The ESO Nearby Abell Cluster Survey .1. Description of the dataset and definition of physical systems

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Main author: 
Katgert, P
IAA authors: 
Katgert, P; Mazure, A; Perea, J; denHartog, R; Moles, M; LeFevre, O; Dubath, P; Focardi, P; Rhee, G; Jones, B; Escalera, E; Biviano, A; Gerbal, D; Giuricin, G
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We describe the results of the ESO Key-programme on ''Structure and Dynamics of Rich Galaxy Clusters'' (which we will henceforth refer to as the ESO Nearby Abell Cluster Survey - or ENACS). We discuss the sample of clusters for which data were obtained, and the observational programme of spectroscopy and photometry that we carried out. The final database contains a total of 5634 galaxies in the directions of 107 clusters from the catalogue by Abell, Corwin and Olowin 1989 (AGO hereafter) with richness R greater than or equal to 1 and mean redshifts z less than or equal to 0.1. For 4465 galaxies the redshift is based solely on absorption lines, for 586 galaxies it is based on both absorption and emission lines, while for the remaining 583 galaxies the redshift is based exclusively on one or more emission lines. For 5615 galaxies an R(25) magnitude was obtained. We discuss in some detail the methods of observation and analysis and determine the quality and reliability of the data from independent, repeated measurements. All absorption-line redshifts with a low S/N-ratio of the peak in the correlation function have been judged on plausibility by combined visual inspection of spectrum and correlation function. This has led to an empirically determined overall reliability of the 5634 accepted redshifts of 0.98. We discuss various methods for defining the compact, physically relevant systems in the 107 'pencil-beam' surveys. We have chosen to apply a fixed velocity gap to separate galaxies that do not form part of the same system. From the summed distribution of the velocity differences between galaxies adjacent in redshift we conclude that, for the average system in our survey, and for our sampling of the velocity distributions, a velocity gap of 1000 km/s is the optimum one for defining the systems. With this gap size, systems are not broken up into sub-systems, and field galaxies are hardly linked to the systems. We present the mean redshifts of the 220 systems (with at least 4 members) identified in the 107 pencil beam surveys, using a fixed gap of 1000 km/s. On average, about 75% of the 5634 galaxies are in the largest system found in the direction of the rich Abell cluster candidate. This shows that, within approximate to 1 h(-1) Mpc of the cluster centre and down to R approximate to 17, field contamination is not negligible for clusters with z less than or similar to 0.1. However, about half of the 25% of galaxies outside the largest system belong to a secondary system along the same line of sight. At the same time, field contamination has produced only a small number of spurious rich clusters within the sample of R greater than or equal to 1, z less than or equal to 0.1 clusters. For about 90% of the nearby rich Abell cluster candidates studied here we find a redshift system that either contains more than half of the total number of redshifts, or that has at least two times as many redshifts as the next largest system. Only in about 10% of the cases does an R greater than or equal to 1, z less than or equal to 0.1 entry in the ACO catalogue appear to be the result of a superposition of two almost equally rich (but relatively poorer) systems. Almost all of the rich and relatively nearby ACO cluster candidates that we studied thus appear to be real rich clusters that represent physical systems.
galaxies, clusters of; galaxies, redshifts; cosmology, observations; dark matter