Ejection of High‐Velocity Stars from the Galactic Center by an Inspiraling Intermediate‐Mass Black Hole

The presence of young stars in the immediate vicinity and strong tidal fieldof SgrA* remains unexplained. One currently popular idea for their originposits that the stars were bused in by an Intermediate-Mass Black Hole (IMBH)which has inspiraled into the Galactic Center a few million years ago. Yu and Tremaine (2003) have argued that in this case some of the old stars inthe SgrA* cusp would be ejected by hard gravitational collisions with the IMBH.Here we derive a general expression for the phase-space distribution of theejected high-velocity stars, given the distribution function of the stars inthe cusp. We compute it explicitly for the Peebles-Young distribution functionof the cusp, and make a detailed model for the time-dependent ejection of starsduring the IMBH inspiral. We find that (1) the stars are ejected in a burstlasting a few dynamical friction timescales; if the ejected stars are detectedby Gaia they are likely to be produced by a single inspiral event, (2) if theinspiral is circular than in the beginning of the burst the velocity vectors ofthe ejected stars cluster around the inspiral plane, but rapidly isotropise asthe burst proceeds, (3) if the inspiral is eccentric, then the stars areejected in a broad jet roughly perpendicular to the Runge-Lenz vector of theIMBH orbit. In a typical cusp the orbit will precess with a period of \sim 10^5years, and the rate of ejection into our part of the Galaxy (as defined by e.g.the Gaia visibility domain) will be modulated periodically. Gaia, together withthe ground-based follow-up observations, will be able to clock manyhigh-velocity stars back to their ejection from the Galactic Center, thusmeasuring some of the above phenomena. This would provide a clear signature ofthe IMBH inspiral in the past 10--20 Myr.Comment: 12 pages, including 7 figure