A critical reassessment of particle Dark Matter limits from dwarf satellites

TL;DR

This paper introduces a new method to more reliably estimate the dark matter density profiles in dwarf spheroidal galaxies, leading to revised, generally lower limits on dark matter annihilation signals from these systems.

Contribution

It develops a novel approach solving the Jeans equation with the dark matter profile as an output, reducing reliance on poorly constrained parameters, and reassesses the dark matter annihilation limits from dwarf galaxies.

Findings

Minimal J-factors are 2-4 times smaller than previous estimates.

Revised limits relax gamma-ray constraints on dark matter annihilation.

Using fixed parametric profiles, the minimal J-factor reduction is limited to about 1.5.

Abstract

Dwarf satellite galaxies are ideal laboratories for identifying particle Dark Matter signals. When setting limits on particle Dark Matter properties from null searches, it becomes however crucial the level at which the Dark Matter density profile within these systems is constrained by observations. In the limit in which the spherical Jeans equation is assumed to be valid for a given tracer stellar population, we study the solution of this equation having the Dark Matter mass profile as an output rather than as a trial parametric input. Within our new formulation, we address to what level dwarf spheroidal galaxies feature a reliable mass estimator. We assess then possible extrapolation of the density profiles in the inner regions and -- keeping explicit the dependence on the orbital anisotropy profile of the tracer population -- we derive general trends on the line-of-sight integral of the density profile squared, a quantity commonly dubbed $J$-factor and crucial to estimate fluxes from prompt Dark Matter pair annihilations. Taking Ursa Minor as a study case among Milky Way satellites, we perform Bayesian inference using the available kinematical data for this galaxy. Contrary to all previous studies, we avoid marginalization over quantities poorly constrained by observations or by theoretical arguments. We find minimal $J$-factors to be about 2 to 4 times smaller than commonly quoted estimates, approximately relaxing by the same amount the limit on Dark Matter pair annihilation cross section from gamma-ray surveys of Ursa Minor. At the same time, if one goes back to a fixed trial parametric form for the density, e.g. using a NFW or Burkert profile, we show that the minimal $J$ can hardly be reduced by more than a factor of 1.5.