Spectroscopic observations of optically selected clusters of galaxies from the Palomar Distant Cluster Survey

We have conducted a redshift survey of 16 cluster candidates from the Palomar Distant Cluster Survey (PDCS) to determine both the density of PDCS clusters and the accuracy of the estimated redshifts presented in the PDCS catalog. We find that the matched-filter redshift estimate presented in the PDCS has an error sigma(z) = 0.06 in the redshift range 0.1 less than or equal to z less than or equal to 0.35 based on eight cluster candidates with three or more concordant galaxy redshifts. We measure the low-redshift (0.1 less than or equal to z less than or equal to 0.35) space density of PDCS clusters to be 31.3(-17.1)(+30.5) x 10(-6) h(3) Mpc(-3) (68% confidence limits for a Poisson distribution) for richness class 1 systems. We find a tentative space density of 10.4(-8.4)(+23.4) x 10(-6) h(3) Mpc(-3) for richness class 2 clusters. These densities compare favorably with those found for the whole of the PDCS and support the finding that the space density of clusters in the PDCS is a factor of similar or equal to 5 above that of clusters in the Abell catalog. These new space density measurements were derived as independently as possible from the original PDCS analysis and therefore demonstrate the robustness of the original work. Based on our survey, we conclude that the PDCS matched-filter algorithm is successful in detecting real clusters and in estimating their true redshifts in the redshift range we surveyed.