Matching the mass function of Milky Way satellites in competing dark matter models

Abstract

Any successful model of dark matter must explain the diversity of observed Milky Way (MW) satellite density profiles, from very dense ultrafaints to large, low density satellites such as Crater~II that appear to be larger their anticipated host dark matter haloes. We consider cold dark matter (CDM), warm dark matter (WDM, 3.3keV thermal relic power spectrum), and a self-interacting dark matter model (SIDM) that induces gravothermal collapse in low mass subhaloes. Predictions for these density profiles are complicated by the limitations of simulation resolution in the stripping of subhaloes by the MW system, therefore we make predictions for satellite properties in these three models using $N$-body simulations combined with a semi-analytic halo stripping algorithm. We find that most CDM and WDM subhaloes of mass $>10^{8}$$M_{\odot}$ are large enough after stripping to fit most satellites; however, the required amount of stripping often requires a stronger tidal field than is available on the subhalo's orbit. The lower concentrations of WDM subhaloes enable this model to explain the required satellite masses with less stripping than is necessary for CDM, and is thus consistent with orbits of larger pericentres. SIDM cores offer the best fits to massive, low density satellites at the expense of predicting many $>10^{9}$$M_{\odot}$ subhaloes to host low density satellites with no observed analogue. We conclude that an SIDM model must have a very high velocity-dependent cross-section in order to match all satellites, and that WDM offers a marginally better fit than CDM to the MW satellite mass function.