Physicists have recently theorized that the merger of two black holes would create gravitational waves that could eject the resultant object from its galaxy. Now such a black hole event has been observed for the first time. Theory predicted that the gravitational waves would be emitted primarily in one direction, pushing the newly enlarged black hole in the opposite, and that is what we seem to be looking at, according to scientists at the Max Planck Institute for Extraterrestrial Physics (MPE).
We can’t see black holes themselves, nor have we yet directly detected gravitational waves. But we can observe the interactions around black holes, in this case the broad emission lines of gases carried with the recoiling black hole as it exits its galaxy, which contrast with the narrow emission lines of the gases the object left behind. These data allowed the object’s speed — a scorching 2650 kilometers per second — to be measured. The recoil caused by the merger is pushing the black hole, which masses several hundred million times the mass of the Sun, completely out of the galaxy it once called home.
What would cause two enormous black holes to encounter each other? The most likely event is a collision between two galaxies. Early calculations and later simulations of such events predicted that such mergers could produce velocities of up to a few hundred kilometers per second, but working out the numbers for spinning black holes produced much higher velocities, up to the several thousand kilometers per second found by Stefanie Komossa’s team at MPE. With speeds like this, exceeding the escape velocity of even massive elliptical galaxies, we have to ponder the consequences for galaxy evolution absent the central black hole. The work also implies an intergalactic population of black holes.
Finding the first ever candidate for a recoiling black hole, thus verifying theory and simulation, is quite a catch. It’s also noteworthy given the distances involved. Komossa’s team first detected X-ray emissions from the black hole’s accretion disk from a gigantic ten billion light years away. The observation of gravitational waves through experiments like LIGO (Laser Interferometer Gravitational-Wave Observatory) and the space-based LISA (Laser Interferometer Space Antenna) may one day soon provide data that will help us refine our model of such events, as well as other black hole activity. We’ll also find out whether Einstein was right that gravitational waves and light waves travel at the same speed.
See this MPE news release for more. The paper is Komossa et al., “A Recoiling Supermassive Black Hole in the Quasar SDSS J092712.65+294344.0?” Astrophysical Journal 678 (May 10, 2008), pp. L81-L84 (abstract)