The velocity dispersion profiles of clusters of galaxies: a cosmological test and the sampling effect

Abstract

In this paper we investigate the velocity dispersion profiles of clusters of galaxies for seven cosmological models. One model is the SCDM model, and the others are six low-density models with the density parameter $Ω=0.1$, 0.2 or 0.3 and with or without a cosmological constant $Λ=1-Ω$. We find that the velocity dispersion profiles depend both on $Ω$ and on $Λ$. For $Λ=0$, the profiles are steeper in a lower-$Ω$ model than in a higher-$Ω$ one. The cosmological constant significantly weakens the dependence on $Ω$: the difference in the profile distributions between two flat models is much smaller than that between the two corresponding open models with the same $Ω$. These results in principle can be used to constrain the cosmological parameters when a large sample of the velocity dispersion profiles is available. Motivated by the practical situation that a sample of $\sim 100$ clusters with $\sim 100$ measured redshifts per cluster is still the best sample available in the foreseen future, we examine carefully to what degree the cosmological parameters can be constrained with the velocity dispersion profiles of such a sample of clusters. The limited sampling around clusters and the limited number of clusters seriously degrade the discriminative power of the velocity dispersion profiles among cosmological models. We find that the five models of $Ω\ge 0.2$ cannot be distinguished by this type of observation. Due to the limited sampling, one should be very cautious in extracting information about the density profile and/or the dynamics around single clusters from the diluted velocity dispersion profiles