Three-Body Kick to a Bright Quasar out of Its Galaxy During a Merger

Abstract

The quasar HE0450-2958 was recently discovered to reside ~7kpc away from a galaxy that was likely disturbed by a recent merger. The lack of a massive spheroid of stars around the quasar raised the unlikely suggestion that it may have formed in a dark galaxy. Here we explain this discovery as a natural consequence of a dynamical kick imparted to the quasar as it interacted with a binary black hole system during a galaxy merger event. The typical stalling radius for a ~10^9 solar mass binary provides a kick of order the escape velocity of the stellar spheroid, bringing the quasar out to around the observed radius before it turns around. This is consistent with the observed low relative velocity between the quasar and the merger-remnant galaxy. The gas carried with the black hole throughout the three-body interaction fuels the quasar for the duration of its journey, ~2x10^7 years. Gravitational radiation recoil could not have produced the required kick.