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.

One way to approach the question is to investigate whether galaxies that are in the process of merging also show evidence of growing SMBHs. Unfortunately, it’s not necessarily easy to determine when a merger is occurring, as recent research that simply tried to count mergers showed. The difficulty it in trying to find distortions in a galaxy’s shape as an indicator of merging. It’s an even tougher problem when a bright AGN is present in one or both proximate galaxies, since an AGN can outshine an entire galaxy.

However, to find evidence that interactions between neighboring galaxies affect occurrence of AGNs, a start could be made simply by looking at pairs of galaxies that are close neighbors, as determined from galaxy surveys. This is exactly what the latest research did:

Galaxy interactions accelerate the growth of supermassive black holes

The COSMOS team has carried out an alternative test that doesn’t require any knowledge of whether there are signs of distortions in optical images. Rather, galaxies are assumed to be in the process of interacting if they have a close neighbor. This requires accurate distance measurements of about 20,000 galaxies in the COSMOS field as provided by the zCOSMOS redshift survey with the European Southern Observatory’s Very Large Telescope. Isolated galaxies provide the comparison sample to establish whether AGNs are more common in galaxies experiencing close encounters. X-ray observations with NASA’s Chandra Observatory cleanly identify those galaxies that harbor an AGN since X-ray emission is a common property of growing SMBHs. Further, X-rays can penetrate obscuring material, such as star-forming regions, usually present in gas-rich galaxy mergers thus providing a broad census of AGN activity.

It is reported in The Astrophysical Journal that galaxies in close pairs are twice as likely to harbor AGNs as compared to galaxies in isolation. This result indicates that mergers, in an early stage, do lead to enhanced black hole growth. Given the moderate frequency of such interactions within the overall galaxy population, these events contribute about 20% to the overall mass buildup of the black hole population. Therefore, additional physical mechanisms are responsible for growing the majority of SMBHs. It is also likely that galaxies undergoing a final coalescence in a merger may play a role. These findings provide further evidence that galaxies and their SMBHs grow together over substantial periods of time.

The criterion for galaxies being sufficiently close together was that the estimated separation be less than about 250,000 light-years and the line-of-sight difference in velocity less than 500 km/sec. Since galaxies in each pair were visually separated, late-stage mergers were excluded. The sample size of galaxy pairs was 562, and there was a control sample of 2726 matched galaxies not in pairs. The presence of an AGN was established based on X-ray emissions measured by the orbiting Chandra X-ray Observatory.

So there you have it: interactions between neighboring galaxies may account for about 20% of SMBH growth. Interactions do seem to have an effect, but don’t account for the majority of growth.

Thus, there must be additional mechanisms that fuel the growth of SMBHs. Actual late-stage mergers, which were excluded from this study, are one obvious mechanism. Various other mechanisms have been suggested. But it’s not as easy as it might seem for intragalactic gas and dust to be swept up by the central black hole. Instead, something has to cancel the angular momentum of the gas and dust and drive large quantities of it rather close to the central black hole, so that it can form an accretion disk before eventually falling into the black hole itself.

Some of the commentaries on research published in July 2011 that dealt with this issue mentioned a few possibilities. These include “disk instabilities and starbursts” (here), and “minor collisions with small, unseen clusters of stars that orbit the main galaxy” (here).

The review article from Science (July 8, 2011), The Coevolution of Galaxies and Supermassive Black Holes: A Local Perspective provides an excellent overview.

J. D. Silverman, P. Kampczyk, K. Jahnke, R. Andrae, S. Lilly, M. Elvis, F. Civano, V. Mainieri, C. Vignali, G. Zamorani, P. Nair, O. Le Fevre, L. de Ravel, S. Bardelli, A. Bongiorno, M. Bolzonella, M. Brusa, N. Cappelluti, A. Cappi, K. Caputi, C. M. Carollo, T. Contini, G. Coppa, O. Cucciati, S. de la Torre, P. Franzetti, B. Garilli, C. Halliday, G. Hasinger, A. Iovino, C. Knobel, A. koekemoer, K. Kovac, F. Lamareille, J. -F. Le Borgne, V. Le Brun, C. Maier, M. Mignoli, R. Pello, E. Perez Montero, E. Ricciardelli, Y. Peng, M. Scodeggio, M. Tanaka, L. Tasca, L. Tresse, D. Vergani, E. Zucca, A. Comastri, A. Finoguenov, H. Fu, R. Gilli, H. Hao, L. Ho, & M. Salvato (2011). The impact of galaxy interactions on AGN activity in zCOSMOS Astrophysical Journal arXiv: 1109.1292v1

Further reading:

Galaxy Interactions Could Cause Overweight Black Holes

The impact of galaxy interactions on AGN activity in zCOSMOS


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