Astronomers have, for the first time, discovered what seems to be a binary set of supermassive black holes. The galactic beasts are orbiting each other about every 100 years, and ultimately they will collide with enough force to trip gravitational-wave detectors on Earth.
Spotting a single supermassive black hole is fairly easy, so much so that astronomers are convinced that one lurks in the center of just about every galaxy. Finding two supermassives locked in a binary orbit is another story. Only about as big as a solar system--though they can weigh as much as a billion suns--spotting them as distinct objects is as difficult as finding the proverbial needle in the haystack.
But that's what a pair of astronomers from the National Optical Astronomical Observatory in Tucson, Arizona, appear to have done. Todd Boroson and Tod Lauer were combing through the Sloan Digital Sky Survey, a collection of images and spectral information on hundreds of thousands of galaxies, when their software flagged a quasar whose light characteristics differed significantly from the rest of the sample. Quasars are the most luminous objects in the universe, and their brightness is thought to be caused by the energy from supermassive black holes consuming tremendous amounts of surrounding gas and dust.
Tomorrow in Nature, the two researchers report finding telltale twin hydrogen lines in the quasar's spectrum, instead of the one line that would emanate from a single black hole. Based on the twin lines, Boroson and Lauer calculate that two supermassives are separated by only 0.3 light-years--one-tenth the distance from the sun to its nearest neighbor--and are orbiting each other at the blinding speed of 6000 kilometers per second. For comparison, the sun's orbital speed around the galaxy's center is about 220 kilometers per second. "Sometimes discoveries depend on recognizing something as interesting even if it wasn't what you were looking for," Boroson says. If this one is confirmed, he adds, it should vastly improve what astronomers know about how supermassives merge and how they light up quasars.
"Wow, that is certainly an interesting discovery," says astronomer Steven Willner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Follow-up observations should reveal "a lot more about the environment around the black holes," he says. Astronomer Alan Marscher of Boston University says the paper does seem to show more than one supermassive black hole, which is new but expected, because two galaxies in the process of merging would contain two supermassive black holes.