Core radii and central densities of 15 rich clusters of galaxies.

Abstract

The projected galaxy distributions and centers are determined for 15 rich regular clusters of galaxies with redshifts up to about 0.1. The distributions are given for approximately the same fixed range of magnitudes. The galaxy distributions in all the clusters are similar and resemble the distribution of a projected, bounded, Emden isothermal gas sphere. The core radii obtained for the distributions exhibit a relatively small dispersion (+/- 15%) around a constant linear value of R = 0.25 Mpc (H0 = 50 km-1 Mpc-1). This result suggests that the extension of the present study to high-redshift clusters, comparing their core radii with the low redshift values, will enable one to place some constraints on the cosmological deceleration parameter, q0. The central densities, in galaxies per cubic Mpc, are also calculated and are found to be similar for the 15 clusters studied. Their values all lie in the range of 200 +/- 100 galaxies per Mpc3, to the limit of 3 mag (+ 1 mag) below the third brightest galaxy of each cluster.