The ESO Nearby Abell Cluster Survey. I. Description of the Dataset and Definition of Physical Systems

Summary - We describe the results of the ESO Key-programme on Structure and Dynamics of Rich Galaxy Clusters' (or ENACS). The final database contains a total of 5634 galaxies in the directions of 107 clusters from the ACO catalogue with richness $R\ge 1$ and mean redshifts $z \le 0.1$. Of the 5634 redshifts, 4465 are based on absorption lines only, 583 on emission lines only and 586 on both. The overall reliability of the 5634 accepted redshifts is 0.98. For 5615 galaxies an $R_{25}$ magnitude was obtained. 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 of 1000 km/s to separate galaxies that do not form part of the same system. 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. On average, about 75~% of the 5634 galaxies are in the largest system found in the direction of the rich Abell cluster candidate. Half of the 25~% of galaxies outside the largest system belong to a secondary system along the same line of sight. 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.