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 Λ cold dark matter (ΛCDM) cosmologies. In this paper, we consider a ΛWDM cosmology in which sterile neutrinos ν_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: ν_s→ν_e,μ,τ+γ. Our findings demand that m_s<3.5keV (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<8.2keV (Virgo A) and m_s<6.3keV (VirgoA+Coma).