The Inner Density Cusp of Cold Dark Matter Halos

Abstract

I report recent results of numerical simulations designed to study the inner structure of Cold Dark Matter (CDM) halos. This work confirms the proposal of Navarro, Frenk & White (NFW) that the shape of LambdaCDM halo mass profiles differs strongly from a power law and is approximately independent of mass. The logarithmic slope of the spherically-averaged density profile, as measured by beta=-dln(rho)/dlnr, decreases monotonically towards the center, becomes shallower than isothermal (beta<2) inside a characteristic radius, r_{-2}, and shows no evidence for convergence to a well-defined asymptotic value (beta_0) at the center. The dark mass contained within the innermost radius resolved by the simulations places strong constraints on beta_0; cusps as steep as r^{-1.5} are clearly ruled out in our highest resolution simulations. A profile where the radial dependence of the slope is a simple power law, beta(r)\propto r^α, approximates the structure of halos better than the NFW profile and so may minimize errors when extrapolating our results inward to radii not yet reliably probed by numerical simulations. We compare the spherically-averaged circular velocity (V_c) profiles to the rotation curves of low surface brightness (LSB) galaxies in the samples of de Blok et al and Swaters et al. The V_c profiles of simulated CDM halos are generally consistent with the rotation curves of LSB galaxies, but there are also some clearly discrepant cases. This disagreement has been interpreted as excluding the presence of cusps, but it may also just reflect the difference between circular velocity and rotation speed likely to arise in gaseous disks embedded within realistic triaxial halos.