How cold is dark matter? Constraints from Milky Way satellites

We test the luminosity function of Milky Way satellites as a constraint for the nature of dark matter particles. We perform dissipationless high-resolution N-body simulations of the evolution of Galaxy-sized halo in the standard cold dark matter model and in four warm dark matter (WDM) scenarios, with a different choice for the WDM particle mass m_w. We then combine the results of the numerical simulations with semi-analytic models for galaxy formation, to infer the properties of the satellite population. Quite surprisingly, we find that even WDM models with relatively low m_w values (2-5 keV) are able to reproduce the observed abundance of ultra faint (M_v < - 9) dwarf galaxies, as well as the observed relation between luminosity and mass within 300 pc. Our results suggest a lower limit of 1keV for thermal WDM, in broad agreement with previous results from other astrophysical observations such as Lyman α forest and gravitational lensing.