A galaxy seems to host two giant black holes, poised to collide in a century
By Daniel Clery, Science.
Excerpt: Unusual radio signals could be long-sought smoking gun of galactic mergers. Astronomers routinely see galaxies crashing into each other and combining. But the final phase of these cosmic mergers has long proved elusive: two supermassive black holes, each once occupying the center of its own galaxy, closely circling each other within a single, combined galaxy. Now, researchers say they have found compelling evidence of such a pairing. A distant galaxy seems to be firing off two beams of radiation from its center at different angles—a sign that a pair of supermassive black holes lurks at its heart. The two behemoths—each with a mass as large as 1 billion Suns—seem to orbit each other every 121 days. ...In as little as 100 years...the black holes should collide, shaking spacetime itself in a titanic burst of gravitational waves. That final burst “would be a really fantastic gravitational wave signal,” says team leader Silke Britzen of the Max Planck Institute for Radio Astronomy. ...astronomers look for periodic signals coming from galactic centers that would indicate orbiting black hole pairs. ...It has proved hard to pick out the definitive signal of binary orbits. Now, Britzen’s team believes it has found such a signal, as described in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society. The candidate signal comes from Markarian 501, a galaxy some 500 million light-years from Earth whose nucleus is so active, it’s referred to as a “blazar.”...