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-α 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 mWDM1.2 keV (2σ) if the WDM consists of early decoupled thermal relics and mWDM5.6 keV (2σ) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-α flux power spectrum, we get mWDM4 keV and mWDM28 keV (2σ) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.