Galaxy Groups

Galaxy groups are gravitationally bound structures with masses in the range between ~ 1013 and a few 1014 solar masses. As such, they fill the gap between galaxy pairs and galaxy clusters. Since most galaxies reside in groups, it is particularly important to understand how the group environment influences the evolution of galaxies and to which extent. It is worth noting that our own Galaxy, together with M31 is part of a loose group of galaxies, the Local Group.

The richness of groups varies from a handful of galaxies up to about a hundred galaxies. Many groups are found to contain a hot plasma that fills the space between galaxies and emits X-ray radiation. This plasma is called Intra-Group Medium (IGM) and is similar to the hot plasma found in galaxy clusters, but has a lower temperature. Its presence is a clear indication that groups are gravitationally bound structures with hot gas trapped in their common potential well.

Small groups are exceptional laboratories for studying gravitational interactions between galaxies. In fact, galaxies within groups move with relative velocities that are of the same order of magnitude of rotational velocities of disk galaxies or of the velocity dispersion of stars within elliptical and spheroidal galaxies. This fact, combined with the closeness of galaxies in space, favour gravitational interactions that lead to profound transformations of the galaxy morphologies and affect their kinematics as well as their as well as their ability to form new stars. Even new galaxies may be formed during these strong interaction events. These are known as "tidal dwarf galaxies" and form out of galaxy disk material, both gas and stars, which condenses in clumps within galaxy tidal tails. Groups could evolve in isolation and merge to form individual bright elliptical galaxies, thus giving rise to the so-called "fossil groups", but they could also fall into galaxy clusters thus enriching the cluster population with already "processed" galaxies.



CG J1720-67.8



At our institute we study in detail the physical and dynamical properties of some of such systems to better understand how they evolve and how groups influence the evolution of galaxies. We select systems that represent various phases in the evolution of groups: rare systems caught in the final stages of their evolution, with all their galaxies involved in strong interactions and next to merge into a single galaxy; dynamically young systems whose galaxies are still well separated but show signs of ongoing interactions, are very active in forming stars and host active galactic nuclei at their centre; intermediate stage systems, partly relaxed, that have a common halo of hot gas but also some strongly interacting galaxies among their members.  For our studies we use data collected at various telescopes around the world, e.g. the Very Large Telescope in Chile, that is operated by the European Southern Observatory. We make use of various observational techniques for imaging and spectroscopy in various spectral windows and of various modelling techniques for the interpretation of our observations.

In addition we are actively involved in a statistical study of obscured star formation in a sample of groups and clusters of galaxies from the local Universe up to redshift z ~ 1 (when the Universe had about a half of its current age) with the aim of studying the evolution of dusty star forming galaxies as a function of the environment. For this study we use groups and clusters that have been identified in the X-rays on a large area of sky (~ 11 deg2) in the framework of the XMM-Newton Large Scale Structure Survey (XMM-LSS) and we employ far-infrared data collected by the Spitzer Space Telescope in the framework of the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey.

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