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

We revisit Lyman-alpha bounds on the dark matter mass in Lambda Warm Dark Matter (Lambda WDM) models, and derive new bounds in the case of mixed Cold plus Warm models (Lambda CWDM), 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-alpha 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 read m(NRP) >= 8 keV (frequentist 99.7% confidence limit) or m(NRP) >= 12.1 keV (Bayesian 95% credible interval) in the pure Lambda WDM 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) < infinity), 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.