Constraining the warm dark matter particle mass with Milky Way satellites

Particle physics theories predict the existence of particles (such as keV mass sterile neutrinos) which could behave as warm dark matter (WDM), producing a cutoff in the linear density power spectrum on the scale of dwarf galaxies. Thus, the abundance of Milky Way satellite galaxies depends on the mass of the warm particle and also scales with the mass of the host galactic halo. We use the GALFORM semi-analytic model of galaxy formation to compare predicted satellite luminosity functions to Milky Way data and determine a lower bound on the thermally produced WDM particle mass. This depends strongly on the Milky Way halo mass and, to some extent, on the baryonic physics assumed. For our fiducial model, we find that for a particle mass of 3.3 keV (the 2 lower limit from an analysis of the Lyman forest by Viel et al.) the Milky Way halo mass is required to be >1.4 10 12 M . For this same fiducial model, we also find that all WDM particle masses are ruled out (at 95 per cent confidence) if the Milky Way halo mass is smaller than 1.1 10 12 M , while if the mass of the Galactic halo is greater than 1.8 10 12 M , only WDM particle masses larger than 2 keV are allowed.