Dark Matter Properties and Halo Central Densities

Abstract

Using an analytic model calibrated against numerical simulations, we calculate the central densities of dark matter halos in a ``conventional'' cold dark matter model with a cosmological constant (LCDM) and in a ``tilted'' model (TLCDM) with slightly modified parameters motivated by recent analyses of Ly-alpha forest data. We also calculate how warm dark matter (WDM) would modify these predicted densities by delaying halo formation and imposing phase space constraints. As a measure of central density, we adopt the quantity D_{V/2}, the density within the radius R_{V/2} at which the halo rotation curve falls to half of its maximum value, in units of the critical density. We compare the theoretical predictions to values of D_{V/2} estimated from the rotation curves of dark matter dominated disk galaxies. Assuming that dark halos are described by NFW profiles, our results suggest that the conventional LCDM model predicts excessively high dark matter densities, unless there is some selection bias in the data toward the low-concentration tail of the halo distribution. A WDM model with particle mass 0.5-1 keV provides a better match to the observational data. However, the modified cold dark matter model, TLCDM, fits the data equally well, suggesting that the solution to the ``halo cores'' problem might lie in moderate changes to cosmological parameters rather than radical changes to the properties of dark matter. If CDM halos have the steeper density profiles found by Moore et al., then neither conventional LCDM nor TLCDM can reproduce the observed central densities.