Baryonic effects on the detectability of annihilation radiation from dark matter subhaloes around the Milky Way

TL;DR

This study uses high-resolution simulations to assess how baryonic processes influence dark matter annihilation signals from Milky Way-like galaxies, revealing significant suppression of subhalo signals and increased smooth halo emission.

Contribution

It provides the first detailed analysis of baryonic effects on dark matter annihilation signals using high-resolution, full-physics simulations of Milky Way analogs.

Findings

Baryonic processes increase smooth halo annihilation emission by a factor of three.

Subhalo fluxes are reduced by nearly an order of magnitude due to tidal effects.

Detection of subhalo signals is unlikely before the inner Galaxy's emission is observed.

Abstract

We use six, high-resolution $\Lambda$CDM simulations of galaxy formation to study how emission from dark matter annihilation is affected by baryonic processes. These simulations produce isolated, disc-dominated galaxies with structure, stellar populations, and stellar and halo masses comparable to those of the Milky Way. They resolve dark matter structures with mass above $\sim 10^6$ $\rm M_{\odot}$ and are each available in both full-physics and dark-matter-only versions. In the full-physics case, formation of the stellar galaxy enhances annihilation radiation from the dominant smooth component of the galactic halo by a factor of three, and its central concentration increases substantially. In contrast, subhalo fluxes are $reduced$ by almost an order of magnitude, partly because of changes in internal structure, partly because of increased tidal effects; they drop relative to the flux from the smooth halo by 1.5 orders of magnitude. The expected flux from the brightest subhalo is four orders of magnitude below that from the smooth halo, making it very unlikely that any subhalo will be detected before robust detection of the inner Galaxy. We use recent simulations of halo structure across the full $\Lambda$CDM mass range to extrapolate to the smallest (Earth-mass) subhaloes, concluding, in contrast to earlier work, that the total annihilation flux from Milky Way subhaloes will be less than that from the smooth halo, as viewed both from the Sun and by a distant observer. Fermi-LAT may marginally resolve annihilation radiation from the very brightest subhaloes, which, typically, will contain stars.