On the Masses of Galaxies in the Local Universe

Abstract: We compare estimates of stellar mass, Mstar, and dynamical mass,Mdyn,for a sample of galaxies from the Sloan Digital Sky Survey (SDSS). We assume dynamical homology (i.e., Mdyn = dispersion*2 * Reff, and we find a tight but strongly non-linear relation: the best fit relation is Mstar = Mdyn0.73, with an observed scatter of 0.15 dex. We also find that, at fixed Mstar, the ratio Mstar/Mdyn depends strongly on galaxy structure, as parameterized by Sersic index, n. The size of the differential effect is on the order of 0.6 dex across 2 < n < 10. The apparent n-dependence of Mstar/Mdyn is similar to expectations from simple models, indicating that assuming homology gives the wrong dynamical mass. We have also derived dynamical mass estimates that explicitly account for differences in galaxies' profiles. Using this `structure-corrected' dynamical mass estimator, M(dyn,n), the best fit relation is Mstar = M(dyn,n)*(0.92 +- 0.08) with an observed scatter of 0.13 dex. While the data are thus consistent with a linear relation, they do prefer a slightly shallower slope. Further, we see only a small residual trend in Mstar/M(dyn,n) with n. We find no statistically significant systematic trends in Mstar/M(dyn,n) as a function of observed quantities (e.g, apparent magnitude, redshift), or as a function of tracers of stellar populations. The net differential bias in Mstar/M(dyn,n) across a wide range of stellar populations and star formation activities is <= 0.12 dex. The very good agreement between stellar mass and structure-corrected dynamical mass strongly suggests that: 1.) galaxy non-homology has a major impact on dynamical mass estimates, and 2. there are not strong systematic biases in the stellar mass-to-light ratios derived from broadband optical SEDs. Further, these results suggest that that the central dark-to-luminous mass ratio has a relatively weak mass dependence.