Investigating the Origins of Dark Matter Halo Density Profiles

Although high-resolution N-body simulations make robust empirical predictionsfor the density distribution within cold dark matter halos, these studies haveyielded little physical insight into the origins of the distribution. Weinvestigate the problem using analytic and semi-analytic approaches. Simpleanalytic considerations suggest that the inner slope of dark matter haloscannot be steeper than alpha=2 (rho ~ r^-alpha), with alpha=1.5-1.7 being amore realistic upper limit. Our analysis suggests that any number of effects,both real (angular momentum from tidal torques, secondary perturbations) andartificial (two-body interactions, the accuracy of the numerical integrator,round-off errors), will result in shallower slopes. We also find that the halosshould exhibit a well-defined relation between r_peri/r_apo and j_theta/j_r. Wederive this relation analytically and speculate that it may be "universal".Using a semi-analytic scheme based on Ryden & Gunn (1987), we further explorethe relationship between the specific angular momentum distribution in a haloand its density profile. For now we restrict ourselves to halos that formprimarily via nearly-smooth accretion of matter, and only consider the specificangular momentum generated by secondary perturbations associated with the colddark matter spectrum of density fluctuations. Compared to those formed inN-body simulations, our ``semi-analytic'' halos are more extended, have flatterrotation curves and have higher specific angular momentum, even though we havenot yet taken into account the effects of tidal torques. Whether the densityprofiles of numerical halos is indeed the result of loss in angular momentumoutside the central region, and whether this loss is a feature of hierarchicalmerging and major mergers in particular, is under investigation.Comment: 50 pages, incl. 9 figs; accepted to Ap