Main Sequence Star Populations in the Virgo Overdensity Region
  We present deep CMDs for two Subaru Suprime-Cam fields in the Virgo Stellar
Stream(VSS)/Virgo Overdensity(VOD) and compare them to a field centred on the
highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading
arm, Branch A of the bifurcation. A prominent population of MS stars is
detected in all three fields and can be traced as faint as g~24 mag. Using
theoretical isochrone fitting we derive an age of 9.1(+1.0;-1.1)Gyr, a median
abundance of [Fe/H]=-0.70 (+0.15; -0.20)dex and d_helio of 30.9+-3.0kpc for the
MS of the Sgr Stream Branch A. The dominant main sequence populations in the
two VSS/VOD fields (Lsun ~265 deg, Bsun ~13 deg) are located at a mean distance
of 23.3+-1.6kpc and have an age ~8.2Gyr and an abundance
[Fe/H]=-0.67(+0.16;-0.12)dex similar to the Sgr Stream stars. These parameters
are also in good agreement with the age of the main population in the
Sagittarius dwarf (8.0+-1.5Gyr). They also agree with the peak in the
metallicity distribution of 2-3Gyr old M-giants, [Fe/H] ~ -0.6dex, in the Sgr
leading arm north. We then compare the results from the VSS/VOD fields with the
Sgr Tidal Stream model by Law & Majewski based on a triaxial Galactic halo that
is calibrated with SDSS Sgr A-branch and 2MASS M-giant stars. We find that the
most prominent feature in the CMDs, the MS population at 23kpc, is not
explained by the model. Instead the model predicts in these directions a low
density filamentary structure of Sgr debris stars at ~9kpc and a slightly
higher concentration of Sgr stars spread from 42-53kpc. At best there is only
marginal evidence for the presence of these populations in our data. Our
findings then suggest that while there are probably some Sgr debris stars
present, the dominant stellar population in the VOD originates from a different
halo structure that has almost identical age and metallicity as some sections
of the Sgr tidal stream.
