INTERNAL VELOCITY AND MASS DISTRIBUTIONS IN SIMULATED CLUSTERS OF GALAXIES FOR A VARIETY OF COSMOGONIC MODELS

The mass and velocity distributions in the outskirts (0.5-3.0 h(-1) Mpc) of simulated clusters of galaxies are examined for a suite of cosmogonic models (two Omega(0) = 1 and two Omega 0 = 0.2 models) utilizing large-scale particle-mesh (PM) simulations (500(3) cells, 250(3) particles and box size of 100 h(-1) Mpc, giving a nominal resolution of 0.2 h(-1) Mpc with the true resolution similar to 0.5 h(-1) Mpc). Through a series of model computations, designed to isolate the different effects, we find that both Omega 0, and P-k (lambda less than or equal to 16 h(-1) Mpc) are important to the mass distributions in clusters of galaxies. There is a correlation between power, P-k, and density profiles of massive clusters; more power tends to point to the direction of a stronger correlation between alpha and M(r < 1.5 h(-1) Mpc) (see eq. [1] for definitions); i.e., massive clusters being relatively extended and small mass clusters being relatively concentrated. A lower Omega(0) universe tends to produce relatively concentrated massive clusters and relatively extended small mass clusters compared to their counterparts in a higher Omega 0 model with the same power. Models with little (initial) small-scale power, such as the HDM model, produce more extended mass distributions than the isothermal distribution for most of the mass clusters. But the CDM models show mass distributions of most of the clusters more concentrated than the isothermal distribution. X-ray and gravitational lensing observations are beginning providing useful information on the mass distribution in and around clusters; some interesting constraints on Omega 0 and/or the (initial) power of the density fluctuations on scales lambda less than or equal to 16 h(-1) Mpc (where linear extrapolation is invalid) can be obtained when larger observational data sets, such as the Sloan Digital Sky Survey, become available. With regard to the velocity distribution, we find two interesting points. First, in 0.5 < r < 3.0 h(-1) Mpc region, all four velocity dispersions (one-dimensional [1D], radial, tangential, line-of-sight) show decreasing distributions as a function of clustercentric distance in the three CDM models; but the HDM model shows just the opposite: weakly increasing velocity dispersions outward. The CDM models can reasonably fit the observed galaxy velocity dispersions in the Coma cluster of galaxies but the HDM model provides a poor fit. Second, we find that for the scales 0.5 < r < 3.0 h(-1) Mpc, the tangential velocity dispersion is always larger than the radial component by a factor of 1.2-1.6 in the CDM models and 1.3-2.0 in the HDM model. In all models the ratio of radial to tangential velocity dispersions is a decreasing function from 0.5 h(-1) Mpc to 3.0 h(-1) Mpc for massive clusters (smaller mass clusters tend to show a minimum for that ratio around 1.5-2.0 h(-1) Mpc in the CDM models). While the velocity dispersions among the three Cartesian directions are isotropic on average, a large scatter (40%) exists in all models. We also examine the infall issue in detail. Lower Omega 0 models are found to have larger turnaround radius for a fixed-mass clump than high Omega 0 models; this conclusion is insensitive to P-k. But we find that the following relation (between the turnaround radius, R(ta), and the mass within R(ta), M(ta)), log(10) R(ta) = a + b log(10) M(ta) (a = -5.2 +/- 0.2, b = 0.40 +/- 0.02, R(ta) and M(ta) are in h(-1) Mpc and h(-1) M., respectively) holds for all the models (the uncertainties in a and b indicate the variations among models). In addition, the relation between the overdensity inside the turnaround radius, delta(ta), and M(ta) is fitted by log(10) delta(ta) = c + d log(10) M(ta) (cf. Table 1 for values of c and d). We show that the spherical top-hat collapse model in an Einstein-de Sitter universe, having delta(ta) = 9 pi(2)/16 = 5.55, gives a fair fit to results (similar to 4-10) of the nonlinear, nonspherical simulations performed here. Lower Omega 0 models have considerably higher delta(ta) similar to 10-30, as expected. Finally, we find that the isothermal approximation (cf. eq. [10]) appears to underestimate the true masses within the Abell radius by 10%-30% with a scatter of similar to 50% around the estimated mean (in the three hierarchical models).