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

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The ESO slice project (ESP) galaxy redshift survey * **

VI. Groups of galaxies

M. Ramella 1, G. Zamorani 2,3, E. Zucca 2,3, G.M. Stirpe 2, G. Vettolani 3, C. Balkowski 4, A. Blanchard 5, A. Cappi 2, V. Cayatte 4, G. Chincarini 6,7, C. Collins 8, L. Guzzo 6, H. MacGillivray 9, D. Maccagni 10, S. Maurogordato 11,4, R. Merighi 2, M. Mignoli 2, A. Pisani 1, D. Proust 4 and R. Scaramella 12

1 Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34131 Trieste, Italy
2 Osservatorio Astronomico di Bologna, via Zamboni 33, I-40126 Bologna, Italy
3 Istituto di Radioastronomia del CNR, via Gobetti 101, I-40129 Bologna, Italy
4 Observatoire de Paris, DAEC, Unité associée au CNRS, D0173 et à l'Université Paris 7, 5 Place J. Janssen, F-92195 Meudon, France
5 Université L. Pasteur, Observatoire Astronomique, 11 rue de l'Université, F-67000 Strasbourg, France
6 Osservatorio Astronomico di Brera, via Bianchi 46, I-22055 Merate (LC), Italy
7 Università degli Studi di Milano, via Celoria 16, I-20133 Milano, Italy
8 Astrophysics Research Institute, Liverpool John-Moores University, Byrom Street, Liverpool L3 3AF, UK
9 Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
10 Istituto di Fisica Cosmica e Tecnologie Relative, via Bassini 15, I-20133 Milano, Italy
11 CERGA, Observatoire de la Côte d'Azur, B.P. 229, F-06304 Nice Cedex 4, France
12 Osservatorio Astronomico di Roma, via Osservatorio 2, I-00040 Monteporzio Catone (RM), Italy

Received 15 July 1998 / Accepted 27 October 1998


In this paper we identify objectively and analyze groups of galaxies in the recently completed ESP survey ([FORMULA] and [FORMULA]; [FORMULA]). We find 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 (corrected for measurement errors and computed at the redshift of the group) is [FORMULA] = 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 [FORMULA] 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 [FORMULA] with respect to non-member galaxies; the slope [FORMULA] 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.

Key words: galaxies: clusters: general – galaxies: distances and redshifts – galaxies: luminosity function, mass function – cosmology: observations – cosmology: large-scale structure of Universe

* based on observations collected at the European Southern Observatory, La Silla, Chile.
** Table 1 is available only in electronic form via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/342/1

Send offprint requests to: Massimo Ramella (ramella@oat.ts.astro.it)

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998