Black holes once existed exclusively in the realm of theory, but astronomers have become increasingly adept at spotting the telltale signs of objects that are hard to spot due to the fact that they suck in any light that happens to cross an event horizon. Black holes have fallen into two separate categories: relatively small ones formed by the collapse of massive stars, and supermassive ones, which appear to lie at the hearts of most galaxies. When provided enough fuel in the form of interstellar gas, these black holes power quasars (also termed active galactic nuclei) but, deprived of input, they tend to sit quietly, much like the one at the center of the Milky Way.

The presence of black holes at the center of galaxies has been made a bit more intriguing by the fact that astronomers have reached the conclusion that many galaxies have been built by mergers of smaller ones. This would suggest that once the chaos of the initial collision settles down, the resulting galaxy should contain two black holes, with gravitational attraction potentially prompting a collision. Today's issue of Nature describes what appears to be the best evidence yet for a supermassive black hole binary, one in which the two objects appear on course for an inevitable collision.

Finding it wasn't exactly easy. The authors relied on data that had been gathered by the Sloan Digital Sky Survey, which currently contains over 900,000 galaxies. The authors selected a subset of the data that contains over 17,000 quasars, and sifted it using a principal components analysis that should identify outliers with unusual spectral properties. Out of that massive data set, a grand total of two objects were identified that appeared to emit light with more than one redshift.

There are two possible ways to get what appears to be a single quasar with more than one redshift. The first is to have two quasars that are far apart but on the same line of sight relative to the earth. Based on the frequency of quasars in the area their data is extracted from, the authors calculate that this should happen with a frequency of about 1.8 x 10-7; since they looked at over 17,000 cases, that leaves a final probability of 3.2 x 10-3. Small, but certainly not out of the realm of the possible.

But the authors go on to describe a number of spectral features that favor the alternative possibility for the double red shift: two objects in close proximity that are moving quickly relative to each other. That's precisely the sort of behavior that you might expect if two supermassive black holes were orbiting each other in close proximity.

Basically, matter moving close to a black hole will move faster, and produce broader spectral lines as a result. Matter that is further out will produce a narrow, sharper spectral line. The new system appears to have two broad-spectrum sources embedded in a single narrow-band emissions source. This could be the product of the two objects (which the authors term "red" and "blue") being close enough to share a single regional gas cloud, while maintaining separate accretion disks.

Assuming this is an accurate description of the system, the authors are able to calculate some of its basic properties. Red weighs in at 108.9 solar masses, with blue tipping the scales at a relatively svelte 107.3 solar masses. The orbital period is fairly brief: below 500 years, and likely to be in the area of a century. That makes for a radius of 0.1 parsec, which is substantially less than a light year.

The authors calculate that red and blue have reached what might be an awkward stage in their inward spiral. They're now too close for basic orbital decay to act efficiently, and not yet close enough together to start producing enough gravity waves to cause the orbit to decay rapidly. That suggests that we have at least a billion years to observe the system before the two objects slam into each other. Hopefully, we'll manage to improve the sensitivity of gravity detectors a bit during that time.

The outstanding question is that if collisions between galaxies are fairly common, why haven't we spotted more of these? Your fingers are sufficient to count the number of other objects that have been proposed to be black hole binaries. The easy answer—binary black holes tend to wind up without matter to fuel a quasar—doesn't seem to make sense, since the collisions should free up lots of interstellar gas and dust. It's a conundrum that may keep the theorists busy for some time.

Nature, 2009. DOI: 10.1038/nature07779

Listing image by P. Marenfeld, National Optical Astronomy Observatory