Searching for dark matter annihilation from individual halos: uncertainties, scatter and signal-to-noise ratios

TL;DR

This paper reviews the uncertainties and variability in predicting gamma-ray signals from dark matter annihilation in individual halos, highlighting the potential of group-scale halos for future detection efforts.

Contribution

It provides a comprehensive analysis of the uncertainties in halo concentration, substructure, and other factors affecting annihilation flux predictions, and evaluates detection prospects considering background noise.

Findings

Uncertainty in halo concentration leads to up to two orders of magnitude variation in flux predictions.

Halo-to-halo scatter in annihilation luminosity is approximately a factor of 2.5.

Group-scale halos are promising targets for upcoming gamma-ray dark matter searches.

Abstract

Individual extragalactic dark matter halos, such as those associated with nearby galaxies and galaxy clusters, are promising targets for searches for gamma-rays from dark matter annihilation. We review the predictions for the annihilation flux from individual halos, focusing on the effect of current uncertainties in the concentration-mass relation and the contribution from halo substructure, and also estimating the intrinsic halo-to-halo scatter expected. After careful consideration of recent simulation results, we conclude that the concentrations of the smallest halos, while well determined at high redshift, are still uncertain by a factor of 4-6 when extrapolated to low redshift. This in turn produces up to two orders of magnitude uncertainty in the predicted annihilation flux for any halo mass above this scale. Substructure evolution, the small-scale cutoff to the power spectrum, cosmology, and baryonic effects all introduce smaller, though cumulative, uncertainties. We then consider intrinsic variations from halo to halo. These arise from variations in concentration and substructure, leading to a scatter of $\sim$ 2.5 in the predicted annihilation luminosity. Finally, we consider the problem of detecting gamma-rays from annihilation, given the expected contributions from other sources. We estimate the signal-to-noise ratio for gamma-ray detection as a function of halo mass, assuming that cosmic rays from star formation are the main noise source in the detection. This calculation suggests that group-scale halos, individually or in stacks, may be a particularly interesting target for the next generation of annihilation searches.