Abstract
Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in gamma-rays. We examine their detectability by present and future gamma-ray observatories. The key innovative features of our analysis are: (i) We take into account the angular size of the dSphs; while nearby objects have higher gamma ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments. (ii) We derive DM profiles and the astrophysical J-factor (which parameterises the expected gamma-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power law distribution, with and without sub-clumps. (iii) We use a Markov Chain Monte Carlo (MCMC) technique to marginalise over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties. (iv) We use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties. Our key findings are: (i) Sub-clumps in the dSphs do not usefully boost the signal; (ii) The sensitivity of atmospheric Cherenkov telescopes to dSphs within 20 kpc with cored halos can be up to ~50 times worse than when estimated assuming them to be point-like. Even for the satellite-borne Fermi-LAT the sensitivity is significantly degraded on the relevant angular scales for long exposures, hence it is vital to consider the angular extent of the dSphs when selecting targets; (iii) No DM profile has been ruled out by current data, but using a prior on the inner dark matter cusp slope 0<=gamma<=1 provides J-factor estimates accurate to a factor of a few if an appropriate angular scale [abridged]
Highlights
The detection of γ -rays from dark matter (DM) annihilation is one of the most promising channels for indirect detection (Gunn et al 1978; Stecker 1978)
This paper extends the earlier study of Walker et al (2011) which showed that there is a critical integration angle where we can obtain a robust estimate of the J-factor, regardless of the value of the central DM cusp slope γ
The main innovative features of our analysis are that: (i) we have considered the effect of the angular size of the dwarf spheroidal galaxies (dSphs) for the first time
Summary
The detection of γ -rays from dark matter (DM) annihilation is one of the most promising channels for indirect detection (Gunn et al 1978; Stecker 1978). Knowledge of the inner slope of the DM profile is of critical importance as most of the annihilation flux comes from that region Lacking this information, several studies have focused on the detectability of these dSphs by current γ -ray observatories such as the satellite-borne Fermi-Large Area Telescope (FermiLAT) and atmospheric Cherenkov telescopes (ACTs) such as the HESS, MAGIC and VERITAS, using a small sample of cusped and cored profiles (generally one of each). Both approaches may be combined, such as in Strigari et al (2007b) and Martinez et al (2009) who rely partially on the results of structure formation simulations to constrain the inner slope and perform a fit to the data to derive the other parameters Such cosmological priors remain sufficiently uncertain that their use is inappropriate for guiding observational strategies. Those who are willing to trust the underlying modelling should proceed to Section 5 where our main results regarding the detectability of dSphs are presented in Figs 12, 15, 16 and 17 (shown later)
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