At the center of nearly every large galaxy lies a supermassive black hole (SMBH), millions or billions of times more massive than the Sun. Not every SMBH needs to be associated with a galaxy, however—it may be possible for a SMBH to escape if it receives a strong kick during a galaxy collision. Astronomers using the Chandra X-ray observatory report they have witnessed one of these unusual events, in which the bright nucleus of a galaxy is not actually located at the galactic center.

As they describe in a forthcoming Astrophysical Journal paper, F. Civano et al. identified a candidate SMBH using the Chandra COSMOS survey. Follow-up observations with the Hubble Space Telescope revealed a compact object moving away from another bright object within the galaxy. Both of these bright objects look like galactic cores, but only one of them appears to contain a black hole, and it's the one that is moving.

The hunt for rogue SMBHs is related to an important question in galactic astronomy: how do SMBHs grow so huge? Since astronomers know small galaxies merge to make larger galaxies, it's likely the black holes at their centers will also merge. Several galaxies appear to contain two SMBHs, lending support to this idea. At this point, the black holes may fall into mutual orbit, but it's possible for one of them to be flung out through an effect known as a gravitational slingshot.

If the SMBHs orbit each other, they emit gravitational radiation, which, according to general relativity, causes their orbits to degrade until they merge. (Gravitational radiation can be observed indirectly through the behavior of pairs of neutron stars, but has yet to be detected directly. Gravitational waves travel at the speed of light and, due to the weakness of gravity, pass through nearly every obstacle.) However, calculations show that a SMBH collision should produce a burst of gravitational waves, which could impart momentum to the single black hole that results, sending it lurching out of the galaxy.

If the thought of black holes roaming the galaxy gives you nightmares, rest assured: to acquire escape speeds high enough to leave the host galaxy, the original black holes must have specific spin and mass combinations, which (while not impossible) are rare. Additionally, ejected SMBHs will continue to shine brightly for tens of millions of years, meaning they are not invisible—we need not fret over rogue black holes sneaking up on us.

The current set of observations was designed to judge between the two possibilities: was SMBH CID-42 thrown out by a gravitational slingshot (in which case another SMBH is lurking in the host galaxy), or was it kicked out after merger by a burst of gravitational waves? The answer to this question helps illuminate how SMBHs grow, and may also explain why some rare galaxies lack an obvious black hole at its core. If the slingshot scenario is more common, then it complicates the coevolution model of SMBH and host galaxy, in which there is a correlation between the growth of the black hole and the galaxy surrounding it.

CID-42 is a bright compact X-ray source, which identifies it as a SMBH. The Chandra X-ray data was sufficiently good to show that it doesn't lie at the galactic center, meaning it had been kicked out somehow. The researchers also found two bright optical sources within the host galaxy, one of which is moving rapidly away from the other. By taking the spectrum of both, they determined one was consistent with a SMBH, while the other was a bright star-forming region. While this doesn't absolutely rule out the slingshot scenario, if there is a second SMBH present, it must be highly obscured—it isn't showing up in X-ray or optical observations, to greater than 99 percent (3σ) confidence.

With just one obvious SMBH, the galaxy seems to provide a clear example of an event that involved a black hole merger followed by a gravity wave-driven rebound.

The astronomers also identified a new X-ray source in another part of the galaxy, but they're not sure what that might be. Part of the difficulty is obtaining good resolution in the observations: the host galaxy of CID-42 is far from the Milky Way, so distinguishing details within the galaxy pushes even good telescopes to their limits. The researchers propose both high-resolution radio and X-ray follow-up observations to determine whether this second source is associated with the SMBH, another star-forming region, or something else entirely.

One example of a recoiling SMBH tells us little about the frequency of these events; slingshots may still be responsible for other escaping black holes. Nevertheless, if a second SMBH in CID-42 is completely ruled out by future observations, it will help clarify both the possible growth mechanisms of black holes in galaxies and provide targets for gravitational wave observatories such as LIGO (the Laser Interferometer Gravitational-wave Observatory).

Astrophysical Journal, 2012. DOI: 10.1088/0004-637X/752/1/49 (About DOIs).