We review old and new observational data for Stephan’s Quintet (SQ), the most famous compact galaxy group. Recently obtained multi-wavelength data has helped to clarify the past and present evolutionary history of this group. If SQ is typical of the compact group phenomenon, then: 1) the groups evolve slowly, resisting merging, due to the injection of energy from high velocity intruders and 2) show low levels of enhanced star formation because individual component ISM’s are quickly stripped via intruder collisions with resultant gas either too hot or cold/diffuse to form many stars. Infall of residual near nuclear gas may also stimulate the development of AGN.

We show that compact groups (CGs) are real physical entities, that almost all HCGs are dynamically associated with generally elongated loose groups of galaxies, that HCGs are compact cores of loose groups, and that they are longer living formations.

Based on the spectroscopic survey of de Carvalho et al. (1997), we analyse the structural and dynamical properties of 17 Hickson Compact Groups (HCG). This analysis probes a region of 0.5° × 0.5° around each group and shows that most are part of larger structures. Our results also suggest that the Hickson sample is composed of different dynamical stages of the group’s evolution. Specifically, we identify three possible evolutionary phases among groups in the sample: loose groups, [core+halo] systems and compact groups, each one presenting a distinct surface density profile. This sequence is consistent with the replenishment scenario for the formation and evolution of compact groups within larger and less dense systems.

In this paper I focus on three topics related to the dynamical evolution of small galaxy groups, for which the input of N-body simulations has been decisive. These are the merging rates in compact groups, the properties of remnants of multiple mergers, and the evolution of disc galaxies surrounded by one or more satellites. The short dynamical times of compact groups make it difficult to understand why such groups are observed at all. N-body simulations have pointed out two possible classes of solutions to this problem. The first one proposes that there is ongoing formation of compact groups, or that the longevity of the group is due to secondary infall. For the second class of solutions the longevity of compact groups is due either to their specific initial conditions, or to a massive common halo, encompassing the whole group. I discuss here these alternatives, together with their respective advantages and disadvantages. I then turn to the structure of remnants of multiple mergers and compare the results of N-body simulations with the properties of observed elliptical galaxies. Finally I discuss the dynamical evolution of a disc galaxy surrounded by one or more spherical satellites.

I will briefly describe ongoing work on numerical simulations intended to reproduce the Cartwheel galaxy, a prototype ring galaxy located in a compact group of galaxies.

We construct models of a galaxy in order to provide a possible identification of the dynamical processes that lead to the formation of structure observed in galactic nuclei. We assume that the center of our model contains a very massive double black hole, surrounded by relatively low-mass particles - star clusters, gas, and dust complexes. Our previous work (Anosova et al. 1994, 1995) snowed that the dynamical evolution of such a model produces many structures similar to those observed in the nuclei of galaxies, including rings and various types of flows and jets. In such models the ’gravitational slingshot’ effect frequently occurs.

We consider a number of such models with different initial parameters. Comparison of our models with the observed structure of NGC 4314 shows good agreement at one stage of the evolution for certain combinations of initial parameters. The model predicts the velocity field observed in NGC 4314.

We present a new mechanism which can explain the observed radial displacement between the stellar and gaseous shells observed around early type galaxies. Our model is based on the well-known phasewrapping process taking into account that a fraction of the interstellar medium is clumpy and consequently it experiences a low dynamical friction during a merging event. Moreover, our recent detection of cold molecular gas associated with the outer shells of Centaurus A is in agreement with the proposed model. This suggests that a considerable fraction of molecular gas in minor mergers may be distributed away from the nuclear regions and remain undetected.

The probability of plunging orbits is enhanced in groups of galaxies and indeed, observations show that ring galaxies, which are believed to form when a galaxy passes through the center of a larger rotating disk, are often found in small groups. Numerical simulations combined with a knowledge of the large-scale H I distribution provide strong constraints on the dynamical history of these systems and on the identity of the intruder. Here we present a numerical model of the Cartwheel galaxy which supports the suggestion that the most distant companion is the intruder. We also present high-resolution H I observations of the more irregular system Arp 119 that reveal a possible connection to the most distant companion.

We have studied the motion of massless particles (stars) bound to a stellar system (a galactic satellite) that moves on a circular orbit in an external field (a galaxy). A large percentage of the stellar orbits turned out to be chaotic, contrary to what happens in the usual restricted three–body problem of celestial mechanics where most of the orbits are regular. The discrepancy is probably due to three facts: 1) Our study is not limited to orbits on the main planes of symmetry, but considers three–dimensional motion; 2) The force exerted by the satellite goes to zero (rather than to infinity) at the center of the satellite; 3) The potential of the satellite is triaxial, rather than spherical.

In celestial mechanics the 3-body problem has a long and rich history, yet the particular problem of 3-galaxy systems has been rather less studied (see review by Valtonen & Mikkola 1991). Most of the up-to-date works have addressed the 3-galaxy problem using a point-like approach, although some explicit-physics simulations have been performed to simulate dynamical friction effects and merging processes (Zheng et al. 1993). These studies have provided important understanding on the general behaviour of observed galaxy triplets (Karachentsev 2000), and some accordance with observations has been obtained.

