A Stringent Limit on the Warm Dark Matter Particle Masses from the
  Abundance of z=6 Galaxies in the Hubble Frontier Fields
  We show that the recently measured UV luminosity functions of ultra-faint
lensed galaxies at z= 6 in the Hubble Frontier Fields provide an unprecedented
probe for the mass m_X of the Warm Dark Matter candidates independent of
baryonic physics. Comparing the measured abundance of the faintest galaxies
with the maximum number density of dark matter halos in WDM cosmologies sets a
robust limit m_X> 2.9 keV for the mass of thermal relic WDM particles at a
1-sigma confidence level, m_X> 2.4 keV at 2-sigma, and m_X> 2.1 keV at 3-sigma.
These constitute the tightest constraints on WDM particle mass derived to date
independently of the baryonic physics involved in galaxy formation. We discuss
the impact of our results on the production mechanism of sterile neutrinos. In
particular, if sterile neutrinos are responsible for the 3.5 keV line reported
in observations of X-ray clusters, our results firmly rule out the
Dodelson-Widrow production mechanism, and yield m_{sterile}> 6.1 keV for
sterile neutrinos produced via the Shi-Fuller mechanism.
