Hypervelocity Stars. III. The Space Density and Ejection History of Main‐Sequence Stars from the Galactic Center

We report the discovery of three new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution—we find 26 stars with v rf > 275 km s −1 and one star with v rf < − 275 km s −1 —shows that HVSs must be short-lived, probably 3-4 M ☉ main-sequence stars. Any population of hypervelocity post-main-sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main-sequence interpretation: longer lived 3 M ☉ HVSs fill our survey volume; shorter lived 4 M ☉ HVSs are missing at faint magnitudes. We infer that there are 96 ± 10 HVSs of mass 3-4 M ☉ within R < 100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5-15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a ~120 Myr old burst of HVSs is also allowed.