  Warm dark matter is consistent with the observations of the large-scale
structure, and it can also explain the cored density profiles on smaller
scales. However, it has been argued that warm dark matter could delay the star
formation. This does not happen if warm dark matter is made up of keV sterile
neutrinos, which can decay into X-ray photons and active neutrinos. The X-ray
photons have a catalytic effect on the formation of molecular hydrogen, the
essential cooling ingredient in the primordial gas. In all the cases we have
examined, the overall effect of sterile dark matter is to facilitate the
cooling of the gas and to reduce the minimal mass of the halo prone to
collapse. We find that the X-rays from the decay of keV sterile neutrinos
facilitate the collapse of the gas clouds and the subsequent star formation at
high redshift.
