B. P. Holden

We present the Bright SHARC (Serendipitous High-Redshift Archival ROSAT Cluster) Survey, which is an objective search for serendipitously detected extended X-ray sources in 460 deep ROSAT PSPC pointings. The Bright SHARC Survey covers an area of 178.6 sq.deg and has yielded 374 extended sources. We discuss the X-ray data reduction, the candidate selection and present results from our on-going optical follow-up campaign. The optical follow-up concentrates on the brightest 94 of the 374 extended sources and is now 97% complete. We have identified thirty-seven clusters of galaxies, for which we present redshifts and luminosities. The clusters span a redshift range of 0.0696<z<0.83 and a luminosity range of 0.065<Lx<8.3e44 erg/s [0.5-2.0 keV] (assuming Ho = 50 km/s/Mpc and qo=0.5). Twelve of the clusters have redshifts greater than z=0.3, eight of which are at luminosities brighter than Lx=3e44 erg/s. Seventeen of the 37 optically confirmed Bright SHARC clusters have not been listed in any previously published catalog. We also report the discovery of three candidate ``fossil groups'' of the kind proposed by Ponman et al. (1994).

We present the first dynamical study of the optically selected Palomar Distant Cluster Survey (PDCS). We have measured redshifts for 17 clusters of galaxies in the PDCS and velocity dispersions for a subset of 11. Using our new cluster redshifts, we redetermine the X-ray luminosities and upper limits. We show that 11 of 12 PDCS clusters we observed are real overdensities of galaxies. Most clusters have velocity dispersions appropriate for clusters of galaxies. However, we find a fraction (about one-third) of objects in the PDCS that have velocity dispersions in the range of groups of galaxies (200 ± 100 km s^-1) but have richnesses appropriate for clusters of galaxies. Within our survey volume of 31.7 × 10⁴ h^-3 Mpc³ (q₀ = 0.1) for richness class 2 and greater clusters, we measure the richness function, X-ray luminosity function (using both the detections and upper limits), and the mass function derived from our velocity dispersions. We confirm that the space density, as a function of richness, of clusters of galaxies in the PDCS is ~5 times that of the Abell catalog. Excluding the above fraction of one-third of objects with low velocity dispersions, we measure a space density ~3 times that of the Abell catalog for equivalent mass clusters of galaxies, raising the possibility that the Abell catalog is incomplete. However, our space density estimates are in agreement with other low-redshift, optically selected cluster surveys such as the EDCC, APM, and EDCC2. Our X-ray luminosity function agrees with other measurements based on both X-ray and optically selected samples, so we find that the PDCS does not miss clusters of galaxies that would be found in an X-ray selected survey. Our resulting mass function, centered on 10¹⁴ M_☉ h^-1, agrees with the expectations from such surveys as the Canadian Network for Observational Cosmology cluster survey, though errors on our mass measurements are too large to constrain cosmological parameters. We do show that future machine-based, optically selected surveys can be used to constrain cosmological parameters.

We present the first dynamical study of the optically selected Palomar Distant Cluster Survey (PDCS). We show that eleven out of twelve PDCS clusters we observed are real over-densities of galaxies. Most clusters have velocity dispersions appropriate for clusters of galaxies. However, we find a fraction (1/3) of objects in the PDCS which have velocity dispersions in the range of groups of galaxies (200 +/- 100 km/s) but have richnesses appropriate for clusters of galaxies. We measure the richness function, X-ray luminosity function (using both the detections and upper limits), and the mass function derived from our velocity dispersions. We confirm that the space density, as a function of richness, of clusters of galaxies in the PDCS is ~5 times that of the Abell catalog. Excluding the above fraction of 1/3 of objects with low velocity dispersions, we measure a space density ~3 times that of the Abell catalog for equivalent mass clusters of galaxies. Our space density estimates are in agreement with other low-redshift, optically-selected cluster surveys such as the EDCC, APM and EDCC2, which raises the possibility that the Abell catalog is incomplete. Our X-ray luminosity function agrees with other measurements based on both X-ray and optically selected samples, so we find that the PDCS does not miss clusters of galaxies that would be found in an X-ray selected survey. Our resulting mass function, centered around 10e14 Msun/h, agrees with the expectations from such surveys as the Canadian Network for Observational Cosmology cluster survey, though errors on our mass measurements are too large to constrain cosmological parameters.

