Galaxies.

We have determined the mass function of loose groups of galaxies in the Las Campanas Redshift Survey. Loose groups of galaxies in the LCRS range in mass from M ∼ 10 12 M ⊙ to 10 15 M ⊙. We find that the sample is complete for masses in the interval 8 · 10 13 − 8 · 10 14 M ⊙. In this interval the mass function of LCRS loose groups is close to that found by Girardi et al. (1998). Comparison of the observed mass function with theoretical mass functions obtained from N-body simulations shows good agreement with a CDM model with density parameters Ω m = 0.3, Ω Λ = 0.7 and the amplitude of perturbations σ 8 = 0.78. For smaller masses the mass function of LCRS loose groups flattens out, differing considerably from the group mass function found by Girardi and Giuricin (2000) and from mass functions obtained by numerical simulations. We suspect that this flattening is due to incompleteness in the LCRS loose group catalogue caused by selection effects, although we cannot fully discount the possibility that it may in fact be an intrinsic property of the loose group mass function. Key words: cosmology: observations – cosmology: large-scale structure of the Universe A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are: missing; you have not inserted them ASTRONOMY AND ASTROPHYSICS The mass function of the Las Campanas loose groups of galaxies