http://arxiv.org/abs/1411.5022 The Fastest Unbound Stars in the Universe James Guillochon (1), Abraham Loeb (1) ((1) Harvard ITC) (Submitted on 18 Nov 2014) The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly 103 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∞≲104 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 M2≳105M⊙. We find that stars that are originally bound to the secondary MBH are frequently ejected with v∞>104 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 (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 ≳0.5 and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving (v∞>104 km s−1) SHS may be as large as 103 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. 20 pages, 18 figures. Submitted to ApJ My comment: Many galaxies are found to have massive black holes (MBH) in their central regions, sometimes several in a single galaxy, perhaps the result of an earlier merger. Stars have been observed near a MBH orbiting at very high speed, which makes sense. We are talking tens of thousands of km/s, maybe even more. ==quote from caption of Figure 1 on page 2== Diagram of primary production channel for SHS. 1: Two galaxies with central black holes merge. 2: Dynamical friction brings the two nuclear clusters and their host MBHs together. 3: The eccentricity of the secondary MBH’s orbit about the primary is excited by asymmetrical scattering of stars that originally orbited the primary MBH. A tightly-bound cluster of stars re- mains bound to the secondary. 4: With each passage of the secondary by the primary, a fraction of stars are ejected as SHS. ==endquote== The idea is that we see a little swarm of stars orbiting our own galaxy's central MBH. So what happens when two galaxies merge and their two MBH gradually close in on each other (together with their two attendant swarms of high-velocity stars) ? The authors show that when the two pass close to each other they strip off some of each other's fast orbiting stars. The ejected stars can in some cases then leave the galaxy with exceptionally high velocity. The authors calculate that semi-relativistic speeds, e.g. 10% of the speed of light, are possible.