The Relation between Morphology and Dynamics of Poor Groups of Galaxies

We investigate the relation between the projected morphology (b/a) and the velocity dispersion (sigma_v) of groups of galaxies using two recently compiled group catalogs, one based on the 2MASS redshift survey and the other on the SDSS Data Release 5 galaxy catalog. We find that the sigma_v of groups is strongly correlated with the group projected b/a and size, with elongated and larger groups having a lower sigma_v. Such a correlation could be attributed to the dynamical evolution of groups, with groups in the initial stages of formation, having small sigma_v, a large size and an elongated shape that reflects the anisotropic accretion of galaxies along filamentary structures. The same sort of correlations, however, could also be reproduced in prolate-like groups, 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 large sigma_v, while groups oriented close to the sky-plane will appear larger in projection, more elongated, and will have smaller sigma_v. We perform tests that relate only to the dynamical evolution of groups (eg., calculating the fraction of early type galaxies in groups) and indeed we find a strong positive (negative) correlation between the group sigma_v (projected major axis) with the fraction of early type galaxies. We conclude that (a) the observed dependencies of the group sigma_v on the group projected size and b/a, should be attributed mostly to the dynamical state of groups and (b) groups 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.

💡 Research Summary

The paper investigates how the projected morphology of poor galaxy groups—quantified by the axial ratio (b/a) and projected size—relates to their internal kinematics, measured by the line‑of‑sight velocity dispersion (σ_v). Using two recent group catalogs, one derived from the 2MASS Redshift Survey and the other from the Sloan Digital Sky Survey Data Release 5, the authors assemble several hundred groups with well‑determined member redshifts. For each group they compute σ_v from the redshift differences of member galaxies, and they determine the projected major and minor axes from the sky positions, thereby obtaining b/a and the projected linear size.

Statistical analysis reveals two robust correlations: (1) σ_v increases with increasing b/a, meaning that groups appearing more circular in projection have larger velocity dispersions, while elongated groups (small b/a) have lower σ_v; (2) σ_v decreases with increasing projected size, so larger‑appearing groups tend to be dynamically colder. These trends could arise from two distinct physical mechanisms.

The first mechanism is dynamical evolution. In the early stages of group formation, galaxies accrete anisotropically along filaments, producing elongated, spatially extended configurations with modest relative motions, thus low σ_v. As the system evolves, gravitational interactions and mergers drive the group toward a more virialized, spherical configuration, raising σ_v. Consequently, the observed σ_v–b/a and σ_v–size relations would trace the degree of virialization.

The second mechanism involves projection effects in intrinsically prolate groups. If galaxies move preferentially along the group’s major axis, a group observed along that axis will appear rounder (high b/a), have a small projected size, and exhibit a large σ_v because the line‑of‑sight component of the motion is maximized. Conversely, a group viewed side‑on will look more elongated, larger in projection, and display a smaller σ_v.

To discriminate between these scenarios, the authors introduce an independent indicator of dynamical maturity: the fraction of early‑type (E/S0) galaxies within each group. Early‑type galaxies are known to dominate in dense, dynamically evolved environments. The analysis shows a strong positive correlation between σ_v and the early‑type fraction, and a strong negative correlation between the projected major axis length and the early‑type fraction. In other words, groups with high σ_v and more circular projections contain a larger proportion of early‑type galaxies, whereas elongated, larger groups host fewer early types. This pattern supports the dynamical‑evolution interpretation, because projection alone would not produce a systematic variation in galaxy morphology.

The authors conclude that (a) the dependencies of σ_v on projected size and axial ratio are primarily driven by the groups’ dynamical state rather than purely geometric projection, and (b) poor galaxy groups in the local universe are not a homogeneous family of virialized systems. Instead, they represent a spectrum of structures at different stages of collapse and virialization, reflecting the ongoing assembly of cosmic web filaments into bound systems. This insight has implications for how group catalogs are used in studies of galaxy evolution, large‑scale structure, and the calibration of mass‑observable relations in cosmology.