Exploring resonantly produced mixed sterile neutrino dark matter models

An unidentified 3.55 keV x-ray line in stacked spectra of galaxies and clusters raises the interesting possibility that it originates from the decay of sterile neutrino dark matter. In this work, we explore mixed sterile neutrino dark matter models that combine cold dark matter and warmer sterile neutrino dark matter produced through lepton number-driven active-to-sterile neutrino transformation. We analyze the sensitivity of the sterile neutrino spectra on active-sterile mixing and on initial neutrino lepton numbers. We explore the parameter space of mixed sterile neutrino dark matter models, and assess their viability based on estimates of the number of subhalos formed as the host sites of satellite galaxies. We find constraints on the active-sterile mixing angle inferred from x-ray observations (less than or similar to 10(-9)) and the fraction of dark matter consisting of 7.1 keV sterile neutrinos (less than or similar to 1). Considering other probes of small-scale structure formation can make these constraints more stringent and future observations of the x-ray line can better constrain these models and their production mechanism.