Abstract
The satellite galaxies of the Milky Way (MW) are effective probes of the underlying dark matter (DM) substructure, which is sensitive to the nature of the DM particle. In particular, a class of DM models have a power spectrum cut-off on the mass scale of dwarf galaxies and thus predict only small numbers of substructures below the cut-off mass. This makes the MW satellite system appealing to constrain the DM properties: feasible models must produce enough substructure to host the number of observed Galactic satellites. Here, we compare theoretical predictions of the abundance of DM substructure in thermal relic warm DM (WDM) models with estimates of the total satellite population of the MW. This produces conservative robust lower limits on the allowed mass, m th, of the thermal relic WDM particle. As the abundance of satellite galaxies depends on the MW halo mass, we marginalize over the corresponding uncertainties and rule out m th ≤ 2.02 keV at 95 percent confidence independently of assumptions about galaxy formation processes. Modelling some of these — in particular, the effect of reionization, which suppresses the formation of dwarf galaxies — strengthens our constraints on the DM properties and excludes models with m th ≤ 2.02 keV in our fiducial model. We also find that thermal relic models cannot produce enough satellites if the MW halo mass is M 200 ≤ 0.6 × 1012 M ☉, which imposes a lower limit on the MW halo mass in CDM. We address several observational and theoretical uncertainties and discuss how improvements in these will strengthen the DM mass constraints.
Highlights
This paper improves on previous work and strengthens the methodology used to constrain the properties of candidate warm DM (WDM) particles in several important ways, which we demonstrate using the thermal relic class of WDM models
This approach to calculating the fraction of viable WDM systems for the first time incorporates the scatter in Nsub at fixed halo mass and the uncertainty in the inferred total Milky Way (MW) satellite population. This is important, as excluding one, or both, of these sources of uncertainty produces constraints on mth that are too strict. We demonstrate this in figure 3 where, for each WDM particle mass, we plot the fraction of haloes with mass M200 = 1012 M that contain enough dark matter (DM) substructure to host the inferred population of MW satellite galaxies
This ensures that the constraints provide a robust lower limit on the mass of the thermal relic WDM particle, improving on the results reported in ref. [21] across the entire MW halo mass range considered
Summary
Simple volume corrections to the observed complement of satellite galaxies have been used already to constrain the viable parameter space of thermal relic WDM models by comparing the number of DM substructures in MW-mass haloes with the number of observed satellites [20, 21].
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