Detailed studies of a new highly evolved group of galaxies

One of the most important problems in cosmology concerns the formation and evolution of galaxies. Recent works support the hierarchical formation theory, in which massive galaxies form by subsequent mergers of smaller systems. Compact groups of galaxies (CGs) appear to play a remarkable role in this respect: They may respresent some of the densest concentrations of galaxies in the Universe and so can be regarded as a unique laboratory for the study of galaxy interactions, tidally triggered starbursts or AGN activity, and the merging of galaxies. Hickson catalogued 100 compact groups from a systematic search of the Palomar Survey prints, selecting them on the basis of population, isolation, and compactness; his groups (HCGs) have been the most systematically observed galaxy systems. CGs have high spatial density, low velocity dispersion (compared to clusters), and small size and therefore short crossing times and, consequently, they should have short coalescence lifetimes. These properties call for a high merging rate and turn CGs into putative factories of elliptical field galaxies. Details of these processes are, however, still unsettled and, particularly, the latest stages of the evolution of CGs are widely unknown - simply due to the fact, that a highly evolved CG was not known. Therefore, it is important to find and study at least one CG that shows characteristics of a group in a very advanced phase of evolution. We believe that we have discovered a quite promising candidate. For this new group we found (ApJ 522, L17, 1999) that it i) is ultradense (median projected separation of galaxies 6.9 kpc (d = 180 Mpc; H0 = 75), ii) has a very low velocity dispersion (67 km/sec), iii) is quite dusty (EB-V up to 1m), iv) has a prominent tidal tail, v) shows emission lines everywhere (starbursts), and vi) is isolated. It appears to be the most evolved group found to date. Therefore it will be rewarding to study this group in detail (and, to some extent, two others which were found by us recently, but are almost unstudied by us till now). We plan to apply for high-resolution imaging, deep photometry, high-resolution spectroscopy, HI and X-ray observations, and intend to calculate theoretical models, to address the following problems: detection and study of tidally triggered structures and the stellar populations present, accurate determination of the kinematics of the group galaxies and of the tidal structures, and detection and investigation of the interstellar/intergalactic medium (cold and hot gas, dust); in addition, a dynamical model will be developed for this group. Given the extent and complexity of the work, we apply for a post-doc position (3 years), the support for a master student grant for 1.5 years, and for the financial funds to cover altogether 4 observational runs.

One of the most important problems in cosmology concerns the formation and evolution of galaxies. Since 60% of galaxies are found in groups, the group environment is of utmost importance for galaxy evolution. The most recent views of hierarchical structure formation in the Universe involve galaxy groups as building blocks of the much larger galaxy clusters and as places where interactions pre-process galaxies, which eventually become part of the cluster population. In particular "compact groups" (CGs) with their high densities and low velocity dispersions offer the most favourable environmental conditions for interaction between group members and for merging processes to take place and lead to profound galaxy transformations. Their evolution is believed to lead to the coalescence of their brightest members in bright elliptical galaxies. However, despite numerous studies conducted mainly on the most famous compilation of CGs, Hicksons`s sample, the evolutionary path and final fate of CGs are far from being well established. - In this project our main goal was to advance our knowledge of the rare-to- observe and therefore less known phases of CG evolution, namely the process of final coalescence, thorough detailed observational studies of at least one example of evolved CGs. Therefore, we focused our attention on the galaxy group CG J1720-67.8 discovered by us a few years ago, which in our preliminary analysis revealed properties, like extreme compactness and very low velocity dispersion, suggestive of an advanced evolutionary stage. We followed a multiwavelength approach, making use of optical, near-infrared, far-infrared and radio data and exploiting modern observational techniques like integral field spectroscopy to investigate the morphological, physical, and kinematical properties of the group as a whole and of its members as well as the distribution and intensity of star formation activity across the group. Comparisons of the data with photoionization models, evolutionary synthesis models and published dynamical models of interacting galaxies were used to interpret our observational results and use them as a clue to the evolutionary history of the system. - As main outcomes of our investigations we confirmed our hypothesis that the group is in a particularly advanced evolutionary stage, we identified in it a few candidate (proto-) dwarf galaxies showing some signs of independent kinematics and we could outline a few possible interactions scenarios, which led to the present group configuration. Last not least the group was revealed as an even more complex system than originally thought.