Direct x-ray constraints on sterile neutrino warm dark matter

Warm dark matter might more easily account for small scale clustering measurements than the heavier particles typically invoked in Lambda cold dark matter (Lambda CDM) cosmologies. In this paper, we consider a Lambda WDM cosmology in which sterile neutrinos nu(s), with a mass m(s) of roughly 1-100 keV, are the dark matter. We use the diffuse x-ray spectrum (total minus resolved point source emission) of the Andromeda galaxy to constrain the rate of sterile neutrino radiative decay: nu(s)->nu(e,mu,tau)+gamma. Our findings demand that m(s)< 3.5 keV (95% C.L.) which is a significant improvement over the previous (95% C.L.) limits inferred from the x-ray emission of nearby clusters, m(s)< 8.2 keV (Virgo A) and m(s)< 6.3 keV (Virgo A+Coma).