Dark Matter Properties and Halo Central Densities

Using an analytic model calibrated against numerical simulations, wecalculate the central densities of dark matter halos in a ``conventional'' colddark matter model with a cosmological constant (LCDM) and in a ``tilted'' model(TLCDM) with slightly modified parameters motivated by recent analyses ofLy-alpha forest data. We also calculate how warm dark matter (WDM) would modifythese predicted densities by delaying halo formation and imposing phase spaceconstraints. 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 tohalf of its maximum value, in units of the critical density. We compare thetheoretical predictions to values of D_{V/2} estimated from the rotation curvesof dark matter dominated disk galaxies. Assuming that dark halos are describedby NFW profiles, our results suggest that the conventional LCDM model predictsexcessively high dark matter densities, unless there is some selection bias inthe data toward the low-concentration tail of the halo distribution. A WDMmodel with particle mass 0.5-1 keV provides a better match to the observationaldata. However, the modified cold dark matter model, TLCDM, fits the dataequally well, suggesting that the solution to the ``halo cores'' problem mightlie in moderate changes to cosmological parameters rather than radical changesto the properties of dark matter. If CDM halos have the steeper densityprofiles found by Moore et al., then neither conventional LCDM nor TLCDM canreproduce the observed central densities.