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Astron. Astrophys. 342, 1-14 (1999)

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9. Summary

In this paper we search objectively and analyze groups of galaxies in the recently completed ESP survey ([FORMULA] and [FORMULA]; [FORMULA]). We identify 231 groups above the number overdensity threshold [FORMULA] =80 in the redshift range 5000 km s-1 [FORMULA] 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 of ESP groups is [FORMULA] = 194 km s-1 (at the redshift of the system and taking into account measurement errors). We verify that our estimates of the average velocity dispersions are not biased by the geometry of the ESP survey which causes most systems to be only partially surveyed.

The groups we find trace very well the geometry of the large scale distribution of galaxies, as they do in shallower surveys. Because groups are also numerous, they constitute an interesting characterization of the large scale structure. The following are our main results on the properties of groups that set interesting "constraints" on cosmological models:

  • the ratio of members to non-members is [FORMULA] = 0.68 [FORMULA] 0.02. This value is in very close agreement with the value found in shallower surveys, once the few richest clusters (e.g. Coma and Virgo) are neglected.

  • the ratio of groups to the number of non-member galaxies is [FORMULA] = 0.13 [FORMULA] 0.01, also in very close agreement with the value found in shallower surveys.

  • the distribution of velocity dispersions of ESP groups is not distinguishable from those of CfA2N and SSRS2 groups.

These results are of particular interest because the ESP group catalog is five times deeper than any other wide-angle shallow survey group catalog and the number of large scale features explored is correspondingly larger. As a consequence, the properties of ESP groups are more stable with respect to possible structure-to-structure variations. The fact that the properties of ESP groups agree very well with those of CfA2N and SSRS2 groups indicates that structure-to-structure variations are not large and that the properties of groups we find can be considered representative of the local universe.

As far as the richest systems (clusters) are concerned, we identify ESP counterparts for 17 out of 20 2-D selected ACO and/or EDCC clusters. Because the volume of ESP is comparable to the volume of individual shallower surveys, it is not big enough to include a fair sample of clusters. The variations from survey to survey in the number and properties of clusters are large.

Turning our attention to properties of galaxies as a function of their environment, we find that:

  • the Schechter luminosity function of galaxies in groups has a brighter [FORMULA] (-19.80) with respect to non-member galaxies ([FORMULA]; the slope [FORMULA] ([FORMULA]) does not change significantly between the two cases.

  • [FORMULA] becomes brighter for members of groups of increasing richness. The parameter [FORMULA] remains almost constant; only in the case of the richest groups we find a marginally significant steepening of the slope [FORMULA].

  • 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).

  • the fraction of [FORMULA]-galaxies in the field is [FORMULA] = 49%; it decreases to [FORMULA] = 46% for poor groups and to [FORMULA] = 33% for richer groups. For the ESP counterparts of ACO and/EDCC clusters [FORMULA] = 25%.

We conclude that luminosity segregation is at work in the ESP survey: galaxies in the dense environment of groups are, on average, brighter than "field" galaxies. Galaxies in groups are also less likely to have detectable emission lines in their spectra. In fact, we find a gradual decrease of the fraction of emission line galaxies among members of systems of increasing richness: the morphology-density relation clearly extends over the whole range of densities from groups to clusters.

As a final note, we identify 12 Seyfert 1 galaxies and 9 Seyfert 2 galaxies. We find that: a) Seyfert galaxies within ESP are members of groups as frequently as other emission line galaxies, and b) Seyfert galaxies show a small but not significant excess of close physical companions relative to the other emission line galaxies. We point out again that the sample of Seyferts is rather small and the statistical uncertainties correspondingly large.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998