The Local Space Density of Optically-Selected Clusters of Galaxies

Abstract

Abridged] We present here new results on the space density of rich, optically-selected, clusters of galaxies at low redshift (z<0.15). These results are based on the application of the matched filter cluster-finding algorithm to 1067 deg2 of the Edinburgh/Durham Southern Galaxy Catalogue (EDSGC). This is the first major application of this methodology at low redshift and in total, we have detected 2109 clusters above a richness cut-off of R_m >= 50 (or Lambda_cl >= 10). We have used extensive Monte Carlo simulations to define the detection thresholds for our algorithm, to measure the effective area of the EDCCII and to determine our spurious detection rate. These simulations have shown that our detection efficiency is strongly correlated with the presence of large-scale structure in the EDSGC data which we believe is due to the assumption of a flat, uniform background. Using these simulations, we are able to compute the space density of clusters in this new survey. We find 83.5 10-6 h-3 Mpc-3 for 100<= R_m < 200 (Lambda_cl~20) systems, 10.1 10-6 h-3 Mpc-3 for 200 <= R_m < 400 (Lambda_cl~40) systems and 2.3 10-6 h-3 Mpc-3 for R_m > 400 (Lambda_cl > 80) systems. These three richness bands roughly correspond to Abell Richness Classes 0, 1 and >=2 respectfully. These new measurements of the local space density of clusters are in agreement with those found at higher redshift (0.2 < z_est < 0.6) in the Palomar Distant Cluster Survey (PDCS; Postman et al. 1996 & Holden et al. 1999) thus confirming that the space density of clusters remains nearly constant out to z~0.6 in agreement with previous optical and X-ray measurements of the space density of clusters.