The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly $10^{ 3}$ km s$^{ -1}$ has provided strong evidence towards the existence of a massive compact object at the galaxy's center. HVS ejected via the disruption of stellar binaries can occasionally yield a star with $v_{ \infty} \lesssim 10^4$ km s$^{ -1}$, 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 $M_2 \gtrsim 10^5 M_{ \odot}$. We find that stars that are originally bound to the secondary MBH are frequently ejected with $v_{ \infty} > 10^4$ km s$^{ -1}$, and occasionally with velocities $\sim 10^5$ km s$^{ -1}$ (one third the speed of light), for this reason we refer to stars ejected from these systems as "semi-relativistic" hypervelocity stars (SHS). 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 undergo a phase in which their orbital eccentricity is $\gtrsim 0.5$ and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving ($v_{ \infty} > 10^{ 4}$ km s$^{ -1}$) SHS may be as large as $10^{ 3}$ Mpc$^{ -3}$. Hundreds of the SHS will be giant stars that could 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 JWST.
Categories: astro-ph.GA astro-ph.CO