Gamma Rays from Annihilations at the Galactic Center in a Physical Dark Matter Distribution

TL;DR

This paper develops a new physical model for dark matter distribution in the Galactic halo using the Jeans equation, predicting gamma-ray signals from DM annihilations that could be detected by Fermi.

Contribution

It introduces the ta-profile based on the Jeans equation with a specific entropy correlation, providing a physically motivated DM density profile for the Galactic halo.

Findings

The ta-profile predicts gamma-ray fluxes within Fermi's detection range.

It offers a less uncertain DM distribution model compared to empirical profiles.

The profile is flatter at the center than NFW but steeper than Einasto, affecting annihilation signal estimates.

Abstract

We discuss the gamma-ray signal to be expected from dark matter (DM)annihilations at the Galactic Center. To describe the DM distribution in the Galactic halo we base on the Jeans equation for self-gravitating, anisotropic equilibria. In solving the Jeans equation, we adopt the specific correlation between the density \rho(r) and the velocity dispersion \sigma^2_r(r) expressed by the powerlaw behavior of the DM `entropy' K= \sigma_r^2/\rho^{2/3} ~ r^\alpha with \alpha ~ 1.25-1.3. Indicated (among others) by several recent N-body simulations, this correlation is privileged by the form of the radial pressure term in the Jeans equation, and yields a main body profile consistent with the classic self-similar development of DM halos. In addition, we require the Jeans solutions to satisfy regular boundary conditions both at the center (finite pressure, round gravitational potential) and in the outskirts (finite overall mass). With these building blocks we derive physical solutions, dubbed `\alpha-profiles'. We find the one with \alpha=1.25, suitable for the Galaxy halo, to be intrinsically flatter at the center relative to the empirical NFW formula, yet steeper than the empirical Einasto profile. So on scales of 10^{-1} deg it yields annihilation fluxes lower by a factor 5 than the former yet higher by a factor 10 than the latter; such fluxes will eventually fall within the reach of the Fermi satellite. We show the effectiveness of the \alpha-profile in relieving the astrophysical uncertainties related to the macroscopic DM distribution, and discuss its expected performance as a tool instrumental to interpret the upcoming gamma-ray data in terms of DM annihilation.