Stellar mass-to-light ratios and the Tully-Fisher relation

Abstract

We have used a suite of simplified spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light ratios (M/Ls) within and among galaxies, amounting to factors of between 3 and 7 in the optical, and factors of 2 in the near-infrared. Under the assumption of a universal spiral galaxy IMF, our models show a strong and robust correlation between stellar M/L and the optical colors of the integrated stellar populations. Using observed maximum disk M/Ls we conclude that a Salpeter Initial Mass Function (IMF) has too many low-mass stars per unit luminosity, but that an IMF similar to the Salpeter IMF at the high-mass end with less low-mass stars (giving stellar M/Ls 30% or more lower than the Salpeter value) is consistent with the maximum disk constraints. We apply the model trends in stellar M/L with color to the Tully-Fisher (TF) relation. We find that the stellar mass TF relation is relatively steep and has modest scatter, and is independent of the passband and color used to derive the stellar masses. The difference in slope between the optical (especially blue) and near-infrared TF relations is due to the combined effects of dust attenuation and stellar M/L variations with galaxy mass. We find that the baryonic TF relation has a slope of 3.5 (with random and systematic errors of +/- 0.2 each). Since we have normalized the stellar M/L to be as high as can possibly be allowed by maximum disk constraints, the slope of the baryonic TF relation will be somewhat shallower than 3.5 if all disks are substantially sub-maximal. [Abridged]