Enhancements to velocity-dependent dark matter interactions from tidal streams and shells in the Andromeda galaxy

TL;DR

This paper investigates how tidal streams and shells in the Andromeda galaxy can enhance dark matter annihilation signals, especially under velocity-dependent cross sections like the Sommerfeld effect, aiding indirect detection efforts.

Contribution

It models the gamma-ray boost from dark matter in tidal structures assuming velocity-dependent annihilation cross sections, providing predictions for Fermi telescope observations.

Findings

Velocity-dependent cross sections significantly increase gamma-ray emission.

Enhancement allows testing of velocity dependence through spatial correlation.

Predicted signals are within Fermi's detection capabilities for certain parameters.

Abstract

Dark matter substructure around nearby galaxies provides an interesting opportunity for confusion-free indirect detection of dark matter. We calculate the boost over a smooth background distribution of dark matter for gamma-ray emission from dark matter self-annihilations in tidal structure in M31, assuming a cross-section inversely proportional to the relative velocities of the dark matter particles as proposed by the Sommerfeld effect. The low velocity of the material in the structure results in a significant increase in gamma-ray emission compared to both the background halo and the predicted emission for a velocity-independent cross section. We also calculate the expected signal for Fermi, for reasonable choices of the dark matter parameters. We find that for a cross section proportional to the inverse-square of the relative velocity, the enhancement to the annihilation rate is sufficient to test the velocity dependence of the cross section by spatial correlation with the stellar component of the stream, given sufficient detector sensitivity.