The Local Space Density of Optically Selected Clusters of Galaxies

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 cutoff of Rm ≥ 50 (or Λcl ≥ 10). This new catalog of clusters is known as the Edinburgh/Durham Cluster Catalogue II (or EDCCII). 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. We believe this is due to the assumption of a flat, uniform background in the matched filter algorithm. 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 ≤ Rm < 200 (Λcl ≃ 20) systems, 10.1 × 10-6 h-3 Mpc-3 for 200 ≤ Rm < 400 (Λcl ≃ 40) systems, and 2.3 × 10-6 h-3 Mpc-3 for Rm > 400 (Λcl > 80) systems. These three richness bands roughly correspond to Abell richness classes 0, 1, and ≥2, respectively. These new measurements of the local space density of clusters are in agreement with those found at higher redshift (0.2 < zest < 0.6) in the Palomar Distant Cluster Survey (PDCS) and therefore remove one of the major uncertainties associated with the PDCS as it had previously detected a factor of 5 ± 2 more clusters at high redshift than expected compared to the space density of low-redshift Abell clusters. This discrepancy is now lessened and, at worst, is only a factor of 4. This result illustrates the need to use the same cluster-finding algorithm at both high and low redshift to avoid such apparent discrepancies. We also confirm 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. Finally, we have compared the EDCCII with the Abell catalog. We detect nearly 60% of all Abell clusters in the EDCCII area regardless of their Abell richness and distance classes. For clusters in common between the two surveys, we find no strong correlation between the two richness estimates. In comparison, ~90% of the EDCCII systems are new, although a majority of them have a richness lower than an Abell richness class of 0 and therefore would be below Abell's original selection criteria. However, we do detect 143 new clusters with Rm ≥ 100 (which corresponds to a richness class of greater than, or equal to, zero) that are not in the Abell catalog, i.e., 63% of the rich EDCCII systems. These numbers lend credence to the idea that the Abell catalog may be incomplete, especially at lower richnesses.