Dark Matter Halos and Disk Rotation Curves

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Cosmological N--body simulations have revealed a remarkable similarity in the structure of dark matter halos formed in hierarchically clustering universes. Regardless of halo mass, cosmological parameters, and power spectrum of initial density fluctuations, the spherically averaged density profiles of dark matter halos have a universal shape. The logarithmic slope of this profile is shallower than isothermal near the center, and steepens gently outwards, becoming steeper than isothermal near the halo virial radius. This profile can be well approximated by a simple formula with only two free parameters: halo mass and ``characteristic'' density, e.g., the density at the radius where the logarithmic slope equals the isothermal value. This characteristic density is proportional to the mean density of the universe at the time of collapse of each system, and decreases systematically with increasing halo mass, reflecting the later collapse of more massive halos. I use these results to examine what constraints can be derived for Cold Dark Matter models from the rotation curves of disk galaxies.