Constraints on sterile neutrino dark matter from XMM--Newton observation   of M33
  Using archival XMM-Newton observations of the diffuse and unresolved components emission in the inner disc of M33 we exclude the possible contribution from narrow line emission in the energy range 0.5-5 keV more intense than 10^{-6}-10^{-5} erg/s. Under the hypothesis that sterile neutrinos constitute the majority of the dark matter in M33, we use this result in order to put constraints on their parameter space in the 1-10 keV mass range. 
Enrico Borriello, Maurizio Paolillo, Gennaro Miele, Giuseppe Longo,   Richard Owen
astro-ph.GA