Spectroscopic Observations of Optically Selected Clusters of Galaxies from the Palomar Distant Cluster Survey

We have conducted a redshift survey of sixteen cluster candidates from thePalomar Distant Cluster Survey (PDCS) to determine both the density of PDCSclusters and the accuracy of the estimated redshifts presented in the PDCScatalog (Postman et. al. 1996). We find that the matched-filter redshiftestimate presented in the PDCS has an error sigma_z = 0.06 in the redshiftrange 0.1 < z < 0.35 based on eight cluster candidates with three or moreconcordant galaxy redshifts. We measure the low redshift (0.1 < z < 0.35) space density of PDCS clustersto be 31.3^{+30.5}_{-17.1} * E-06 h^3 Mpc^-3 (68% confidence limits for aPoisson distribution) for Richness Class 1 systems. We find a tentative spacedensity of 10.4^{+23.4}_{-8.4}* E-06 h^3 Mpc^-3 for Richness Class 2 clusters.These densities compare favorably with those found for the whole of the PDCSand support the finding that the space density of clusters in the PDCS is afactor of ~5 above that of clusters in the Abell catalog (Abell 1958; Abell,Corwin, and Olowin 1989). These new space density measurements were derived asindependently as possible from the original PDCS analysis and therefore,demonstrate the robustness of the original work. Based on our survey, weconclude that the PDCS matched-filter algorithm is successful in detecting realclusters and in estimating their true redshifts in the redshift range wesurveyed.Comment: 23 pages with 4 figures and 3 seperate tables. To be published in the November Issue of the Astronomical Journa