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 mx and its relative abundance Fwdm=Ωwdm/Ωdm using a suite of cosmological hydrodynamical simulations in 28 C+WDM configurations. We find that the 3σ bounds in the mx − Fwdm parameter space approximately follow Fwdm ~ 0.35 (keV/mx)−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 ~ 6.5 keV (resp. 3.5 keV) are consistent with BOSS data at the 2σ (resp. 3σ) 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-α 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 T0 and γ, then our 3σ C+WDM {bounds strengthen to Fwdm ~ 0.20 (keV/mx)−1.37, and the lightest resonantly-produced sterile neutrinos consistent with our extended data set have masses of ~ 7.0 keV at the 3σ level. In particular, using dedicated hydrodynamical simulations, we show that} a hypothetical 7 keV sterile neutrino produced in a lepton asymmetry of  = | nνe − ne | / s = 8 × 10−6 is consistent at 1.9 σ (resp. 3.1 σ) 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.