Structure Formation with Cold + 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 $Ω_{\rm cold}=0.6$, $Ω_ν=0.3$, and $Ω_{\rm baryon}=0.1$. We present analytic fits to the C+HDM power spectra for both cold and hot ($ν$) components, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adequately, these simulations included six times as many neutrino particles as cold particles. Our simulation 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 fit 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 Mpc, the C+HDM streaming velocities are considerably larger than CDM. Thus C+HDM looks promising as a model of structure formation.