We present new observations of Hα emission of the Hickson Compact Group 18 (HCG 18) obtained with a scanning Fabry-Perot interferometer. The velocity field does not show motions of individual group members but instead a complex common velocity field of H18b, c and d. The gas distribution is very clumpy with blobs coinciding with group members but located also elsewhere. Comparing Hα with H ɪ data we reached the conclusion that HCG 18 seems to be an irregular galaxy containing several clumps of star formation sites.

We report on a broad-band R surface photometric and low resolution spectroscopic study of a set of galaxies located in small groups and pairs of galaxies considered to be in low density environment. Groups span a wide range in density and show different morphological mix. We are analyzing systemic velocities, photometric parameters and spectral properties of the dominant galaxies in the groups. Our aim is to investigate the connection between interaction and induced galaxy activity, from star formation to AGN activity.

Dwarf satellite galaxies and globular clusters bound to luminous (in particular early type) galaxies, can be steered by interactions to orbits penetrating deep into the potential of their parent galaxy, where the outer stars are eroded and the dwarf satellite is completely destroyed in a relatively short time, resulting in very faint disturbances (soft merging) in the form of faint rings in the parent galaxy.

With special image processing methods we have found faint structures in E and SO galaxies, which for many reasons (mass, statistics, etc.) can be interpreted as observational evidence of this type of soft merging. This debris of dwarf satellites can provide a considerable part of the mass of the parent galaxy.

We give a short review of optical observations of the objects classified by Vorontsov-Vel’yaminov as “nests” of galaxies, describing their luminosity, color, structure and star formation properties. Some ideas on the nature of single dwarf “nests” are discussed.

Four southern compact groups, the Hickson Compact Groups HCG 22 and HCG 26 (Hickson 1982), and the groups AM 1238–396 and ESO 410-G(024—026) have been imaged in HI, along with 12cm continuum using the Australia Telescope Compact Array (ATCA). The initial findings for the latter two groups are presented here. While ESO 410-G is not in fact a physical group, due to the discordant redshifts amongst the members, it is however presented here.

Overall for all the groups we find no other sources of HI in the field that might indicate that these are part of a larger loose group structure. This is not always the case, as with HCG 23 (Williams 1995) and HCG 95 (Hutchmeier 1999), both of whom find additional Hɪ sources within the primary beam accordant with the compact group’s velocity.

The HI is also clearly associated with each of the individual member galaxies in all cases, except for HCG 26, which envelopes the whole group as was also shown by Williams and van Gorkom (Williams 1995).

A class of compact, isolated high–velocity clouds which plausibly represents a homogeneous subsample of the HVC phenomenon in a single physical state was objectively identified by Braun and Burton (1999). Six examples of the CHVCs, unresolved in single–dish data, have been imaged with the Westerbork Synthesis Radio Telescope. The high–resolution imaging reveals the morphology of these objects, including a core/halo distribution of fluxes, signatures of rotation indicating dark matter, and narrow linewidths constraining the kinetic temperature of several opaque cores. In these regards, as well as in their kinematic and spatial deployment on the sky, the CHVC objects are evidently a dynamically cold ensemble of dark–matter–dominated H ɪ clouds accreting onto the Local Group in a continuing process of galactic evolution.

Interactions between galaxies have spectacular effects on gas dynamics, and small galaxy groups are a privileged place to investigate them. In particular, they could test the existence of cold H2 gas as dark matter in the outer parts of galaxies. HI observations have revealed that galaxies in small groups are deficient in atomic gas, like in richer galaxy clusters such as Virgo, although in a lesser extent. Galaxy interactions could be the cause of this deficiency, stripping the gas out of galaxies and enriching the inter-cluster medium (ICM) in hot gas, which in turn strips gas through ram pressure. Alternatively, the gas present at the formation of the group could have been heated to its virial temperature, and be observed now as X-rays. The dynamical processes related to this extended gas in small galaxy groups are reviewed.

We have observed Hɪ emission and radio continuum emission from the compact group of galaxies HCG 95 with the Very Large Array (VLA)1. Two continuum sources coincide in with galaxies in this group: HCG 95 B (3.9 mJy) and HCG95C (6 mJy). Hɪ emission and absorption was detected in galaxy HCG 95 C. In addition we detected two so far unknown dwarf galaxies by their Hɪ emission within 3.5 arcmin of the group center. We did not detect galaxy b (with ç = 8000 kms−1 it is obviously a foreground object) and galaxy d — an edge-on Sc galaxy. This group definitely is Hɪ deficient compared with the average Hɪ content expected for spiral galaxies of the same luminosity and type. The first-ranked elliptical galaxy HCG 95 A might be responsible for the observed Hɪ deficiency in this group.

