Black Hole Growth: Galactic Mergers Found 'Not Guilty'

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Location, location, location — it’s as true for our universe as it is for real estate. We live in a nice, quiet suburban galaxy, with a well-behaved black hole at the center. Most galaxies are like that, as it happens, but there are a few rougher neighborhoods where the black holes cause their share of mayhem in the form of active galactic nuclei (AGNs).

AGNs are bright spots sometimes found at the center of these unique galaxies. They are the result of voracious black holes — squatting at the center like giant spiders — devouring huge amounts of matter (usually gas clouds, but also the occasional star).

As that matter falls into the black hole, it heats up and releases radiation (light) that can be detected by our astronomical instruments.

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But how do the black holes attract their prey? Waiting for a gas cloud to stumble into their vicinity isn’t a very efficient means of hunting. Scientists had thought that these black holes with way-too-healthy appetites came about because of merging galaxies, which forced material into the black holes whenever they collided.

Now a new study — the largest of its kind to date — indicates that such is not the case. Something else must be giving rise to AGNs.

Indeed, while scientists talk about “cosmic collisions,” bringing to mind images of catastrophic impact, galactic mergers aren’t really all that “violent” at all. As Nicole Gugliucci reported for Discovery News last year:

No stars actually collide in the process. They are so small on galactic scales and spread so far apart that they simply change their gravitational dance around their galaxies. Surely, some get slowly flung away in long tidal arms as seen in the wide-field image, but that is hardly apocalyptic. The gas within galaxies, however, does collide, and an avalanche of star formation commences. Newborn stars heat up surrounding dust clouds, causing them to glow in the infrared, thus being detected by Spitzer. Massive stars born in the merger explode in violent supernovae, leaving behind gas that glows with x-rays to be picked up by Chandra. The galaxies’ older stellar tenants still shine on in visible light to be detected by Hubble, completing the galactic picture.

So galactic mergers might like to get a little rowdy now and then, just to let off steam, but don’t blame them for those overfed black holes at the center of active galaxies.

According to a new paper in the Astrophysical Journal by scientists at Germany’s Max Planck Institute for Astronomy, there is “no significant link between AGN activity and galactic mergers for at least the past eight billion years.” It’s far more likely to be due to more mundane processes: “instabilities within galaxies, collisions of molecular clouds, or tidal disruption by other galaxies.”

The scientists rounded up the usual suspects — 140 active galaxies in all — and compared them with a control group of 1200 inactive galaxies. They used data in the optical regime collected by the Hubble Space Telescope, as well as x-ray data collected by ESA’s XMM-Newton space telescope to identify the active galaxies. Then they tested the whole shebang for signs of galactic mergers.

According to study co-author Knud Jahnke, “Instead of the neat, geometric spiral or smooth elliptical shapes you usually see in Hubble images, colliding galaxies typically look distorted and warped.”

Now they just needed to figure out whether a galaxy was distorted or not, in order to determine whether misshapen galaxies were more likely to have AGNs. So they formed a line up — or an “identity parade,” per the press release — with the telltale AGNs removed from the images to eliminate bias. Then they asked 10 experts on galaxies from several different institutions to determine which were distorted.

The result: “no significant correlation between a galaxy’s activity and its distortion.” In other words, merging galaxies aren’t responsible for the rapid growth of black holes at the center of active galaxies — at least not in the data they surveyed, covering the last 8 billion years.

Given that our universe is 13.7 billion years old, that accounts for more than half our cosmic history. It’s possible that there could be a connection between galactic mergers further back in time, but that’s the focus of a planned follow-up study based on data from Hubble and the James Webb Space Telescope, slated for launch sometime after 2014. Think of it as filing for an appeal. Until then, this new verdict stands.

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