Lyman-α constraints on warm and on warm-plus-cold dark matter models

We revisit Lyman-α bounds on the dark matter mass in Λ Warm Dark Matter (ΛWDM) models, and derive new bounds in the case of mixed Cold plus Warm models (ΛCWDM), using a set up which is a good approximation for several theoretically well-motivated dark matter models. We combine WMAP5 results with two different Lyman-α data sets, including observations from the Sloan Digital Sky Survey. We pay a special attention to systematics, test various possible sources of error, and compare the results of different statistical approaches. Expressed in terms of the mass of a non-resonantly produced sterile neutrino, our bounds readm_NRP ⩾ 8 keV (frequentist 99.7% confidence limit) or m_NRP ⩾ 12.1 keV (Bayesian 95% credible interval) in the pure ΛWDM limit. For the mixed model, we obtain limits on the mass as a function of the warm dark matter fraction F_WDM. Within the mass range studied here (5 keV < m_NRP < ∞), we find that any mass value is allowed when F_WDM < 0.6 (frequentist 99.7% confidence limit); similarly, the Bayesian joint probability on (F_WDM, 1/m_NRP) allows any value of the mass at the 95% confidence level, provided that F_WDM < 0.35.