Detailed studies of a new highly evolved group of galaxies

The formation and evolution of galaxies is one of the most important problems in cosmology. The most recent views of hierarchical structure formation in the Universe involve galaxy groups as building blocks of clusters and places where interactions pre-process galaxies. The study of groups` evolution is to be considered a milestone in the process of understanding the evolution of larger scale structures. Compact galaxy groups (CGs) offer the most favourable environmental conditions for interactions and merger processes to take place and lead to profound galaxy transformations. Subsequent mergers within CGs are thought to lead to the final coalescence of groups into bright elliptical galaxies. Although a few "fossil" groups have been identified, the details of processes of group evolution and coalescence are not yet clear. In particular the rarity of known systems in the verge of merging makes it exceedingly difficult to study in sufficient detail the passage from CG to elliptical galaxy. For this reason it is important to find at least one such an evolved group, which can be studied as a prototype of the final evolutionary phases of CGs. The most well known and studied sample of CGs, compiled by Hickson and originally comprising 100 groups, encompasses several evolutionary stages. However, none of these groups, with the possible exception of HCG 31, can be said to be really undergoing final coalescence. Recently, we discovered a very promising candidate of evolved CG. We found that this system showed all properties typical of CGs and additionally was exceptional in compactness, low velocity dispersion, and star formation activity (ApJ 522, L17, 1999). Since this appeared to be the most evolved group found to date, we started a three-years project (FWF, P15065-N02) aimed at a very detailed multiwavelength photometric and spectroscopic study of this system, named by us CG J1720-67.8. We collected and analysed a considerable amount of data in the optical, near-IR, and radio regimes and made good progress in the study of CG J1720-67.8`s members and their undergoing merging process. We also identified and investigated a number of tidal dwarf galaxy candidates belonging to the system. However, CG J1720-67.8 has proven even more complex a system than we originally thought. The interpretation of our most recent observations aimed at revealing the interaction history and likely future evolution of the group requires a comparison with detailed dynamical models. Therefore, to complete our analysis of the evolution of this prototypical CG by means of detailed modelling, we ask for a one-year extension of our project (1 post-doc position).

The formation and evolution of galaxies is one of the most important problems in cosmology. The important role of galaxy evolution within the group environment is underlined by the most recent views of hierarchical structure formation in the Universe that involve galaxy groups as building blocks of clusters and places where interactions pre-process galaxies. Compact galaxy groups (CGs) offer the most favourable environmental conditions for interactions and merger processes to take place and lead to profound galaxy transformations. Subsequent mergers within CGs are thought to lead to the final coalescence of groups into bright elliptical galaxies. Although a few "fossil" groups have been identified, the details of processes of group evolution and coalescence are not yet clear. In particular the rarity of known systems in the verge of merging makes it exceedingly difficult to study in sufficient detail the passage from a CG to an elliptical galaxy. For this reason, the related project P15065- N02 had been devoted to a very detailed multiwavelength photometric and spectroscopic study of such an evolved group, discovered by us and named CG J1720-67.8 (ApJ 522, L17, 1999). As a follow-up of the above study, this one-year project, P17772-N02, has focussed on the application of theoretical models and numerical simulations in order to fully exploit previously collected observational data and achieve a deeper comprehension of the history of this group. A two-fold approach has been followed. I) Modern techniques of evolutionary synthesis of stellar populations have been applied to reproduce as far as possible the observed properties of the galaxies in the group. Some attention has been devoted to the treatment of dust extinction during the comparison between models and observational data. The results are consistent with two of the galaxies having experienced a strong burst of star formation 40 to 180 million years ago, thus posing a constraint to the age of their latest gravitational interaction. An older, relatively strong, episode of star formation with an age of about 1 billion years, has characterized the history of the most massive member of the group. The hypothesis that this event was associated with a galaxy-galaxy merger cannot be ruled out. Additional outcomes of the modelling has been the estimate of the stellar masses of the galaxies and of the percentages of luminosity and stellar masses produced during their latest bursts of star formation. II) The above results as well as the observed configuration of the galaxies projected on the plane of the sky, have been useful to constrain dynamical models of the group. Through the application of the N-body hydrodynamical simulations we found that the presently observed galaxy configuration is inconsistent with a gravitational interaction involving only the two spiral galaxies of the group. The role played by the more massive, early-type companion galaxy in the interaction, that had remained unclear from the previous analysis of the observational data, appears to have been fundamental.