Seminarium "High Energy, Cosmology and Astro-particle Physics (HECA)"

Low-mass galaxies are powerful tools with which to investigate departures from the standard cosmological paradigm in models that suppress the abundance of small dark matter substructures. One of the simplest metrics that can be used to compare different models is the abundance of satellite galaxies in the Milky Way. Viable dark matter models must produce enough substructure to host the observed number of Galactic satellites. Here, we scrutinize the predictions of the neutrino Minimal Standard Model (nuMSM), a well-motivated extension of the Standard Model of particle physics in which the production of sterile neutrino dark matter is resonantly enhanced by a lepton asymmetry in the primordial plasma. This process enables the model to evade current constraints associated with non-resonantly produced dark matter. Independently of assumptions about galaxy formation physics and the Milky Way halo mass we rule out, with at least 95 per cent confidence, all parametrizations of the nuMSM with Ms ≤ 1 keV. Incorporating physically motivated prescriptions of baryonic processes and modelling the effects of reionization strengthens our constraints and we exclude all models with Ms ≤ 3 keV. Our fiducial constraints do not rule out the putative 3.55 keV X-ray line, if it is indeed produced by the decay of a sterile neutrino. In contrast with other work, we find that the constraints from satellite counts are substantially weaker than those reported from X-ray non-detections.