We present the Bright SHARC (Serendipitous High-Redshift Archival ROSAT Cluster) Survey, which is an objective search for serendipitously detected extended X-ray sources in 460 deep ROSAT PSPC pointings. The Bright SHARC Survey covers an area of 178.6 sq.deg and has yielded 374 extended sources. We discuss the X-ray data reduction, the candidate selection and present results from our on-going optical follow-up campaign. The optical follow-up concentrates on the brightest 94 of the 374 extended sources and is now 97% complete. We have identified thirty-seven clusters of galaxies, for which we present redshifts and luminosities. The clusters span a redshift range of 0.0696<z<0.83 and a luminosity range of 0.065<Lx<8.3e44 erg/s [0.5-2.0 keV] (assuming Ho = 50 km/s/Mpc and qo=0.5). Twelve of the clusters have redshifts greater than z=0.3, eight of which are at luminosities brighter than Lx=3e44 erg/s. Seventeen of the 37 optically confirmed Bright SHARC clusters have not been listed in any previously published catalog. We also report the discovery of three candidate ``fossil groups'' of the kind proposed by Ponman et al. (1994).Comment: Minor revisions: References updated and typos corrected. Shortened by use of emulateapj.st

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 σ_z = 0.06 in the redshift range 0.1 ≤ z ≤ 0.35 based on eight cluster candidates with three or more concordant galaxy redshifts. We measure the low-redshift (0.1 ≤ z ≤ 0.35) space density of PDCS clusters to be 31.3 × 10^-6 h³ Mpc^-3 (68% confidence limits for a Poisson distribution) for richness class 1 systems. We find a tentative space density of 10.4 × 10^-6 h³ 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 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.

We present here initial results on the X-ray cluster luminosity function (XCLF) from the Bright Serendipitous High-Redshift Archival Cluster (SHARC) sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray-luminous clusters in the redshift range 0.3 ≤ z ≤ 0.83 (median z = 0.42) and 1.1 × 10⁴⁴ ≤ L_X ≤ 8.3 × 10⁴⁴ ergs s^-1 (0.5-2.0 keV). We present a preliminary selection function for the Bright SHARC Survey based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC Survey XCLF and find it to be fully consistent with a nonevolving XCLF to L_X 5 × 10⁴⁴ ergs s^-1 and z 0.7. At L_X > 5 × 10⁴⁴ ergs s^-1, we find evidence for a deficit of clusters compared to that expected from a nonevolving XCLF. We detect only one such cluster in the redshift range 0.3 ≤ z ≤ 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. The statistical significance of this deficit is 96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC Survey and the 160 deg² survey of Vikhlinin et al. This joint survey covers 260 deg² and contains only one confirmed 0.3 ≤ z ≤ 0.7, L_X > 5 × 10⁴⁴ ergs s^-1 cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al.). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.

We have conducted a redshift survey of sixteen 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 (Postman et. al. 1996). 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 < z < 0.35 based on eight cluster candidates with three or more concordant galaxy redshifts. We measure the low redshift (0.1 < z < 0.35) space density of PDCS clusters to be 31.3^{+30.5}_{-17.1} * E-06 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^{+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 PDCS and support the finding that the space density of clusters in the PDCS is a factor of ~5 above that of clusters in the Abell catalog (Abell 1958; Abell, Corwin, and Olowin 1989). 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.

We present here initial results on the X-ray Cluster Luminosity Function (XCLF) from the Bright SHARC sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray luminous clusters in the range 0.3 < z < 0.83 (median z = 0.42) and 1.1 < Lx(44) < 8.5 erg/s [0.5-2.0 keV]. We present a preliminary selection function for the Bright SHARC based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC XCLF and find it to be fully consistent with a non-evolving XCLF to Lx(44) = 5 erg/s and z = 0.7. At Lx(44) > 5 erg/s, we find evidence for a deficit of clusters compared to that expected from a non-evolving XCLF. We detect only one such cluster in the redshift range 0.3 < z < 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. (1999). The statistical significance of this deficit is 96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC and the 160 sq. deg. survey of Vikhlinin et al (1998a). This joint survey covers 260 sq. degs. and contains only one confirmed 0.3 < z < 0.7, Lx(44) > 5 erg/s cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al. 1999). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.