How Cold Is Cold Dark Matter? Small-Scales Constraints from the Flux Power Spectrum of the High-Redshift Lyman-αForest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman- flux power spectrum of 55 high-resolution HIRES spectra at 2. 0<z<6. 4. From the HIRES spectra, we obtain a lower limit of mₖ₃₌1. 2 keV (2) if the WDM consists of early decoupled thermal relics and mₖ₃₌5. 6 keV (2) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman- flux power spectrum, we get mₖ₃₌4 keV and mₖ₃₌28 keV (2) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.