The Fastest Unbound Stars in the Universe

Abstract

The discovery of hypervelocity stars (HVSs) leaving our galaxy with speeds of nearly 10(3) km s(-1) 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 v(infinity) less than or similar to 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 greater than or similar to 10(5)M(circle dot). We find that stars that are originally bound to the secondary MBH are frequently ejected with v(infinity) > 10(4) km s(-1), and occasionally with velocities similar to 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 (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 greater than or similar to 0.5 and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving (v(infinity) > 10(4) km s(-1)) SHSs may be as large as 10(3) Mpc(-3). 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.