A successful targeted search for hypervelocity stars

Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following the discovery of the first HVS by Brown and collaborators, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0 +/- 305120 and SDSS J091759.5 +/- 672238, traveling with Galactic rest-frame velocities of at least +558 +/- 12 and +638 +/- 12 km s(-1), respectively. Assuming the HVSs are B8 main-sequence stars, they are at distances of similar to 75 and similar to 55 kpc, respectively, and have travel times from the Galactic center consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg(2) survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major infall event at the Galactic center in the last similar to 160 Myr.