The ESO slice project (ESP) galaxy redshift survey: VI. Groups of galaxies

In this paper we identify objectively and analyze groups of galaxies in the recently completed ESP survey (23(h)23(m) less than or equal to alpha(1950) less than or equal to 01(h)20(m) and 22(h)30(m) less than or equal to alpha(1950) less than or equal to 22(h)52(m); -40 degrees 45' less than or equal to delta(1950) less than or equal to -39 degrees 45'). We find 231 groups above the number overdensity threshold delta rho/rho = 80 in the redshift range 5000 km s(-1) less than or equal to cz less than or equal to 60000 km s(-1). These groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same cz range. The median velocity dispersion (corrected for measurement errors and computed at the redshift of the group) is sigma(ESP,median) = 194 km s(-1). We show that our result is reliable in spite of the particular geometry of the ESP survey (two rows of tangent circular fields of radius theta = 15 arcmin), which causes most systems to be only partially surveyed. In general, we find that the properties of ESP groups are consistent with those of groups in shallower (and wider) catalogs (e.g. CfA2N and SSRS2). As in shallower catalogs, ESP groups trace very well the geometry of the large scale structure. Our results are of particular interest because the depth of the ESP survey allows us to sample group properties over a large number of structures. We also compare luminosity function and spectral properties of galaxies that are members of groups with those of isolated galaxies. We find that galaxies in groups have a brighter M* with respect to non-member galaxies; the slope ai is the same, within the errors, in the two cases. We find that 34% (467/1360) of ESP galaxies with detectable emission lines are members of groups. The fraction of galaxies without detectable emission lines in groups is significantly higher: 45% (783/1725). More generally, we find a gradual decrease of the fraction of emission line galaxies among members of systems of increasing richness. This result confirms that the morphology-density relation found for clusters also extends toward systems of lower density.