2016ApJ...825L...1M

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 (WDM) 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 of m_X >= 2.9 keV for the mass of thermal relic WDM particles at a 1σ confidence level, m_X >= 2.4 keV at 2σ, and m_X >= 2.1 keV at 3σ. These constraints are independent of the baryonic physics involved in galaxy formation and constitute the tightest constraints on WDM particle mass derived to date. 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.