The Fastest Unbound Stars in the Universe

The discovery of hypervelocity stars (HVSs) leaving our galaxy with speeds of nearly 10³ 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}_∞ } ≲ {{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}≳ {{10}⁵}{{M}_⊙ }. We find that stars that are originally bound to the secondary MBH are frequently ejected with {{v}_∞ }\gt {{10}⁴} km s^-1, and occasionally with velocities ∼10⁵ 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 ≳0.5 and their periapse distance is tens of the primary’s Schwarzschild radius, the space density of fast-moving ({{v}_∞ }\gt {{10}⁴} km s^-1) SHSs may be as large as 10³ 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.