BY DR EMILY BALDWIN
Posted: 05 March, 2009
Astronomers from the National Optical Astronomy Observatory (NOAO) in Arizona have found the best evidence yet for a pair of massive black holes orbiting around each other in a distant galaxy.
While it has been postulated that twin black holes might exist, it took an innovative, systematic search to find the signature of such a rare pair. It is well known that material falling into a black hole emits light in characteristic patterns - emission lines - that carry the information about the speed and direction of the black hole and the material falling into it. But if two black holes are present, they would orbit each other before merging, an event that would also be imprinted into the emission lines.
Artist's impression of the newly discovered binary black hole system. Each black hole is surrounded by a disc of material gradually spiraling into its grasp, releasing radiation from x-rays to radio waves. The two black holes complete an orbit around their centre of mass every 100 years, traveling with a relative velocity of 6000 kilometres per second. Image: p. Marenfeld and NOAO/AURA/NSF.
NOAO astronomers used data from the Sloan Digital Sky Survey to search for the dual black hole signature among 17,500 quasars discovered by the survey. Quasars are the most luminous type of active galaxies, powered by the accretion of material that is dragged into an orbit around the black hole. There are over 100,000 known quasars, with most being found in the Sloan Digital Sky Survey and at distances of billions of light years.
The newly identified black holes present the most confident evidence for a binary system, for the two monsters are separated by just 1/10 of a parsec, or 1/3 of a light year. Orbital velocity measurements reveal the two black holes differ in size: the smaller one has a mass 20 million times that of the Sun while the larger one is 50 times bigger.
“The double set of broad emission lines is pretty conclusive evidence of two black holes,” says former NOAO Director Todd Boroson. “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.”
Boroson and colleague Tod Lauer eliminated the possibility that the quasars were just a line of sight coincidence by demonstrating that they were at the same redshift, and that there was a signature of only one host galaxy. Although massive black holes are known to lurk in the centres of all large galaxies, what happens to these central black holes when these galaxies collide and merge has remained a mystery. Theory predicts that they will orbit each other and eventually merge into an even larger black hole, and the new discovery may be a stepping stone to uncovering that evolutionary step.
The results are published in this week’s edition of the journal Nature.