Three-Body Kick to a Bright Quasar Out of Its Galaxy during a Merger

The quasar HE0450-2958 was recently discovered to reside ~7kpc away from agalaxy that was likely disturbed by a recent merger. The lack of a massivespheroid of stars around the quasar raised the unlikely suggestion that it mayhave formed in a dark galaxy. Here we explain this discovery as a naturalconsequence of a dynamical kick imparted to the quasar as it interacted with abinary black hole system during a galaxy merger event. The typical stallingradius for a ~10^9 solar mass binary provides a kick of order the escapevelocity of the stellar spheroid, bringing the quasar out to around theobserved radius before it turns around. This is consistent with the observedlow relative velocity between the quasar and the merger-remnant galaxy. The gascarried with the black hole throughout the three-body interaction fuels thequasar for the duration of its journey, ~2x10^7 years. Gravitational radiationrecoil could not have produced the required kick.Comment: 5 pages, 2 figures, ApJ Letters, in pres