When Galaxies Collide

Large galaxies grow through collisions of many smaller ones (see the Perspective by Coppi). When two galaxies collide, the giant supermassive black holes that sit in their centers eventually meet and spin around one another as a binary system. In the absence of any braking forces, the black holes would continue to orbit one another for at least billions of years. However, large galaxy cores host single black holes, so other astrophysical processes must help the black hole pairs coalesce more rapidly. Mayer et al. (p. 1874, published online 7 June) performed hydrodynamical simulations which show that gas within merging galaxies slows their black holes enough so that they can bind together within just 1 million years. In simulating the decay of a binary black hole system within a gas-rich galaxy that has recently formed from the merger of two smaller spirals, the authors tracked the black holes before their coalescence to within a few light years from each other. Max et al. (p. 1877, published online 17 May) have obtained very-high-resolution infrared images of a nearby pair of spiral galaxies called NGC 6240 that have already collided--their stars and gas wrapping are around one another. Using adaptive optics techniques on the Keck telescope in Hawaii, they pinpoint the positions of two black holes that once dotted the centers of the original galaxies. Around the black holes, cones of gas and new stars are seen that may have formed in the wake of the black holes as they spiraled in toward one another. This separation indicates the effects of dynamical friction stirring the gas as it mixes together. CREDIT: MAYER ET AL.