Black hole collision may have set off fireworks in the Milky Way

Science : Evidence suggests that the now quiescent, supermassive black hole at the center of the Milky Way experienced a period of intense activity a few million years ago that produced some of the highest-energy radiation in the universe. A pair of gamma ray-emitting gas bubbles that seem to have been fueled by a violent event at the galactic core were discovered last year, along with more newborn stars and less elderly stars than had been expected, writes Ron Cowen for Science. Kelly Holley-Bockelmann of Vanderbilt University and her colleagues say this activity may have been caused by a collision between the Milky Way and the remains of a satellite galaxy that housed an intermediate-mass black hole. Gas orbiting within the innermost 5000 light years of the galaxy would have been pushed into the center, and some of that would have fallen into the central black hole and generated the gamma ray-emitting gas bubbles. Part of the gas would have become raw material for the young stars that have been observed there, and the Milky Way’s black hole and the satellite’s smaller one could have propelled old stars outward from the center as the two black holes merged. If that was the case, the outward-flung stars would have formed a ring of high-velocity stars a few thousand light-years from the center, and they could be detected by the Hubble Space Telescope.