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 40 \arcmin \times 40 \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 ^ { -1 } ) and lie near the CMD locus of Virgo .
The implied distance to the Virgo substructure from the candidates is 14 \pm 3 kpc .
We derive mean kinematics from these 16 stars , finding a radial velocity V _ { GSR } = 153 \pm 22 km s ^ { -1 } and proper motions ( \mu _ { \alpha } \cos \delta, \mu _ { \delta } ) = ( -5.24 , -0.91 ) \pm ( 0.43 , 0.46 ) mas yr ^ { -1 } .
From the mean kinematics of these members , we determine that the Virgo progenitor was on an eccentric ( e \sim 0.8 ) orbit that recently passed near the Galactic center ( pericentric distance R _ { p } \sim 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 ( \sim 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 .