Dark matter annihilation radiation in hydrodynamic simulations of Milky Way haloes

TL;DR

This study uses high-resolution hydrodynamic simulations of Milky Way-like galaxies to predict dark matter annihilation signals, revealing deviations from standard models and broad agreement with gamma-ray observations at large galactic latitudes.

Contribution

First detailed hydrodynamic simulations of Milky Way halos showing deviations from standard dark matter profiles and implications for annihilation radiation predictions.

Findings

Dark matter halos are more contracted than NFW profiles suggest.

Inner halo regions are nearly spherical with minimal offset from stellar centers.

Predicted gamma-ray signals broadly match observations at high galactic latitudes.

Abstract

We obtain predictions for the properties of cold dark matter annihilation radiation using high resolution hydrodynamic zoom-in cosmological simulations of Milky Way-like galaxies (APOSTLE project) carried out as part of the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) programme. Galactic halos in the simulation have significantly different properties from those assumed in the "standard halo model" often used in dark matter detection studies. The formation of the galaxy causes a contraction of the dark matter halo, whose density profile develops a steeper slope than the Navarro-Frenk-White (NFW) profile between $r\approx1.5$ kpc and $r\approx10$ kpc. At smaller radii, $r\lesssim1.5$ kpc, the halos develop a flatter than NFW slope. This unexpected feature may be specific to our particular choice of subgrid physics model but nevertheless the dark matter density profiles agree within 30% as the mass resolution is increased by a factor 150. The inner regions of the halos are almost perfectly spherical (axis ratios $b/a > 0.97$ within $r=1$ kpc) and there is no offset larger than 45 pc between the centre of the stellar distribution and the centre of the dark halo. The morphology of the predicted dark matter annihilation radiation signal is in broad agreement with $\gamma$-ray observations at large Galactic latitudes ($b\gtrsim3^\circ$). At smaller angles, the inferred signal in one of our four galaxies is similar to that which is observed but it is significantly weaker in the other three.