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By an enormous burst of gravitational waves that accompanies the merger of two black holes, a newly formed black hole has been booted out of its parent galaxy at thousands of kilometres per second, confirming theories that extreme ejection events like this can occur and aren’t only plausible in supercomputer simulations. When two black holes merge, waves of gravitational radiation ripple outward through the galaxy at the speed of light. Because the waves are emitted mainly in one direction, the black hole is forced to recoil in the opposite direction. The result is that the black hole is catapulted out from its normal location in the nucleus of the galaxy, and if the kick velocity is high enough, the black hole can completely escape the gravitational clutches of its parent galaxy. The discovery of a black hole obeying these rules is the first direct observation of its kind, and the astrophysicists working on the project, lead by Stefanie Komossa from the Max Planck Institute for Extraterrestrial Physics (MPE), have confirmed that the several 100 million solar mass black hole was ejected at a speed of 2650 kilometres per second at a distance of 10 billion light years. The black hole’s accretion disc gas is expected to continue to feed the recoiling black hole for millions of years to come.
Artist impression of a black hole and its accretion disc being ejected from a galaxy. Optical image: Hubble Space Telescope; illustration: Max Planck Institute for Extraterrestrial Physics. The new discovery is important as it indirectly proves that black holes do merge, and that these events are sometimes accompanied by large kicks. But another implication is that there must be galaxies without black holes in their nuclei, as well as black holes which float forever in space between the galaxies, which raises a set of new questions: Did galaxies and black holes form and evolve jointly in the early Universe? Or was there a population of galaxies which had been deprived of their central black holes? And if so, how was the evolution of these galaxies different from that of galaxies that retained their black holes? By consolidating theoretical ideas with direct observations from the ground and from space, the astrophysicists are preparing to answer these questions. But whatever the outcome, this first observation will have far-reaching consequences for our understanding of galaxy formation and evolution in the early Universe.
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