Contamination of stellar-kinematic samples and uncertainty about dark matter annihilation profiles in ultrafaint dwarf galaxies: the example of Segue I

TL;DR

This paper investigates how contamination in stellar-kinematic samples affects dark matter density estimates in ultrafaint dwarf galaxies, highlighting the potential for significant overestimation of gamma-ray signals from dark matter annihilation.

Contribution

It demonstrates that contamination can cause large overestimations of J-factors and emphasizes the need for careful analysis of stellar membership to accurately interpret dark matter signals.

Findings

Contamination can overestimate J-factors by orders of magnitude.

Segue I's J-factor estimates are particularly sensitive to stellar membership uncertainties.

Robust dark matter inferences require explicit contamination analysis.

Abstract

The expected gamma-ray flux coming from dark matter annihilation in dwarf spheroidal (dSph) galaxies depends on the so-called `J-factor', the integral of the squared dark matter density along the line-of-sight. We examine the degree to which estimates of J are sensitive to contamination (by foreground Milky Way stars and stellar streams) of the stellar-kinematic samples that are used to infer dark matter densities in `ultrafaint' dSphs. Applying standard kinematic analyses to hundreds of mock data sets that include varying levels of contamination, we find that mis-classified contaminants can cause J-factors to be overestimated by orders of magnitude. Stellar-kinematic data sets for which we obtain such biased estimates tend 1) to include relatively large fractions of stars with ambiguous membership status, and 2) to give estimates for J that are sensitive to specific choices about how to weight and/or to exclude stars with ambiguous status. Comparing publicly-available stellar-kinematic samples for the nearby dSphs Reticulum~II and Segue~I, we find that only the latter displays both of these characteristics. Estimates of Segue~I's J-factor should therefore be regarded with a larger degree of caution when planning and interpreting gamma-ray observations. Moreover, robust interpretations regarding dark matter annihilation in dSph galaxies in general will require explicit examination of how interlopers might affect the inferred dark matter density profile.