Deriving the velocity distribution of Galactic dark matter particles from the rotation curve data

Abstract

The velocity distribution function (VDF) of the hypothetical weakly interacting massive particles (WIMPs), currently the most favored candidate for the dark matter in the Galaxy, is determined directly from the circular speed (``rotation'') curve data of the Galaxy assuming isotropic VDF. This is done by ``inverting''---using Eddington's method---the Navarro-Frenk-White universal density profile of the dark matter halo of the Galaxy, the parameters of which are determined by using the Markov chain Monte Carlo technique from a recently compiled set of observational data on the Galaxy's rotation curve extended to distances well beyond the visible edge of the disk of the Galaxy. The derived most-likely local isotropic VDF strongly differs from the Maxwellian form assumed in the ``standard halo model'' customarily used in the analysis of the results of WIMP direct-detection experiments. A parametrized (non-Maxwellian) form of the derived most-likely local VDF is given. The astrophysical ``g factor'' that determines the effect of the WIMP VDF on the expected event rate in a direct-detection experiment can be lower for the derived most-likely VDF than that for the best Maxwellian fit to it by as much as 2 orders of magnitude at the lowest WIMP mass threshold of a typical experiment.