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For active-sterile mixing to be responsible for the full relic abundance ofdark matter additional new physics is needed beyond the keV-scale sterileneutrino itself. The extra ingredient we consider here is the presence ofself-interactions among the sterile neutrinos. We examine whetheractive-to-sterile conversion is amplified enough in this scenario that theobserved abundance of dark matter can be obtained with a subconstraint mixingangle. This turns out never to be the case in the region we explore: eitherself-interactions have too small an impact and cannot escape bounds on the massand mixing angle, or they have too great an impact and cause dark matter to beoverproduced. The sharp transition from marginal to excessive effectivenessoccurs because a resonance criterion is met in the effective in-medium mixingangle. Once the system goes resonant the game is as good as over, asnonlinearity in the Boltzmann equation leads to runaway production of sterileneutrinos, beginning at a plasma temperature of a few hundred MeV and typicallyending at a few tens of MeV. The scenario is therefore ruled out largely by itsown dynamics. In this study we focus exclusively on mediators heavier than$\sim 1$ GeV; future work will extend the analysis to lighter mediators,allowing for contact to be made with the kinds of scenarios motivated by issuesof small-scale structure.

Self-interacting sterile neutrino dark matter: the heavy-mediator case