The Cosmological Significance of Disk Galaxy Rotation Curves

Abstract

We use the rotation curves of more than 100 disk galaxies to examine whether the structure of their surrounding dark halos is consistent with the universal density profile proposed by Navarro, Frenk & White (NFW profiles). Rotation curve shape is a strong function of galaxy surface brightness: low-surface brightness galaxies (LSBs) have slowly rising rotation curves while in high-surface brightness systems the rotation speed rises sharply and stays flat or even declines beyond the optical radius. These observations are consistent with NFW halo profiles, with the possible exception of a few LSBs where the rotation curves are better described by shallower central density profiles. Consistency with observational trends requires that halos have lower characteristic densities than expected in the standard biased cold dark matter (CDM) scenario, but roughly compatible with COBE-normalized, low-density, flat CDM universes. The data also imply that disk mass-to-light ratios increase gently with luminosity, and that the halo circular velocity, V_200, is not directly proportional to the disk rotation speed, V_rot. Slowly rotating disks are surrounded by halos of higher circular velocities, whereas faster rotators (V_rot > 150 km/s) are all surrounded by halos of similar mass, corresponding to V_200 ~ 200 km/s. We speculate that this is because the efficiency of assembly of baryons into galaxies is high in massive halos, leading to disks too massive to be stable in systems that exceed the ``critical'' V_200 ~ 200 km/s. This modeling also provides a natural explanation for the distribution of sizes and rotation speeds of disk galaxies. (Abridged.)