The Overdensity in Virgo, Sagittarius Debris, and the Asymmetric Spheroid

Abstract

We investigate the relationship between several previously identified Galactic halo stellar structures in the direction of Virgo using imaging and spectroscopic observations of F turnoff stars and blue horizontal branch stars from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We show that the Sagittarius dwarf leading tidal tail does not pass through the solar neighborhood; it misses the Sun by more than 15 kpc, passing through the Galactic plane outside the Solar Circle. It also is not spatially coincident with the large stellar overdensity S297+63-20.5 in the Virgo constellation. S297+63-20.5 has a distinct turnoff color and kinematics. Faint (g ~ 20.3) turnoff stars in S297+63-20.5 have line-of-sight, Galactic standard of rest velocities V(GSR)= 130 +/- 10 km/s, opposite in sign to infalling Sgr tail stars. The path of the Sgr leading tidal tail is also inconsistent with the positions of some of the nearer stars with which it has been associated, and whose velocities have favored models with prolate Milky Way potentials. We additionally show that the number densities of brighter (g ~ 19.8) F turnoff stars are not symmetric about the Galactic center, and that this discrepancy is not primarily due to the S297+63-20.5 moving group. Either the spheroid is asymmetric about the Galactic center, or there are additional substructures that conspire to be on the same side of the Galaxy as S297+63-20.5. The S297+63-20.5 overdensity in Virgo is likely associated with two other previously identified Virgo substructures: the Virgo Stellar Stream (VSS) and the Virgo Overdensity (VOD). However, the velocity difference between the VSS and S297+63-20.5 and the difference in distance estimates between the VOD and S297+63-20.5 must be reconciled.

10 figures, ApJ in press

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