Modeling the Stellar Populations in the Canis Major Overdensity: The Relation between the Old and Young Populations

摘要

We analyze the stellar populations of the Canis Major stellar overdensity using quantitative color-magnitude diagram (CMD) fitting techniques. The analysis is based on photometry obtained with the Wide Field Imager at the 2.2 m telescope at La Silla for several fields near the probable center of the overdensity. 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] ~ -1.0, a distance of ~7.5 kpc, a line-of-sight depth σlos of 1.5 ± 0.2 kpc, and a characteristic age range of 3-6 Gyr. However, the spread in ages and the possible presence of a ~10 Gyr old population cannot 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 metallicity similar to the old stars, they are equidistant and therefore cospatial with the old stars; if the young stars have close to solar metallicity they are more distant (~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 cospatial.