Evolution of Clusters in Cold Plus Hot Dark Matter Models

Abstract

We use N-body simulations to study evolution of galaxy clusters over the redshift interval 0 <= z <= 0.5 in cosmological models with a mixture of cold and hot dark matter (CHDM). Four different techniques are utilized: the cluster-cluster correlation function, axial ratios and quadrupoles of the dark matter distribution in individual clusters, and virial properties. We find that the correlation function for clusters of the same mass limit was larger and steeper at high redshifts. The slope increases from 1.8 at z=0 to 2.1 at z=0.5. Comoving correlation length r_c scales with the mass limit M within comoving radius 1.5 h^-1 Mpc and the redshift z as r_c ~= 20(1+z)(M/M_*)^1/3, where M_* = 3*10^14 h^-1 M_sun. When the correlation length is normalized to the mean cluster separation d_c, it remains almost constant: r_c~=(0.45-0.5) d_c. For small masses (M_clust <2*10^14 h^-1 M_sun) there is an indication that r_c goes slightly above the relation with the constant of proportionality being ~= 0.55-0.6. Anisotropy of density distribution in a cluster shows no change over redshift with axial ratios remaining constant around 1.2. In other words, clusters at present are as elongated as they were at the epoch of their first appearance. While the anisotropy of clusters does not change with time, the density profile shows visible evolution: the slope of density profile changes from gamma ~= -3.5 at z=0.5 to gamma ~= -2.5 at the present. We find that the core of a cluster remains essentially the same over time, but the density of the outlying regions increases noticeably. The virial relation M ~ v^2 is a good approximation, but there is a large fraction of clusters with peculiar velocities greater than given by this relation, and clusters with the same rms velocities have smaller masses in the past, a factor of 2 at z=0.5.