  We use the Ly-alpha forest power spectrum measured by the SDSS and
high-resolution spectroscopy observations in combination with cosmic microwave
background and galaxy clustering constraints to place limits on a sterile
neutrino as a dark matter candidate in the warm dark matter (WDM) scenario.
Such a neutrino would be created in the early universe through mixing with an
active neutrino and would suppress structure on scales smaller than its free
streaming scale. We ran a series of high-resolution hydrodynamic simulations
with varying neutrino mass to describe the effect of a sterile neutrino on the
Ly-alpha forest power spectrum. We find that the mass limit is m_s >14 keV at
95% c.l. (10keV at 99.9%), which is nearly an order of magnitude tighter
constraint than previously published limits and is above the upper limit
allowed by X-ray constraints, excluding this candidate as dark matter in this
model. The corresponding limit for a neutrino that decoupled early while in
thermal equilibrium is 2.5keV (95% c.l.).
