Structure Formation with Cold plus Hot Dark Matter

Abstract

We report results from high-resolution particle-mesh (PM) N-body simulations of structure formation in an Ω = 1 cosmological model with a mixture of cold plus hot dark matter (C+HDM) having Ω cold = 0.6, Ω v = 0.3, and Ω baryon = 0.1. We present analytic fits to the C+HDM power spectra for both cold and hot (neutrino) components, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adeq uately, these simulations included 6 times as many neutrino particles as cold particles. Our simulations boxes were 14, 50, and 200 Mpc cubes (with H 0 = 50 km s -1 Mpc -1 ); we also did comparison simulations for cold dark matter (CDM) in a 50 Mpc box. C+HDM with linear bias factor b = 1.5 is consistent both with the COBE data and with the galaxy correlations we calculate. We find the number of halos as a function of mass and redshift in our simulations; our results for both CDM and C+HDM are well fitted by a Press-Schechter model. The number density of galaxy-mass halos is smaller than for CDM, especially at redshift z > 2, but the numbers of cluster-mass halos are comparable. We also find that on galaxy scales the neutrino velocities and flatter power spectrum in C+HDM result in galaxy pairwise velocities that are in good agreement with the data, and about 30% smaller than in CDM with the same biasing factor. On scales of several tens of megaparsecs, the C+HDM streaming velocities are considerably larger than CDM. As a result, the "cosmic Mach number" in C+HDM is about a factor of 2 larger than in CDM, and probably in better agreement with observations. Thus C+HDM looks promising as a model of structure formation. The presence of a hot component requires the introduction of a single additional parameter beyond standard CDM - the light neutrino mass or, equivalently, Ω v - and allows the model to fit essentially all the available cosmological data remarkably well. The τ neutrino is predicted to have a mass of about 7 eV, compatible with the MSW explanation of the solar neutrino data together with a long-popular particle physics model. We outline a number of additional tests to which the C+HDM model should be subjected.