The Relation Between Morphology and Dynamics of Poor Groups of Galaxies

Abstract

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.

The properties of early-type galaxies in the Ursa Major cluster

“…It was found that the fraction of massive early-type galaxies (E/S0) and dwarf elliptical galaxies is higher in a dynamically more evolved system with high velocity dispersion (Trentham & Tully 2002;Tovmassian et al 2004;Tovmassian & Plionis 2009). Especially in the case of the NGC 3992 subgroup, which is the most dynamically evolved Ursa Major cluster subgroup, considering its high frequency of early-type galaxies, morphological transformation to early-type galaxies by interactions and merging between galaxies may have occurred more frequently in the early stages of its dynamical evolution than in the other subgroups.…”

Groups of galaxies in the SDSS Data Release 7

“…Thus in studies of groups the proper choice of samples is important. To avoid contamination in their study Tovmassian & Plionis (2009) selected a sample of groups in a narrow richness interval to study the dynamics of poor groups.…”

“…Fortunately, many samples of groups are homogeneous as stated by authors and tested by other investigators. Tovmassian & Plionis (2009) studied the homogeneity of our SDSS DR5 groups (Paper II) and concluded that the sample is homogeneous with respect to virial properties. Nurmi et al (in preparation) studied N-body simulations of dark matter halos comparing the properties of simulated halos with our volume-limited groups of galaxies.…”

“…Tovmassian & Plionis (2009) studied the orientation and shape of the SDSS DR5 groups and found evidence that "groups of galaxies do not constitute a family of objects in dynamical equilibrium but rather are at various stages of their virialisation process".…”

Galaxy evolution in Local Group analogs

“…Tovmassian & Plionis 2009, and references therein) suggest that groups in the local universe are a family of cosmic structures presently at various stages of their virialization processes. The morphology-density relation found at high density regimes (Dressler 1980) is at work also at the group scale (Postman & Geller 1984).…”