ON THE MASSES OF GALAXIES IN THE LOCAL UNIVERSE

We compare estimates of stellar mass, M(*), and dynamical mass, M(d), for a sample of galaxies from the Sloan Digital Sky Survey. Under the assumption of dynamical homology (i.e.,(M) over tilde (d) similar to sigma(2)(0)R(e), where sigma(0) is the central velocity dispersion and R(e) is the effective radius), we find a tight but strongly nonlinear relation between the two mass estimates: the best-fit relation is M(*) proportional to (M) over tilde (0.73)(d) , with an observed scatter of 0.15 dex. We also find that, at fixed M(*), the ratio M(*)/(M) over tilde (d) depends strongly on galaxy structure, as parameterized by the 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 M(*)/(M) over tilded is qualitatively and quantitatively similar to expectations from simple, spherical and isotropic dynamical models, indicating that assuming homology gives the wrong dynamical mass. To explore this possibility, we have also derived dynamical mass estimates that explicitly account for differences in galaxies' structures. Using this structure-corrected dynamical mass estimator, M(d,n), the best-fit relation is M(*) proportional to M(d,n)(0.92 +/- 0.01(+/- 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 M(*)/M(d,n) with n. We find no statistically significant systematic trends in M(*)/M(d,n) as a function of observed quantities (e.g., apparent magnitude, redshift), or as a function of tracers of stellar populations (e.g., H alpha equivalent width, mean stellar age), nor do we find significantly different behavior for different kinds of galaxies (i.e., central versus satellite galaxies, emission versus non-emission galaxies). At 99% confidence, the net differential bias in M(*)/M(d,n) across a wide range of stellar populations and star formation activities is less than or similar to 0.12 dex (approximate to 40%). The very good agreement between stellar mass and structure-corrected dynamical mass strongly suggests, but does not unambiguously prove, that (1) galaxy non-homology has a major impact on dynamical mass estimates, and (2) there are no strong systematic biases in the stellar mass-to-light ratios derived from broadband optical spectral energy distributions. Further, accepting the validity of both our stellar and dynamical mass estimates, these results suggest that the central dark-to-luminous mass ratio has a relatively weak mass dependence, but a very small scatter at fixed mass.