a sign of the rapid burst model of growth mentioned above. This is typically just a relatively short phase in the life of the galaxy-black hole combination – on the order of a hundred million years or so. That's based on the observation that only about 1% (very roughly) of large galaxies are in this phase, over the 13.7-billion year age of the visible universe. Whether this represents the only mode of growth, or even the bulk of it, is the big unknown. And of course, if there are SMBHs that grow by modes other than rapid accretion, we won't even detect them as AGN.

The standard model of AGNs, which is pretty well accepted by the astrophysical community, is that rapid accretion of interstellar gas and dust around a SMBH is what powers the AGN's "engine". Presumably, then, the AGN goes quiet when most of the available gas and dust has been consumed. But that leaves the question of what initiates the process in the first place. Since there are still many AGNs that are active in the universe out to z=1, so that the galaxies involved have been growing for at least 5 billion years since the early days of the universe, AGNs could not have been active for their entire lives. Therefore, something happened at some point to trigger the activity we observe now.

Astrophysicists want to know what that something is. At least initially, there is much more gas and dust spread throughout the galaxy than in the center. Something has to happen to cause that matter to lose its angular momentum so it can fall into the center. One popular hypothesis has been that this process is triggered by mergers between mature galaxies of roughly equal size, as the gas and dust perturbed by the merger falls inward and is swept up by the central black holes (which might merge themselves). Up until now, there has not been a large-scale investigation of this hypothesis.

Now we have one: The bulk of the black hole growth since z~1 occurs in a secular universe: No major merger-AGN connection. (Available at the arXiv: 1009.3265v2.)

A sample of 140 AGNs was selected for examination. Another sample of 1264 inactive galaxies, carefully matched in size, distance, etc. was also selected for comparison. The only reliable indication of an ongoing merger is a visible distortion of the object's shape, so this is taken as a proxy for the occurrence of a merger. However, the galaxies observed could be undergoing "minor" mergers that don't result in visible distortion (considering how far away most selected objects are). And on the other hand, there's no way to be sure that an object's observable distortion is due to a merger. So the conservative view is that this research is looking at the correlation between galaxy activity and distortion of shape.

There are two specific questions addressed by the research: (1) How many AGN have a distorted structure that appears to be the result of a galactic merger? (2) Do AGNs show any significant difference in terms of visible distortion from otherwise comparable inactive galaxies?

The first question is about whether mergers that produce distortions are a necessary condition for an AGN. Since fewer than 15% of AGNs have visible distortion, the answer is clearly "no". The second question concerns whether a merger that produces distortion is sufficient to trigger an AGN. Since there was no significant difference between AGNs and a control set of non-AGNs in terms of frequency of visible distortion, it seems that whatever causes a distorted form (such as a merger) is not a significant cause for triggering an AGN.

Bottom line: Not only are distortion-producing mergers unnecessary for triggering an AGN, they do not even seem to be a significant cause. One way to think of it is as a visible symptom of some underlying process that might otherwise be hard to detect. (A medical example would be a cancer, whose presence might be indicated by physical symptoms or biochemical markers in the blood.) In the present case, it appears that having a distorted form isn't a symptom usually exhibited by a galaxy when an AGN is present - and in fact, it doesn't predict the presence of an AGN at all.

It is important to be able to identify reliable symptoms, because a galaxy may have an AGN that is not readily detectable directly. Many AGNs are not intense radio sources, presumably because they do not have significant jet structures. And unless we are viewing the galaxy more or less face-on, radiation at shorter wavelengths can be blocked by a thick torus of gas and dust surrounding the central engine of the AGN.

Not all important questions are answered by this study. For example, galaxy mergers that do not significantly distort galactic structure – perhaps involving the cannibalism of a small galaxy by a large one – might play an important role in triggering an AGN.

The results of this research are surprising, because they seem to rule out distortion-producing galaxy mergers as an important cause of AGNs – the previous general assumption. However, it shouldn't be concluded that galaxy collisions can never produce AGNs, let alone SMBHs. There is still the question of whether a SMBH can form "from scratch" without some sort of "seed". It could be that very large black holes formed in the very first instants after the big bang, as "primordial" black holes. (See here, for example. Further possibility for the formation of seed black holes are discussed here.)

However, a simulation study reported last year in Nature (here) showed that in the early universe, SMBHs could form directly from galaxy collisions. But conditions at that time were very different – there was much more gas around that hadn't formed into stars, and a much larger single mass of gas could accumulate without forming stars. Time permitting, as usual, I'd like to discuss this research in another post.

This post was chosen as an Editor's Selection for
Cisternas, M., Jahnke, K., Inskip, K., Kartaltepe, J., Koekemoer, A., Lisker, T., Robaina, A., Scodeggio, M., Sheth, K., Trump, J., Andrae, R., Miyaji, T., Lusso, E., Brusa, M., Capak, P., Cappelluti, N., Civano, F., Ilbert, O., Impey, C., Leauthaud, A., Lilly, S., Salvato, M., Scoville, N., & Taniguchi, Y. (2011).
The Astrophysical Journal, 726 (2) DOI: 10.1088/0004-637X/726/2/57

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