Loose groups of galaxies in the Perseus–Pisces survey^*

Dipartimento di Fisica dell'Università, via Celoria 16, I-20133 Milano, Italy Present address: Ludwig–Maximilians–Universität, Sektion Physik, Lehrstuhl Wagner, Theresienstr. 37, D-80333 München, Germany

Send offprint request to: R. Trasarti–Battistoni

Received: 5 August 1997
Accepted: 24 December 1997

Abstract

We present a large catalog of loose groups of galaxies in the Southern Galactic Hemisphere, selected from the Perseus–Pisces redshift Survey (PPS). Particular care is taken in order to obtain group samples as homogeneous as possible to previously published catalogs. All our catalogs contain about 200 groups, significantly more than in most previous studies where group samples were obtained from galaxy data sets of comparable quality to (but smaller extent than) PPS. Groups are identified with the adaptive Friends–Of–Friends (FOF) algorithm of Huchra & Geller (1982), with suitable normalizations and at . The luminosity function (LF) normalization appropriate for PPS yields a number density threshold for the adopted D₀, instead of used in previous studies of other samples. However, the customary choice of D₀ obtained (through the LF) from a fixed mass overdensity , well motivated in theory, suffers from important observational uncertainties and sample–to–sample variations of the LF normalization, and from major uncertainties in the relation between galaxy density n and mass density ρ . We discuss how to self–consistently match FOF parameters among different galaxy samples. We then separately vary several FOF and sample parameters, and discuss their effect on group properties. Loose groups in PPS nicely trace the large scale structure (LSS) in the parent galaxy sample. The group properties vary little with different redshift corrections, redshift cut–off, and galaxy LF, but are rather sensitive to the adopted links D₀ and V₀. More precisely, the typical group size (velocity dispersion) is linearly related to the adopted distance (velocity) link, while it is rather insensitive to the adopted velocity (distance) link. Physical properties of groups in PPS and in directly comparable samples show good agreement. There is a complex interplay among LSS features, sample depth, FOF grouping procedure, and group properties.