This is a report on the detection of HI emission from three ‘new’ dwarf galaxies Perseus A, Perseus B, and Camelopardalis D in the IC342/Maffei group of galaxies and of Draco A. The actual number of (probable) member galaxies of this group increases to 19 galaxies. Its velocity dispersion is 86 kms−1. With a distance of 2.2±0.5 Mpc this group is the nearest to the Local Group and might have considerable dynamical influence on the Local Group.

Compact groups are intermediate between single and binary galaxy systems, and clusters of galaxies. Because of their small numbers of members, and the small separations between the members of each group, they are good candidates for the study of galaxy interactions and mergers. In particular, high resolution imaging of these systems in neutral hydrogen can yield important data on interactions between galaxies.

The radio telescopes of the Australian National Telescope Facility (ATNF) are especially well suited to the study of neutral hydrogen gas in these systems. The Parkes 64-m telescope has a 15 arcminute FWHM beam at 21 centimeters wavelength which is sufficient to include the sky area of a typical compact group. Parkes was used for detection of HI in nine southern groups and for determination of their approximate velocities, if not previously known. Follow-up observations with the ATNF Compact Array synthesis telescope provided high resolution (20 arcsecond) images with appropriate frequency resolution to determine velocity fields for the gas distribution.

We present a global study of Hɪ spectral line mapping for 16 Hickson Compact Groups (HCGs) combining new and unpublished VLA data, plus the analysis of the Hɪ content of individual galaxies. Sixty percent of the groups show morphological and kinematical signs of perturbations (from multiple tidal features to concentration of the Hɪ in a single enveloping cloud) and sixty five of the resolved galaxies are found to be Hɪ deficient with respect to a sample of isolated galaxies. In total, 77% of the groups suffer interactions among all its members which provides strong evidence of their reality. We find that dynamical evolution does not always produce Hɪ deficiency, but when this deficiency is observed, it appears to correlate with a high group velocity dispersion and in some cases with the presence of a first-ranked elliptical. The X-ray data available for our sample are not sensitive enough for a comparison with the Hɪ mass; however this study does suggest a correlation between Hɪ deficiency and hot gas since velocity dispersions are known from the literature to correlate with X-ray luminosity.

X-ray observations of groups have provided considerable new insight into these cosmologically important systems. Recent ROSAT surveys of loose and compact groups suggest that ≃ 50% of all nearby groups contain a hot intragroup medium, including many spiral-rich groups. However, spiral-only groups tend not to be X-ray detected. Spectroscopic studies of X-ray groups indicate that these systems contain a large number of faint galaxies. The existence of a large population of faint galaxies, combined with the extended X-ray halos, is consistent with X-ray detected groups being real, bound systems. The nature of non-X-ray detected groups, which do not contain an extensive faint galaxy population, is still unclear.

ROSAT observations have also revealed the existence of individual elliptical galaxies with X-ray properties and total masses similar to those of X-ray groups. These objects may be the merger remnants of X-ray groups or ‘failed groups’ (i.e. overdensities that formed only a single galaxy). The number density of these objects suggests they make a non-negligible contribution to the mass density of the universe.

We present the largest survey to date of the X-ray properties of loose groups. We derive relations between X-ray luminosity, temperature and velocity dispersion, and also examine the surface brightness profiles of these systems. We find significant departures from the trends seen in galaxy clusters, which we interpret as arising from the effect of galaxy winds, and we briefly compare the properties of these loose groups with those of compact groups.

The study of the relationship between the hot gas in groups and the galaxies they contain can help to constrain the evolution of both galaxies and groups. Here we present evidence that the intergalactic medium in groups has been strongly affected by preheating associated with galaxy formation which mostly preceded group collapse. The presence of this effect appears to be unrelated to the morphology of group galaxies, which supports models in which galaxy types are not primordial. We also present preliminary evidence that early-type galaxies in groups are not underluminous in the X-ray compared to isolated galaxies, suggesting that their dark halos may not have been substantially stripped.

IC 1262 and NGC 6159 have been selected for high resolution observations because their X-ray luminosity observed in the ROSAT All Sky Survey (Voges 1992) was higher than that observed from other normal, early type galaxies. The HRI observations confirm both the identification of the X-ray sources with the two galaxies (see Fig. 1; point sources in the field, clearly associated with optical counterparts, further ensure that the absolute pointing is correct), and the very high luminosities. Moreover, they indicate very extended emission, out to ≥ 7′ and ≥ 4′ radius (≥ 400 and ≥ 220 kpc at the galaxies’ distances), respectively, with a relatively regular morphology (although not azimuthally symmetric at large radii), and a smoothly decreasing surface brightness profile. At the center of IC 1262, however, several peculiar features are present (see Fig. 3 and later discussion).

