Hypervelocity stars. I. The spectroscopic survey

We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half-complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the two most recently discovered HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508 +/- 12 and +418 +/- 10 km s(-1), respectively. The other late B-type objects in our survey are consistent with a population of post-main-sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H](Wk) =-1: 3 and velocity dispersion 108 +/- 5 km s(-1). It is interesting to note that the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors, possibly extremely low mass objects. Two of our objects are B supergiants in the Leo A dwarf, providing the first spectroscopic evidence for star formation in this dwarf galaxy within the last similar to 30 Myr.