We present the results of the spectral classiÐcation of the 82 brightest galaxies in a sample of 17 compact groups. We verify that the active galactic nuclei (AGNs) are preferentially located in the most early-type and luminous galaxies of the groups, as is usually observed in the Ðeld. But these AGNs also appear to be systematically concentrated toward the central parts of the groups. Our observations suggest a correlation between activity types, morphologies, and densities of galaxies in the compact groups. This is consistent with a scenario in which galaxies of compact groups evolve by interacting with their environment and are currently in a quiet phase of their activity.

Using a sample of 91 galaxies distributed over 27 compact groups (CGs) of galaxies, we define an index that allows us to quantify their level of activity due to an active galactic nucleus (AGN) or star formation. By combining the mean activity index with the mean morphological type of the galaxies in a group, we are able to quantify the evolutionary state of the groups. We find that they span an evolutionary sequence that correlates with the spatial configuration of the galaxies in the CG. We distinguish three main configuration types: A, B, and C. Type A CGs show predominantly low velocity dispersions and are rich in late-type spirals that show active star formation or harbor an AGN. Type B groups have intermediate velocity dispersions and contain a large fraction of interacting or merging galaxies. Type C comprises CGs with high velocity dispersions, which are dominated by elliptical galaxies that show no activity. We suggest that evolution proceeds A j B j C. Mapping the groups with different evolution levels in a diagram of radius versus velocity dispersion does not reveal the pattern expected based on the conventional fast merger model for CGs, which predicts a direct relation between these two parameters. Instead, we observe a trend contrary to expectation: the evolutionary state of a group increases with velocity dispersion. This trend seems to be related to the masses of the structures in which CGs are embedded. In general, the evolutionary state of a group increases with the mass of the structure. This suggests either that galaxies evolve more rapidly in massive structures or that the formation of CGs embedded in massive structures predated the formation of CGs associated with lower mass systems. Our observations are consistent with the structure formation predicted by the CDM model (or ÃCDM), only if the formation of galaxies is a biased process.

No abstract

We present the results of the classification of spectral activity types for 193 galaxies from a new sample of 49 compact groups of galaxies in the southern hemisphere (SCGs). The SCGs is a new sample of compact groups selected in automated fashion from a digitized galaxy catalogue, covering an area of ∼ 5200 sq deg around the South Galactic Pole. It is complete up to m∼ 14.5 in

We have conducted a new spectroscopic survey to characterize the nature of nuclear activity in Hickson Compact Groups (HCGs) galaxies and establish its frequency. We have obtained new intermediate resolution optical spectroscopy for 200 member-galaxies and corrected for underlying stellar population contamination using galaxy templates. Spectra for 11 additional galaxies have been acquired from the ESO and 6dF public archives and emission line ratios have been taken from the literature for 59 galaxies more. Here we present the results of our classification of the nuclear activity for 270 member-galaxies, which belong to a well defined sample of 64 HCGs. We found a large fraction of galaxies, 63%, with emission lines. Using standard diagnostic diagrams, 45% of the emission line galaxies were classified as pure AGNs, 23% as Transition Objects (TOs) and 32% as Star Forming Nuclei (SFNs). In the HCGs, the AGN activity appears as the most frequent activity type. Adopting the interpretation that in TOs a Low Luminosity AGN coexists with circumnuclear star formation, the fraction of galaxies with an AGN could rise to 42% of the whole sample. The low frequency (20%) of SFNs confirms that there is no star formation enhancement in HCGs. After extinction correction we found a median AGN Hα luminosity of 7.1×10 39 erg s −1 , which implies that AGNs in HCG have a characteristically low luminosity. This result added to the fact, that there is an almost complete absence of Broad Line AGNs in Compact Groups (CGs) as found by Martínez et al. (2008a) and corroborated in this study for HCGs, is consistent with very few gas left in these galaxies. In general, therefore, what may characterize the level of activity in CGs is a severe deficiency of gas.

We present the results of a morphological study based on NIR images of 25 galaxies, with different levels of nuclear activity (star formation or AGN), in 8 Compact Groups of Galaxies (CGs). We perform independently two different analysis: a study of the deviations of isophotal levels from pure ellipses and a study of morphological asymmetries. The results yielded by the two analysis are highly consistent. For the first time, it is possible to show that deviations from pure ellipses are produced by inhomogeneous stellar mass distributions related to galaxy interactions and mergers.We find evidence of mass asymmetries in 74% of the galaxies in our sample. In 59% of these cases, the asymmetries come in pairs, and are consistent with tidal effects produced by the proximity of companion galaxies. The symmetric galaxies are generally small in size or mass, inactive, and have an early-type morphology. They may have already lost their gas and least attached envelop of stars to their more massive companions.In 20% of the galaxies we find evidence for cannibalism: a big galaxy swallowing a smaller companion. In 36% of the early-type galaxies the color gradient is positive (blue nucleus) or flat. Summing up these results, as much as 52% of the galaxies in our sample could show evidence of an on going or past mergers. Our observations also suggest that galaxies in CGs merge more frequently under "dry" conditions (that is, once they have lost most of their gas).

We present the analysis of the spectra of 62 galaxies in 15 compact groups. The galaxies were classified in four activity classes: galaxies without emission, starburst nucleus galaxies (SBNGs), luminous AGNs and low-luminosity AGNs (LLAGNs). The star formation in the HCG starbursts is more intense than in normal spirals, but comparable to those in the field SBNGs. Their mean gas metallicity is solar and they do not follow the metallicity-luminosity relation traced by the early-type SBNGs in the field, suggesting that most of them are late-type SBNGs. This morphology preference coupled to the observation that the HCG starbursts are predominantly in the halo of the groups is consistent with the idea that compact groups are embedded in sparser structures. The stellar metallicities of the non starburst galaxies are relatively high for their luminosities. In these galaxies the equivalent widths of the metal absorption lines are slightly narrower than normal while the Balmer absorption lines are relatively strong. All these galaxies could be ``post-starburst'' at an advanced stage of evolution. Our observations are supporting a scenario where the core of the groups are slowly collapsing evolved systems embedded in more extended structures (Ribeiro et al. 1998). In the core of the groups, the interactions were more frequent and the galaxies evolved at a more rapid rate than in their halos.Comment: 36 pages, 10 encapsulated postscript figures. to appear in ApJ, October 20, 1998 issue, Vol. 506 #

No abstract