STRUCTURE FORMATION WITH COLD PLUS HOT DARK-MATTER

We report results from high-resolution particle-mesb (PM) N-body simul ations of structure formation in an OMEGA = 2 cosmological model with a mixture of cold plus hot dark matter (C + HDM) having OMEGA(cold) = 0.6, OMEGA(v) = 0.3, and OMEGA(baryon) = 0.1. We present analytic fits to the C+HDM power spectra for both cold and hot (neutrino) component s, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adequately, these simulations included 6 times as many neutrino particles as cold particles. Our sim ulations boxes were 14, 50, and 200 Mpc cubes (with H-0 = 50 km s-1 Mp c-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 bot h 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 si mulations; our results for both CDM and C+HDM are well fitted by a Pre ss-Schechter model. The number density of galaxy-mass halos is smaller than for CDM, especially at redshift z > 2, but the numbers of cluste r-mass halos are comparable. We also find that on galaxy scales the ne utrino velocities and flatter power spectrum in C+HDM result in galaxy pairwise velocities that are in good agreement with the data, and abo ut 30% smaller than in CDM with the same biasing factor. On scales of several tens of megaparsecs, the C+HDM streaming velocities are consid erably 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 bett er agreement with observations. Thus C + HDM looks promising as a mode l 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, OMEGA(v)-and allows the model t o fit essentially all the available cosmological data remarkably well. The tau neutrino is predicted to have a mass of about 7 eV, compatibl e 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.