The DPOSS II compact group survey: first spectroscopically confirmed candidates

Context. This paper presents the results of a pilot redshift survey of 18 candidate compact groups from the distant DPOSS survey that extends the available surveys of compact groups of galaxies to z ∼ 0.2 in redshift, mainly Hickson Compact Groups and Southern Compact Groups. Aims. The goal of our survey was to confirm group membership via redshift information and to measure the characteristic parameters of a representative, albeit small, sample of DPOSS survey groups. Methods. Of the 18 candidates observed, seven are found to be indeed isolated compact groups, i.e. groups with 3 or more concordant members and with no neighbouring known cluster, while 7 are chance projection configurations on the sky. Three remaining candidates, despite having 3 or more concordant member galaxies, are located in the neighbourhood of known clusters, while another candidate turned out to be a dense sub-condensation within Abell 0952. Results. The median redshift of our 7 confirmed groups is z ∼ 0.12, to be compared with a median redshift of 0.03 for the local sample of compact groups by Hickson. The typical group size is ∼ 50 kpc, and the median radial velocity dispersion is 167 km s−1, while typical crossing times range from 0.005 H 0 to 0.03 H −1 0 with a median value of 0.018 H −1 0 , all similar to the values usually found in the literature for such structures in the local universe. The average mass-to-light ratio for our groups, M/LB, is 92h, higher than the value found for nearby Hickson compact groups but lower than that found for loose groups. Our results suggest that, once full redshift information for its members becomes available, the DPOSS sample will provide a reference sample to study the properties of compact groups beyond the local universe.