2015ApJ...806..124G

The discovery of hypervelocity stars (HVSs) leaving our galaxy with speeds of nearly 10³ km/s has provided strong evidence of the existence of a massive compact object at the galaxy's center. HVSs ejected via the disruption of stellar binaries can occasionally yield a star with km/s; here we show that this mechanism can be extended to massive black hole (MBH) mergers, where the secondary star is replaced by a MBH with mass. We find that stars that are originally bound to the secondary MBH are frequently ejected with km/s, and occasionally with velocities Å10⁵ km/s (one third the speed of light). For this reason we refer to stars ejected from these systems as ''semi-relativistic'' hypervelocity stars (SHSs). Bound to no galaxy, the velocities of these stars are so great that they can cross a significant fraction of the observable universe in the time since their ejection (several Gpc). We demonstrate that if a significant fraction of MBH mergers undergoes a phase in which their orbital eccentricity is ><≥0.5 and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving ( km/s) SHSs may be as large as 10³/Mpc3. Hundreds of SHSs will be giant stars that can be detected by future all-sky infrared surveys such as WFIRST or Euclid and proper motion surveys such as LSST, with spectroscopic follow-up being possible with the James Webb Space Telescope.