Can sterile neutrinos be ruled out as warm dark matter candidates?
  We present constraints on the mass of warm dark matter (WDM) particles from a combined analysis of the matter power spectrum inferred from the Sloan Digital Sky Survey \lya flux power spectrum at 2.2<z<4.2, cosmic microwave background data, and the galaxy power spectrum. We obtain a lower limit of m~10 keV (2 sigma) if the WDM consists of sterile neutrinos and m~2 keV (2 sigma) for early decoupled thermal relics. If we combine this bound with the constraint derived from x-ray flux observations in the Coma cluster, we find that the allowed sterile neutrino mass is ~10 keV (in the standard production scenario). Adding constraints based on x-ray fluxes from the Andromeda galaxy, we find that dark matter particles cannot be sterile neutrinos, unless they are produced by a nonstandard mechanism (resonant oscillations, coupling with the inflaton) or get diluted by some large entropy release. 
Matteo Viel, Julien Lesgourgues, Martin G. Haehnelt, Sabino Matarrese,   Antonio Riotto