Finding a needle in a haystack might be easy compared to finding two very similar black holes closely orbiting each other in a distant galaxy.

Astronomers from the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, have found what looks like two massive black holes orbiting each other in the center of one galaxy. It has been postulated that twin black holes might exist, but it took an innovative, systematic search to find such a rare pair.

The newly identified black holes appear to be separated by only 1/10 of a parsec (1/3 of a light-year) - a tenth of the distance from Earth to the nearest star (other than the Sun).

This discovery of the most plausible binary black hole candidate ever found may lead to a greater understanding of how massive black holes form and evolve at the centers of galaxies. The NOAO astronomers' results are published in this week's edition of the journal Nature.

After a galaxy forms it is likely that a massive black hole can also form at its center. Since many galaxies are found in clusters of galaxies, individual galaxies can collide with each other as they orbit in the cluster.

The mystery is what happens to these central black holes when galaxies collide and ultimately merge together. Theory predicts that they will orbit each other and eventually merge into an even larger black hole.

The signature of a black hole in a galaxy has been known for many years. The material falling into a black hole emits light in narrow wavelength regions forming emission lines which can be seen when the light is dispersed into a spectrum.

These emission lines carry the information about the speed and direction of the black hole and the material falling into it. If two black holes are present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. This signature has now been found.

Former NOAO Director Todd Boroson and NOAO Astronomer Tod Lauer used data from the Sloan Digital Sky Survey, made with a 2.5-meter diameter telescope at Apache Point in southern New Mexico, to look for this characteristic dual black hole signature among 17,500 quasars discovered by the survey.

More than 100,000 quasars are known, with most being found in the Sloan Digital Sky Survey and at distances of billions of light-years.

Quasars are the most luminous versions of the general class of objects known as active galaxies, which can be a hundred times brighter than our Milky Way galaxy, and powered by the accretion of material into supermassive black holes in their nuclei.

The matter falling into the black hole doesn't go directly in, but orbits around the black hole forming a flat accretion disc, much like the soap scum on water circling an open drain.

It has long been thought that all large galaxies must have a massive black hole in their center and that some galaxies must have two or more black holes, at least until the black holes merge.

The black holes would be so close together that it would be nearly impossible to see them or their accretion disks separately. However, the light emitted from the accretion disks, and the galaxy containing the black hole, ought to be identifiable.

Boroson and Lauer had to especially careful to eliminate the possibility that they were seeing two galaxies, each with its own black hole, superimposed on each other. To try to eliminate this superposition possibility, they determined that the quasars were at the same redshift-determined distance and that there was a signature of only one host galaxy.

If the two quasars were independent objects at different distances, the spectral signatures of both host galaxies should have been seen, and each would have a different redshift and thus a different distance, even though they would be in the same line of sight. Determining the spectral signature was critical, as it would be impossible to see the host galaxies directly against the glare of the quasar.

"The double set of broad emission lines is pretty conclusive evidence of two black holes," Boroson argues.

"If in fact this were a chance superposition, one of the objects must be quite peculiar. One nice thing about this binary black hole system is that we predict that we will see observable velocity changes within a few years at most. We can test our explanation that the binary black hole system is embedded in a galaxy that is itself the result of a merger of two smaller galaxies, each of which contained one of the two black holes."

The smaller black hole has a mass 20 million times that of the Sun; the larger one is 50 times bigger, as determined by the their orbital velocities.