Particle Annihilation in Cold Dark Matter Micropancakes

Abstract

Cold primordial particle dark matter forms with a distribution in six-dimensional phase space closely approximating a three-dimensional sheet. Folds in the mapping of this sheet onto configuration space create ubiquitous sheetlike caustics (``micropancakes''). A typical WIMP dark matter halo has many micropancakes, each with a scale comparable to the halo itself, a width about $10^{-8}$ of the halo size and a typical maximum density up to about $10^4$ times the halo mean. It is demonstrated here that the total annihilation rate of dark matter particles is dominated by particles close to these micropancakes, so radiation is emitted predominantly from highly contorted two-dimensional surfaces rather than a filled volume. The total annihilation rate of particles is about a factor of 5 higher than predicted from N-body simulations, which cannot resolve these features. Micropancakes also produce sharp line discontinuities in the surface brightness of annihilation radiation.