Substructure Boosts to Dark Matter Annihilation from Sommerfeld Enhancement

TL;DR

The paper investigates how Sommerfeld enhancement and small-scale substructure jointly boost dark matter annihilation signals in galactic halos, and it provides a framework to quantify the total boost across subhalos via a structure quantity $\mathcal{L}(M)$ that factors the particle physics and density-field contributions.It shows that the combined boosts can reach $10^2$ to $10^9$ for a wide range of Yukawa-like dark-sector parameters, with resonances and extremely cold substructures driving the largest gains, largely independent of inner-density cusp details.Applying these boosts to dwarf spheroidal galaxies and subhalos in Via Lactea II, the study predicts gamma-ray fluxes that, in many models, dominate over the extragalactic background and could be detectable by Fermi, depending crucially on the specific particle-physics parameters.These results imply strong prospects for indirect DM detection in dSphs and subhalos and emphasize the inseparability of particle physics from astrophysical DM distribution when Sommerfeld enhancement is significant.

Abstract

The recently introduced Sommerfeld enhancement of the dark matter annihilation cross section has important implications for the detection of dark matter annihilation in subhalos in the Galactic halo. In addition to the boost to the dark matter annihilation cross section from the high densities of these subhalos with respect to the main halo, an additional boost caused by the Sommerfeld enhancement results from the fact that they are kinematically colder than the Galactic halo. If we further believe the generic prediction of CDM that in each subhalo there is an abundance of substructure which is approximately self-similar to that of the Galactic halo, then I show that additional boosts coming from the density enhancements of these small substructures and their small velocity dispersions enhance the dark matter annihilation cross section even further. I find that very large boost factors ($10^5$ to $10^9$) are obtained in a large class of models. The implications of these boost factors for the detection of dark matter annihilation from dwarf Spheroidal galaxies in the Galactic halo are such that, generically, they outshine the background gamma-ray flux and are detectable by the Fermi Gamma-ray Space Telescope.

Figures (15)

• Figure 1: Sommerfeld enhancement as a function of the particle physics parameters for two different values of the relative velocity $\beta$. Because of the scaling properties of equation (\ref{['eq:sommdiff2']}) the behavior of the Sommerfeld enhancement as a function of $\log_{10}\beta$ can actually be read off along diagonals in the plane.
• Figure 2: Sommerfeld enhancement factor as a function of the particle physics parameters assuming a one-particle Maxwell-Boltzmann velocity distribution with velocity dispersion $\sigma_v$ for four different orders of magnitude of the velocity dispersion (all velocities are relative to the speed of light).
• Figure 3: Top: Normalization of the $\mathcal{L} \propto \rho_s^2\, r_s^3/D^2$ relation of equation (\ref{['eq:lumrhosrsNFW']}) for the GNFW profile. The functional dependence of the normalization on the exponent $\gamma$ in the profile given in equation (\ref{['eq:NFW']}) is given in equation (\ref{['eq:normgammaNFW']}). Bottom: Fraction of the emission that comes from the region within $r_s$.
• Figure 4: Top: Normalization of the $\mathcal{L} \propto \rho_{-2}^2\, r_{-2}^3/D^2$ relation of equation (\ref{['eq:lumrhosrsEinasto']}) for the Einasto profile. The functional dependence of the normalization on $\alpha$ in the profile given in equation (\ref{['eq:einasto']}) is given in equation (\ref{['eq:normgammaEinasto']}). Bottom: Fraction of the emission that comes from the region within $r_{-2}$.
• Figure 5: The total (defined in Eq. [\ref{['eq:totalboost']}]) and reduced boost (defined in Eq. [\ref{['eq:redboost']}]) for subhalos of two different mass scales. The dark matter profile of all the subhalos is assumed to be well-described by an NFW profile.