Constraining Sterile Neutrino Warm Dark Matter with Chandra Observations
  of the Andromeda Galaxy
  We use the Chandra unresolved X-ray emission spectrum from a 12'-28' (2.8-6.4
kpc) annular region of the Andromeda galaxy to constrain the radiative decay of
sterile neutrino warm dark matter. By excising the most baryon-dominated,
central 2.8 kpc of the galaxy, we reduce the uncertainties in our estimate of
the dark matter mass within the field of view and improve the signal-to-noise
ratio of prospective sterile neutrino decay signatures relative to hot gas and
unresolved stellar emission. Our findings impose the most stringent limit on
the sterile neutrino mass to date in the context of the Dodelson-Widrow model,
m_s < 2.2 keV (95% C.L.). Our results also constrain alternative sterile
neutrino production scenarios at very small active-sterile neutrino mixing
angles.
