Abstract. The results of the redshift measurements and of the detailed surface photometry in BVR of the compact group ShCG 376 are presented. The radial velocity dispersion, the virial mass, the total luminosity, the M/L ratio, and the crossing time of the group are estimated. The group consists of eight accordant redshift spiral galaxies. Four (or possibly five) of the group members have emission-line spectra. Such morphological content and the number of emission-line galaxies are very atypical for compact galaxy groups. There are signs of interaction between some members of the group. It is suggested that the irregular shape of the brightest galaxy No. 4 is probably due to interaction with other members of the group, particularly, the emission line galaxy No. 6 with a discordant redshift (∆v = 2600 km s −1 ). It is speculated that the latter galaxy may be a infalling intruder to the group.

We estimate the distribution of intrinsic shapes of UZC–SSRS2 groups of galaxies from the distribution of their apparent shapes. We measure the projected group axial ratio using the moments of their discrete galaxy distribution. Then, using the non‐parametric kernel method to estimate the smooth apparent axial ratio distribution, we numerically invert a set of integral equations to recover the corresponding intrinsic distribution under the assumption that groups are either oblate or prolate spheroids. We find that the prolate spheroidal model fits very well the UZC–SSRS2 group distribution with a true mean axial ratio 〈β〉≃ 0.3 and σβ≃ 0.15. This shows that groups of galaxies are significantly more elongated, both on the plane of the sky and in three dimensions, than clusters of galaxies. The poorest groups that we consider, those with four members, are even more elongated than the overall population with 85 per cent of the groups having β≲ 0.4.

Counts of galaxies are made in the vicinity of Hickson compact groups (HCGs). Loose group galaxies are detected in ∼70% of nearby groups with . It is shown that members of the loose group z ! 0.025 associated with a certain HCG are located in a relatively narrow strip oriented along the elongation of the HCG. Such a peculiar distribution of the loose group galaxies and the found correlation of the radial velocity dispersion of the loose group galaxies with the elongation of the group allows us to conclude that the loose group galaxies are rotating around a common gravitational center. This is a clue for the solution of the known puzzles related to HCGs: the lack of strong radio and luminous far-infrared sources, a surprisingly large spiral fraction, the lack of blue elliptical galaxies, and so on. Only those of such elongated formations in which the bright members happen to be oriented close to each other on the sky are detected as compact groups.

We investigate the relation between the projected morphology and the velocity dispersion of groups of galaxies using two recently compiled group catalogs, one based on the 2MASS redshift survey (Crook et al. 2007) and the other on the SDSS Data Release 5 galaxy catalog (Tago et al. 2008). We analyse a suitable subsample of groups from each catalog selected such that it minimizes possible systematic effects. We find that the velocity dispersion of groups is strongly correlated with the group projected shape and size, with elongated and larger groups having a lower velocity dispersion. Such a correlation could be attributed to the dynamical evolution of groups, with groups in the initial stages of formation, before virialization is complete, having small velocity dispersion, a large size and an elongated shape that reflects the anisotropic accretion of galaxies along filamentary structures. However, we show that the same sort of correlations could also be reproduced in prolate-like groups, irrespective of their dynamical state, if the net galaxy motion is preferentially along the group elongation, since then the groups oriented close to the line of sight will appear more spherical, will have a small projected size and high velocity dispersion, while groups oriented close to the sky-plane will appear larger in projection, more elongated, and will have smaller velocity dispersion. Although both factors must play a role in shaping the observed correlations, we attempt to disentangle them by performing tests that relate only to the dynamical evolution of groups (ie., calculating the fraction of early type galaxies in groups and the projected group compactness). Indeed we find a strong positive (negative) correlation between the group velocity dispersion (group projected major axis) with the fraction of early type galaxy members. We conclude that (a) the observed dependencies of the group velocity dispersion on the group projected size and shape, should be attributed mostly to the dynamical state of groups and (b) groups of galaxies in the local universe do not constitute a family of objects in dynamical equilibrium, but rather a family of cosmic structures that are presently at various stages of their virialization process.

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