Main-Sequence Star Populations in the Virgo Overdensity Region

We present deep color-magnitude diagrams 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 main sequence 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 −1.1 Gyr, a median abundance of [Fe/H]=−0.70 −0.20 dex and a heliocentric distance of 30.9 ± 3.0 kpc for the main sequence of the Sgr Stream Branch A. The dominant main sequence populations in the two VSS/VOD fields (Λ ≈ 265◦, B ≈ 13◦) are located at a mean distance of 23.3 ± 1.6 kpc and have an age ∼ 8.2 Gyr and an abundance [Fe/H]=−0.67 −0.12 dex similar to the Sgr Stream stars. These statistically robust parameters, derived from photometry of 260 main sequence stars, are also in good agreement with the age of the main population in the Sagittarius dwarf galaxy (8.0± 1.5 Gyr). They also agree with the peak in the metallicity distribution of 2 − 3 Gyr old M-giants, [Fe/H]≈ −0.6 dex, 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 shape that is empirically calibrated with SDSS Sgr A-branch and 2MASS M giant stars. We find that the most prominent feature in the CMDs, the main sequence population at 23 kpc, is not explained by the model. Instead the model predicts in these directions a low density filamentary structure of Sgr debris stars at ∼ 9 kpc and a slightly higher concentration of Sgr stars spread over a heliocentric distance range of 42 − 53 kpc. 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 Virgo Overdensity originates from a different halo structure that has almost identical age and metallicity as some sections of the Sgr tidal stream. Subject headings: Galaxies: Individual: Name: Sagittarius, Galaxy: Abundances, Galaxies: Stellar Content, Galaxy: Halo, Galaxy: Structure