14 research outputs found
Morphological and Luminosity Content of Poor Galaxy Groups
We find that the fraction of early-type galaxies in poor groups (containing
from 4 to 10 members) is a weakly increasing function of the number of the
group members and is about two times higher than in a sample of isolated
galaxies. We also find that the group velocity dispersion increases weakly with
the fraction of early-type galaxies. Early-type galaxies in poor groups are
brighter in the near-infrared with respect to isolated ones by 0.75 mags (in K)
and to a lesser degree (by 0.5 mags) also in the blue. We also find early-type
galaxies in groups to be redder than those in the field. These findings suggest
that the formation history for early-type galaxies in overdense regions is
different from that of in underdense regions, and that their formation in
groups is triggered by merging processes.Comment: 5 pages, 3 figures, ApJ Lett. in pres
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.Comment: ApJ accepted, 8 pages, 8 figure
Richness Dependence of the Recent Evolution of Clusters of Galaxies
We revisit the issue of the recent dynamical evolution of clusters of
galaxies using a sample of ACO clusters with z<0.14, which has been selected
such that it does not contain clusters with multiple velocity components nor
strongly merging or interacting clusters, as revealed in X-rays. We use as
proxies of the cluster dynamical state the projected cluster ellipticity,
velocity dispersion and X-ray luminosity. We find indications for a recent
dynamical evolution of this cluster population, which however strongly depends
on the cluster richness. Poor clusters appear to be undergoing their primary
phase of virialization, with their ellipticity increasing with redshift with a
rate de/dz ~ 2.5, while the richest clusters show an ellipticity evolution in
the opposite direction (with de/dz ~ -1.2), which could be due to secondary
infall. When taking into account sampling effects due to the magnitude-limited
nature of the ACO cluster catalogue we find no significant evolution of the
cluster X-ray luminosity, while the velocity dispersion increases with
decreasing redshift, independent of the cluster richness, at a rate dsigma/dz ~
-1700 km/sec.Comment: 10 pages, MNRAS in pres