Neutrinos as cluster dark matter

Abstract

The dynamical mass of clusters of galaxies, calculated in terms of modified Newtonian dynamics, is a factor of two or three times smaller than the Newtonian dynamical mass but remains significantly larger than the observed baryonic mass in the form of hot gas and stars in galaxies. Here I consider further the suggestion that the undetected matter might be in the form of cosmological neutrinos with mass on the order of 2 eV. If the neutrinos and baryons have comparable velocity dispersions and if the two components maintain their cosmological density ratio, then the electron density in the cores of clusters should be proportional to temperature to the 3/2 power, as appears to be true in non-cooling flow clusters. This is equivalent to the ``entropy floor'' proposed to explain the steepness of the observed luminosity-temperature relation, but here preheating of the medium is not required. Two fluid (neutrino-baryon) hydrostatic models of clusters, in the context of MOND, reproduce the observed luminosity-temperature relation of clusters. If the beta law is imposed the gas density distribution, then the self-consistent models predict the general form of the observed temperature profile in both cooling and non-cooling flow clusters.