2025MNRAS.541.3713N

Low-mass galaxies provide a powerful tool with which to investigate departures from the standard cosmological paradigm in models that suppress the abundance of small dark matter (DM) structures. One of the simplest metrics to compare different models is the abundance of Milky Way (MW) satellite galaxies. Viable models must produce enough substructure to host the observed number of Galactic satellites. Here, we scrutinize the predictions of the neutrino minimal standard model ($\nu$MSM), a well-motivated extension of the standard model of particle physics in which sterile neutrino DM production 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 DM. Independently of assumptions about galaxy formation physics we rule out, with at least $95$ per cent confidence, all parametrizations of the $\nu$MSM with sterile neutrino mass, $M_\mathrm{s}{}\le \text{keV}[1.4]$. Incorporating physically motivated prescriptions of baryonic processes and the effects of reionization strengthen our constraints to exclude all parametrizations with $M_\mathrm{s}{}\le \text{keV}[4]$. Unlike other literature, 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; however, some of the most favoured parameter space is excluded. If the MW satellite count is higher than we assume, or if the MW is less massive than $M_{200}^{\text{MW}}=7\times 10^{11}\ \text{M}_{\odot }$, we rule out the $\nu$MSM as the origin of the 3.55 keV excess. Importantly, we find that the stringency of satellite count constraints reported in other work is overestimated.