Lyman-alpha forests cool warm dark matter

Abstract

The free-streaming of keV-scale particles impacts structure growth on scales\nthat are probed by the Lyman-alpha forest of distant quasars. Using an\nunprecedentedly large sample of medium-resolution QSO spectra from the ninth\ndata release of SDSS, along with a state-of-the-art set of hydrodynamical\nsimulations to model the Lyman-alpha forest in the non-linear regime, we issue\none of the tightest bounds to date, from Ly-$\\alpha$ data alone, on pure dark\nmatter particles : $m_X > 4.09 \\: \\rm{keV}$ (95% CL) for early decoupled\nthermal relics such as a hypothetical gravitino, and correspondingly $m_s >\n24.4 \\: \\rm{keV}$ (95% CL) for a non-resonantly produced right-handed neutrino.\nThis limit depends on the value on $n_s$, and Planck measures a higher value of\n$n_s$ than SDSS-III/BOSS. Our bounds thus change slightly when Ly-$\\alpha$ data\nare combined with CMB data from Planck 2016. The limits shift to $m_X > 2.96 \\:\n\\rm{keV}$ (95% CL) and $m_s > 16.0 \\: \\rm{keV}$ (95% CL). Thanks to SDSS-III\ndata featuring smaller uncertainties and covering a larger redshift range than\nSDSS-I data, our bounds confirm the most stringent results established by\nprevious works and are further at odds with a purely non-resonantly produced\nsterile neutrino as dark matter.\n