Baryons and Dark Matter in Disk Galaxies

The goal is to measure the properties of dark halos of spiral galaxies from the decomposition of HI rotation curves, in order to derive parameters for their dark halos (e.g. the dark matter density and core radius) as a function of galaxy luminosity. Separating the luminous and dark matter components of the rotation curve is an old and degenerate problem. Dynamical estimates of the surface density of the baryonic disk are often been used to break the degeneracy. This usually gives low density submaximal disks with compact dark matter halos that dominate the rotation curve almost everywhere. The surface density of the disk comes from the vertical velocity dispersion and the scale height of the disk: these two parameters must be for the same stellar population. In practice, the scale height is usually for old disk stars, while the measured velocity dispersion is for the combined light of the young and old disk populations. The young stars are bright and kinematically cold, and the combined spectrum leads to an underestimate of the stellar surface density. From high S/N data, it is possible to measure the velocity dispersion of the old hot disk without contamination from the young cold disk. Two dynamical tracers are used for some large nearby spirals: high resolution integrated light spectra of the stellar disk, and velocities of several hundred individual disk planetary nebulae. The two tracers agree well, and give maximal disk surface densities. This validates our approach to the scaling laws for dark halos, which showed how the halo density and core radius scale with luminosity. The halo density gives an estimate of the assembly time of halos of different masses. The baryon content of dwarfs indicates that halos with circular
velocities below about 40 km/s are almost completely dark. (Based on work with J. Kormendy, S. Aniyan, M. Arnaboldi, O. Gerhard et al.)