Mass-to-light Ratios of Spatially Resolved Stellar Populations in M31

A galaxy’s stellar mass-to-light ratio (M*/L) is a useful tool for converting luminosity to stellar mass (M*). However, the practical utility of M*/L inferred from stellar population synthesis (SPS) models is limited by mismatches between the real and assumed models for star formation history (SFH) and dust geometry, both of which vary within galaxies. We present an analysis of the spatial variations in M*/L and their dependence on color, SFH, and dust across the disk of the nearby spiral galaxy M31. We derive a map of M* from color-magnitude diagrams of resolved stars in the Panchromatic Hubble Andromeda Treasury (PHAT) survey, then combine this M* map with the observed surface brightness to measure color vs. M*/L relations (CMLRs) in the optical and mid-infrared. We show that the CMLR residuals correlate with recent SFH, such that quiescent regions are offset to higher M*/L than star-forming regions at a fixed color. Finally, we find a flatter optical CMLR than any SPS-based CMLRs in the literature. We show that this is an effect of dust geometry, which is typically neglected but should be accounted for when using optical data to map M*, particularly for highly inclined galaxies.