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_solar 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_solar HVSs fill our survey volume; shorter lived 4 M_solar HVSs are missing at faint magnitudes. We infer that there are 96+/-10 HVSs of mass 3-4 M_solar 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.