Upper limits on neutrino masses from the 2dFGRS and WMAP: the role of priors

Abstract

Solar, atmospheric, and reactor neutrino experiments have confirmedneutrino oscillations, implying that neutrinos have non-zero mass,but without pinning down their absolute masses.While it is established that the effect of neutrinos on the evolution of cosmicstructure is small, the upper limits derived from large-scale structure could help significantly to constrain the absolute scale of the neutrino masses. In a recent paper the 2dF Galaxy Redshift Survey (2dFGRS) teamprovided an upper limit mν, tot < 2.2 eV,i.e. approximately 0.7 eV for each of the three neutrino flavours or,phrased in terms of their contribution to the matter density, Ων/Ωm < 0.16. Here we discuss this analysis in greater detail, consideringissues of assumed `priors' like the matter density Ωm and the bias of the galaxy distribution with respect to the dark matterdistribution. As the suppression of the powerspectrum depends on the ratio Ων/Ωm, we findthat the out-of-fashion mixed dark matter model, with Ων = 0.2,Ωm = 1 and no cosmological constant, fits both the 2dFGRS powerspectrum and the CMB data reasonably well, but only for a Hubble constant H0 < 50 km s−1 Mpc−1. As a consequence, excluding low values of the Hubble constant, e.g. with the HST Key Project, is important in order to get a strong upper limit on the neutrino masses. We also comment on the improved limit obtained by the WMAP team, and point out that the main neutrinosignature comes from the 2dFGRS and the Lyman α forest.