Constraints from Ly-α forests on non-thermal dark matter including resonantly-produced sterile neutrinos

We use the large BOSS DR9 sample of quasar spectra to constrain two cases of non-thermal dark matter models. cold-plus-warm dark matter (C+WDM) where the warm component is a thermal relic, and sterile neutrinos resonantly produced in the presence of a lepton asymmetry (RPSN). We establish constraints on the thermal relic mass m(x) and its relative abundance F-wdm = Omega(wdm) / Omega(dm) using a suite of cosmological hydrodynamical simulations in 28 C+WDM configurations. We find that the 3 sigma bounds in the m(x) - F-wdm parameter space approximately follow F-wdm similar to 0.35(keV / m(x))(-1.37) from BOSS data alone. We also establish constraints on sterile neutrino mass and mixing angle by further producing the non-linear flux power spectrum of 8 RPSN models, where the input linear power spectrum is computed directly from the particles distribution functions. We find values of lepton asymmetries for which sterile neutrinos as light as similar to 6.5 keV (resp. 3.5 keV) are consistent with BOSS data at the 2 sigma (resp. 3 sigma) level. These limits tighten by close to a factor of 2 for values of lepton asymmetries departing from those yielding the coolest distribution functions. Our Lyman-alpha forest bounds can be additionally strengthened if we include higher-resolution data from XQ-100, HIRES and MIKE that allow us to probe smaller scales. At these scales, the measured flux power spectrum exhibits a suppression that can be due to Doppler broadening, IGM pressure smoothing or free-streaming of WDM particles. In order to distinguish between these mechanisms, thermal history at redshifts z >= 5 should be determined. In the current work, we show that if one extrapolates temperatures from lower redshifts via broken power laws in T-0 and gamma, then our 3 sigma C+WDM bounds strengthen to F-wdm similar to 0.20(keV / m(x))(-1.37), and the lightest resonantly-produced sterile neutrinos consistent with our extended data set have masses of similar to 7.0 keV at the 3 sigma level. In particular, using dedicated hydrodynamical simulations, we show that a hypothetical 7 keV sterile neutrino produced in a lepton asymmetry of L = broken vertical bar n(ve) - n((v) over bare)broken vertical bar / s = 8 x 10(-6) is consistent at 1.9 sigma (resp. 3.1 sigma) with BOSS (resp. BOSS + higher-resolution) data, for the thermal history models tested in this work. More information about the state of the IGM at redshifts 5-6 will allow one to conclude whether the small-scale suppression of the flux power spectrum is due to such sterile neutrino or to thermal effects.