5. The group catalog
We identify 231 groups within the redshift limits km s-1 . These groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same cz range.
In Table 1 we present our group catalog. For each group we list the ID number (column 1), the number of members (column 2), the coordinates and (columns 3 and 4 respectively), the mean radial velocity cz in km s-1 corrected for Virgo infall and galactic rotation (column 5), and the velocity dispersion (column 6). We compute the velocity dispersion following the prescription of Ledermann (1984) for an unbiased estimator of the dispersion (see previous section). We also take into account the cosmological expansion of the universe and the measurement errors according to the prescriptions of Danese et al. (1980). The errors we associate to the redshifts are those output by the RVSAO cross-correlation procedure multiplied by a factor 1.6. This factor brings the cross-correlation error in rough agreement with the external error estimated from repeated observations (Vettolani et al. 1998 - here we do not distinguish between emission and absorption line redshifts). Table 1 is available only in electronic form via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsweb.u-strasbg.fr/Abstract.html.
In the case of 24 groups, the correction of for the measurement errors leads to a negative value. In column 6 of Table 1 we give the error as an upper limit to for these groups.
Not all galaxies in the region of the sky covered by the ESP survey have a measured redshift. Of the original target list, 444 objects are not observed, and 207 objects have a noisy spectrum, insufficient for a reliable determination of the redshift. In Table 1 we give, for each group, the ratio of these objects to the number of members (column 7). In computing these rates, we assign to each group the objects without redshift whose (, ) coordinates fall within the angular region defined by the union of the circular regions obtained by projecting on the sky circles of linear radius centered on all group members. There are groups that are separated along the line-of-sight but that overlap once projected on the sky. If an object without redshift lies within the overlap region, we assign the object to both groups.
There are 67 groups that do not contain any object of the photometric catalog without measured redshift. On the other hand, in the case of 51 groups the number of objects without redshift equals, or exceeds, the number of members. These groups are mostly triplets and quadruplets. Only 14 out of the 51 (possibly) highly incomplete groups have 5. Most of these groups are located in the relatively small region B of the redshift survey (Vettolani et al. 1998), which is the least complete (completeness level = 71%).
Finally, we estimate that only 8 out of 231 groups are entirely contained within one OPTOPUS field. By "entirely contained" we mean that none of the circles of projected linear radius centered on the member galaxies crosses the edges of the OPTOPUS fields.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998