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-$\alpha$ 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 $m_{\mathrm{WDM}}\gtrsim 1.2\mathrm{keV}$ ($2\sigma$) if the WDM consists of early decoupled thermal relics and $m_{\mathrm{WDM}}\gtrsim 5.6\mathrm{keV}$ ($2\sigma$) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-$\alpha$ flux power spectrum, we get $m_{\mathrm{WDM}}\gtrsim 4\mathrm{keV}$ and $m_{\mathrm{WDM}}\gtrsim 28\mathrm{keV}$ ($2\sigma$) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.