The nearby (D = 11 Mpc) sparse group of galaxies, Leo-I, is in many respects unique. It is the nearest group containing both bright spirals (M96 and M95) and a bright elliptical (M105). A giant (diameter ca. 200 kpc) intergalactic Hɪ ring orbits the central M105/NGC3384 galaxy pair and appears to interact with M96. If M96 is really in the group core, the Leo-I group provides an unusually “clean” route to determining the Hubble constant. In our 22 ksec ASCA SIS exposure of M96 we have detected diffuse X-ray emission extending more than 10 arcminutes North of M96, in the direction of the Hɪ ring. The morphology and spectral characteristics of the diffuse emission shows that M96 has recently interacted with the Hɪ ring, indicating that M96, the Hɪ ring and the central galaxy M105 are at the same distance within a few percent.

We report the results of a detailed statistical analysis of the Mid and Far IR (MIR/FIR) emission properties in a mixed morphology (E+S) sample of galaxy pairs from the Karachentsev (1972) Catalogue. The sample is large and diverse enough (≃ 130 pairs) to permit discrimination of pairs by morphological types and interaction classes. It samples a large enough volume to enable a non-parametric estimate of a Far-Infrared Luminosity Function (FIRLF). We find average factors of 3 and 5 enhancement in FIR and 25μm luminosities of the late-type pair components relative to an isolated galaxy control sample from Karachentseva (1973) Catalogue. This is interpreted as the MIR/FIR signature of the link between interaction and star formation. Although the spirals in (E+S) pairs fail to show a trend towards higher FIR luminosity with decreasing companion separation, a high-order comparison that takes into account information on the interaction classes, morphological types and the HI content suggests that:

1. • a clear correlation between the IR luminosities and interaction classes is present suggesting that a subsample of the (E+S) pairs is more closely related to recent events of star formation,

2. • the ratio of present to past star formation (as measured by the IR luminosities scaled to LB luminosity), increases measurably along the sequence Sa-Sc in paired spirals,

3. • no Hɪ depletion in the star formation-enhanced spirals in mixed pairs is found, and

4. • the high mass (M ≥ 10 M⊙) star formation rate (SFR) in paired Sc spirals is higher than that of the isolated Sc’s by a factor ~ 2 – 3.

Due to their very high spatial density and relatively low velocity dispersion, Hickson’s compact groups of galaxies (hereafter HCGs, Hickson et al. 1989a, H89a) are ideal test grounds where to study the physics of galaxy interaction.

Tidal shocks induced by galaxy interaction are expected to trigger rapid bursts of star formation in the circumnuclear region of galaxies. The increases in density and temperature caused locally by such shocks produce an enhancement in both the total FIR luminosity and in the ratio between the FIR fluxes at 60 and 100 μm. These enhancements have been actually observed in the circumnuclear regions of both interacting and ”putative” merging galaxies and should therefore be found also in all galactic environments where the frequency of interaction is higher than in the field. However, while interactions are more frequent among HCGs members than in the field (e.g. Hickson 1994, Shaker et al. 1999), there are conflicting arguments on whether HCGs present or do not present a FIR excess. As stressed by many authors, the critical point is the IRAS data resolution which prevents to map the spatial distribution of the FIR emission. We investigate the FIR properties of a selected sample of HCGs by sing the new high spatial resolution data from Allam et al. 1996 (A96). In particular, we address the problem whether or not there is any FIR enhancement in the compact groups and the reasons of that. This is a short presentation of the analysis performed and of the results obtained (Merluzzi et al. 1999).

I discuss the steering role played by small groups of galaxies concerning sources and structures on widely different scales: the very compact quasars, and the extended atmospheres of hot intracluster gas. The evolution of such sources out to substantial z is ruled by astrophysical processes taking place in early small groups.

A galaxy system must have a minimum velocity dispersion for its mass to be greater than the sum of the masses of its galaxies. Nearly half of the nearby Hickson compact groups (HCGs) have too low a velocity dispersion in comparison with the rotational velocities of their spiral galaxies and internal velocity dispersions of their early types.

A detailed study of the low velocity dispersion group, HCG 16 – the only known group of late-type galaxies with diffuse intergalactic X-ray emitting hot gas – reveals that half of the diffuse X rays are associated with foreground/background sources and the remaining gas is clumpy and mostly associated with the bright galaxies of the group. The large-scale environment of the group suggests that HCG 16 lies where a cosmological filament falls perpendicularly onto a large-scale sheet.

The observed frequency of compact groups is lower than predicted from the extended Press-Schechter formalism, which also predicts that most 1013 M⊙ objects in the Universe must be fairly old and hence have already coalesced into single objects, reminiscent of elliptical galaxies over-luminous in X-rays that are now being discovered.

Thus, the low survival time of dense groups against the merging instability is no longer a worry for compact groups, as they form in large enough numbers. I show why other arguments against the reality of HCGs no longer hold, partly because of the biases of Hickson’s sample.