Constraints on the properties of warm dark matter using the satellite
  galaxies of the Milky Way
  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,
$m_\mathrm{th}$, 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 $m_\mathrm{th} \leq 2.02\,
\mathrm{keV}$ at 95 per cent 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 $m_\mathrm{th}
\leq 3.99\, \mathrm{keV}$ in our fiducial model. We also find that thermal
relic models cannot produce enough satellites if the MW halo mass is
$M_{200}\leq 0.6\times 10^{12}\, \mathrm{M_\odot}$, 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.
