Most galaxies are centered on massive black holes, and some seem to have an even more remarkable core: black hole binaries, or two black holes tightly circling each other as the result of the merger of two galaxies. Now Hiroshi Sudou, an astronomer at Gifu University, and colleagues at other institutions in Japan have what they believe is the leading candidate for the first observational confirmation of a supermassive black hole binary. The group reports their findings in the 23 May issue of Science.
Sudou and his colleagues set out to find a binary system by zeroing in on active galaxies--those in which the central black hole gobbles gas and stars from an accretion disk and spews powerful jets of energy. The jets emit radio waves, and Sudou and his team hoped to spot a telltale elliptical motion in a jet that would indicate its black hole was circling another massive object. To get the precision required to spot such motion, they needed a second radio galaxy to provide a stable reference point. After searching the catalogs, they focused on a pair of radio galaxies in the constellation Andromeda known as 3C 66B (previously identified as a possible black hole binary) and the far more distant 3C 66A. Using the Very Long Baseline Array, a collection of 10 radio telescopes scattered from Hawaii to Puerto Rico, Sudou and colleagues made six observations. "It was risky to devote so much observation time to one object," says paper co-author Yoshiaki Taniguchi, an astronomer at Tohoku University in Sendai. But the gamble paid off. Over the course of a year, 3C 66B traveled "in a very beautiful ellipse," says Sudou.
That yearly periodicity raises red flags for Donald Backer, an astronomer at University of California, Berkeley. "It's good science, it's intriguing, but that 1-year period is a little scary," because it might be related to Earth's orbit. Sudou and his colleagues say their analysis rules that out, but Backer still worries that there may be some error in their software that produces such a neat annual period.
Sudou and his colleagues hope that continued observations will answer such questions. Particularly, they are hoping to see the ellipse shrink, which would indicate that the two black holes are drawing closer together. David Merritt, an astrophysicist at Rutgers University in Piscataway, New Jersey, says barring systematic errors, that kind of observation, "would be convincing proof" of a black hole binary.