Loose Groups of Galaxies in the Perseus-Pisces Survey

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 D_0=0.231 Mpc/h and V_0=350 km/s at cz_0=1000 km/s. The luminosity function (LF) normalization \phi_*=0.02 (h/Mpc)^3} appropriate for PPS yields a number density threshold \delta n/n\approx 180 for the adopted D_0, instead of \delta n/n\approx 80 used in previous studies of other samples. 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. Group properties vary little with different redshift corrections, redshift cut-off, and galaxy LF, but are rather sensitive to the adopted links D_0 and V_0. More precisely, the typical (i.e., median) 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.(abridged)