The Origin of the Virgo Stellar Substructure
  We present three-dimensional space velocities of stars selected to be
consistent with membership in the Virgo stellar substructure. Candidates were
selected from SA 103, a single 40x40 arcmin field from our proper motion (PM)
survey in Kapteyn's Selected Areas (SAs), based on the PMs, SDSS photometry,
and follow-up spectroscopy of 215 stars. The signature of the Virgo
substructure is clear in the SDSS color-magnitude diagram (CMD) centered on SA
103, and 16 stars are identified that have high Galactocentric-frame radial
velocities (V_GSR > 50 km/s) and lie near the CMD locus of Virgo. The implied
distance to the Virgo substructure from the candidates is 14+/-3 kpc. We derive
mean kinematics from these 16 stars, finding a radial velocity V_GSR = 153+/-22
km/s and proper motions (mu_alpha*cos(delta), mu_delta) = (-5.24,
-0.91)+/-(0.43, 0.46) mas/yr. From the mean kinematics of these members, we
determine that the Virgo progenitor was on an eccentric (e ~ 0.8) orbit that
recently passed near the Galactic center (pericentric distance R_p ~ 6 kpc).
This destructive orbit is consistent with the idea that the substructure(s) in
Virgo originated in the tidal disruption of a Milky Way satellite. N-body
simulations suggest that the entire cloud-like Virgo substructure (encompassing
the "Virgo Overdensity" and the "Virgo Stellar Stream") is likely the tidal
debris remnant from a recently-disrupted massive (~10^9 M_sun) dwarf galaxy.
The model also suggests that some other known stellar overdensities in the
Milky Way halo (e.g., the Pisces Overdensity and debris near NGC 2419 and SEGUE
1) are explained by the disruption of the Virgo progenitor.
