Segue 2: A Prototype of the Population of Satellites of Satellites
  We announce the discovery of a new Milky Way satellite Segue 2 found in the
data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE).
We followed this up with deeper imaging and spectroscopy on the Multiple Mirror
Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius
of 34 pc and a systemic velocity of -40$ km/s. Our MMT data also provides
evidence for a stream around Segue 2 at a similar heliocentric velocity, and
the SEGUE data show that it is also present in neighboring fields. We resolve
the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about
7 km/s. This object shows points of comparison with other recent discoveries,
Segue 1, Boo II and Coma. We speculate that all four objects may be
representatives of a population of satellites of satellites -- survivors of
accretion events that destroyed their larger but less dense parents. They are
likely to have formed at redshifts z > 10 and are good candidates for fossils
of the reionization epoch.
