We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a
heliocentric radial velocity of +853+-12 km/s, the largest velocity ever
observed in the Milky Way halo. The star is either a hot blue horizontal branch
star or a B9 main sequence star with a heliocentric distance ~55 kpc. Corrected
for the solar reflex motion and to the local standard of rest, the Galactic
rest-frame velocity is +709 km/s.
Because its radial velocity vector points 173.8 deg from the Galactic center,
we suggest that this star is the first example of a hyper-velocity star ejected
from the Galactic center as predicted by Hills and later discussed by Yu &
Tremaine. The star has [Fe/H]~0, consistent with a Galactic center origin, and
a travel time of <80 Myr from the Galactic center, consistent with its stellar
lifetime. If the star is indeed traveling from the Galactic center, it should
have a proper motion of 0.3 mas/yr observable with GAIA. Identifying additional
hyper-velocity stars throughout the halo will constrain the production rate
history of hyper-velocity stars at the Galactic center.Comment: 4 pages, submitted to ApJ Letter