We show that the recently measured UV luminosity functions of ultra-faint lensed galaxies at z \approx 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 } \geq 2.9 keV for the mass of thermal relic WDM particles at a 1- \sigma confidence level , m _ { X } \geq 2.4 keV at 2- \sigma , and m _ { X } \geq 2.1 keV at 3- \sigma .
These constitute the tightest constraints on WDM particle mass derived to date from galaxy abundance 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 } \gtrsim 6.1 keV for sterile neutrinos produced via the Shi-Fuller mechanism .