We analyze the stellar populations of the Canis Major stellar over-density , using quantitative color-magnitude diagram ( CMD ) fitting techniques .
The analysis is based on photometry obtained with the Wide Field Imager at the 2.2m telescope at La Silla for several fields near the probable center of the over-density .
A modified version of the MATCH software package was applied to fit the observed CMDs , enabling us to constrain the properties of the old and young stellar populations that appear to be present .
For the old population we find [ Fe/H ] \sim -1.0 , a distance of \sim 7.5 kpc and a line-of-sight depth \sigma _ { los } of 1.5 \pm 0.2 kpc and a characteristic age range of 3-6 Gyrs .
However , the spread in ages and the possible presence of a \sim 10 Gyr old population can not be constrained .
The young main-sequence is found to have an age spread ; ages must range from a few hundred Myr to 2 Gyr .
Because of the degeneracy between distance and metallicity in CMDs the estimates of these parameters are strongly correlated and two scenarios are consistent with the data : if the young stars have a similar metallicity to the old stars , they are equidistant and therefore co-spatial with the old stars ; if the young stars have close to solar metallicity they are more distant ( \sim 9 kpc ) .
The relatively low metallicity of the old main-sequence favors the interpretation that CMa is the remnant of an accreted dwarf galaxy .
Spectroscopic metallicity measurements are needed to determine whether the young main-sequence is co-spatial .