Dynamical Constraints on Disk Masses

While the total interior mass of a galaxy is reasonably well determined by a good rotation curve, the relative contributions from disk, bulge and halo are only weakly constrained by one-dimensional data. Barred galaxies are intrinsically more complicated, but provide much tighter constraints on the disk masses and support the idea that most of the mass in the inner parts of bright galaxies is in their stars. There appears to be no systematic difference in dark matter content between barred and unbarred galaxies, consistent with the theoretical result that the global stability of galaxies with dense centers does not depend on their halo fraction. The rotation curve shapes of lower luminosity and low-surface-brightness galaxies, on the other hand, indicate significant mass in the DM halo even near their centers. We find that most DM halos appear to have large cores, inconsistent with the predictions from cosmological simulations. We also show that such large-core halos can result from compression by disk infall of physically reasonable initial halos. Maximum disks, while apparently required by the data, do seem to present some puzzles; most notably they re-open the old disk-halo “conspiracy” issue and incorrectly predict that surface brightness should be a second parameter in the Tully-Fisher relation. 1. Surface brightness and luminosity The question of what fraction of the central attraction should be attributed to dark matter (DM) within the disk of a spiral galaxy is still unresolved. Most of the controversy surrounds the higher luminosity, high surface brightness (HSB) galaxies which I argue here have very little DM in their inner regions. As the rotation curves of most LSB and low-luminosity galaxies, on the other hand, do not have the shapes predicted from their light distributions, a significant DM contribution is required in their inner parts. While this conclusion has been established from much careful work on individual galaxies, trends suggesting increasing DM content towards both types of galaxy can also be found in a statistical analysis of a large galaxy sample. If the surface density of a disk of mass M decreases exponentially in its outer parts with scale length R0, the rotation speed arising from the disk alone