Hypervelocity Stars from a Supermassive Black Hole–Intermediate-mass Black Hole Binary

Abstract

Abstract In this paper we consider a scenario in which the currently observed hypervelocity stars in our Galaxy have been ejected from the Galactic center as a result of dynamical interactions with an intermediate-mass black hole (IMBH) orbiting the central supermassive black hole (SMBH). By performing three-body scattering experiments, we calculate the distribution of the ejected stars’ velocities given various parameters of the IMBH–SMBH binary: IMBH mass, semimajor axis, and eccentricity. We also calculate the rates of change of the BH binary orbital elements due to those stellar ejections. One of our new findings is that the ejection rate depends (although mildly) on the rotation of the stellar nucleus (its total angular momentum). We also compare the ejection velocity distribution with that produced by the Hills mechanism (stellar binary disruption) and find that the latter produces faster stars on average. Also, the IMBH mechanism produces an ejection velocity distribution that is flattened toward the BH binary plane, while the Hills mechanism produces a spherically symmetric one. The results of this paper will allow us in the future to model the ejection of stars by an evolving BH binary and compare both models with Gaia observations, for a wide variety of environments (galactic nuclei, globular clusters, the Large Magellanic Clouds, etc.).