However galaxies are not point-like particles, but rather consist of a large number of stars which can be approximated as point-like particles. Qualitative and quantitative differences result in the dynamics when the 3-galaxy problem is addressed self-consistently; i. e. when galaxies are able to redistribute energy and angular momentum among their stars. Since not using self-consistent galaxies necessarily casts some doubts on earlier results, we address here the 3-galaxy problem self-consistently and compare some results with observations. A full report on this work is in preparation.

Several studies have addressed the topic of spiral interactions in pairs or in small groups (Barnes 1998). However there is no specific study, to our knowledge, of interacting triplets of spirals. Here we present a preliminary report of ongoing research on the dynamics of triplets that includes galaxies of different morphology. In particular we follow a triplet of equal-mass spirals until a merger remnant that resembles an elliptical-like galaxy has been formed.

It is known that the HI component of spirals is a good tracer of interactions. Indeed, several triplets (e.g. in M81 group) that do not appear to be perturbed in optical images do so in H I maps. Although a self-consistent treatment of gas requires the inclusion of e.g. pressure forces, we use here a test-particle approach to study it during a triple interaction of spirals; this can yield information about the large-scale kinematical features of gaseous tails.

Statistics of spin orientations in binary galaxies may be used as a tool to study the formation process of galaxies. Helou (1984) found that the spins of spiral galaxies in binary systems are anticorrelated, and he suggested that it may have to do with the inclination dependence of the galaxy merger process. We study this dependence in small groups of galaxies. An N-body code is developed which uses an inclination dependent dynamical friction law to affect mergers of galaxies. It is found that some correlation results in this way in groups, which have initially random spin orientations.

By using a few body code with a frictional force we study the dynamics of small galaxy groups. The results have been compared with observational data on triples and binaries by Karachentsev et al. We start with four to five galaxy groups initially and take samples of the groups when the membership has dropped to two or three. It is possible to find merger evolution models which agree with observations. One interesting factor is the mass-to-light ratio of the observed systems: we find the best fit with the models when the mass-to-light ratio is about 50 in solar units.

The formation of a multiple black hole system via merging processes of galaxies is simulated. Starting from the hierarchical halo merger model by Lacey and Cole (1993) and the general assumption that supermassive black holes (> 106M⊙) are common in the nuclei of galaxies we followed the formation of a four black hole system. Ultimately, instability of the four body system led to 835 symmetric two-sided slingshot ejections out of 10,000 simulations. This result was applied to the most straightforward application of the slingshot process, namely to the origin of extended radio sources.

The evolution of three and four black hole clusters which arise in mergers of small galaxy groups have been simulated. Typically, the chaotic dynamics of these multiple black hole systems leads to a decay via the slingshot process. Orbits of 100,000 initial configurations have been calculated and the final configurations have been classified in seven different categories. Examples of the orbits in each category are illustrated.

We have investigated the behavior of a general Newtonian three body system using the simplest initial conditions. We calculated the evolution of about a million bound three body systems. Within our resolution these cover all the possible initial states. All these systems eventually decay with one body being ejected from the residual binary system. The overall pattern representing the decay time scales projected on a homology map is dominated by resonances. In general appearance it reminds us of a traditional Masaai warrior shield.

Theories of nuclear activity mechanisms in disk galaxies (such as Seyfert galaxies) range from intrinsic (e.g. bars) to external (tidal perturbation by passing companions). The Byrd et al. (1986, 1987) simulation survey determined the tidal perturbation necessary to induce nuclear activity. Observational surveys have raised doubt as to the tidal explanation of Seyfert-type activity in galaxies with some suggesting a weak or nonexistent correlation between tidal perturbation and Seyfert activity compared to a normal sample. The simulations show a several hundred million year delay in the appearance of activity after perturbation and a similar duration which can explain these null results. For double galaxies, we derive from the tidal hypothesis that: (1) There should be an excess of pairs where both are active over pairs with one active. (2) Nuclear activity will be preferentially excluded from pairs which are unequal and favorably distributed to pairs where the members are equal. (3) The size of pair members to create activity can be smaller if the pair members are equal in size than if they are unequal. (4) Pairs where one or both members are active should have a smaller velocity difference times separation than if both are normal. All the predictions are seen in observations of activity in Karachentsev’s (1983) list of double galaxies. Optimally, statistical studies of nuclear activity and companions should include their sizes, separations, and radial velocities relative to the primary. The study by Dultzin-Hacyan et al. (1999) which finds an excess of large companions around Seyfert 2 galaxies is a step in the right direction.

Star formation processes in galaxies are far from being understood. Resonance rings of star formation are a common phenomenon of early-type barred spiral galaxies. Most rings are regions of enhanced star formation via gas cloud collisions near resonances, under the action of a gravitational bar perturbation. A small number of rings may form in response to a mild tidal interaction with a nearby companion. In either case, resonance rings are natural dynamical experiments in star formation stellar association time evolution. We study star formation in two galaxies where resonance rings probably have these two different modes of origin. NGC 3081 has four bi-symmetric star-forming rings that are probably connected to bar-driven gas dynamics, while NGC 4622 is a non-barred spiral where a strong one-sided ring may be connected to a past tidal interaction. We model the young clusters in the rings as two-fold and one-fold periodic orbits, respectively. We show that age indicators, Hα emission and the color index B – I, vary with position angle around the rings in a two-fold and one-fold sense as predicted by our models. The success of these models shows that future high resolution observations can be used to separate the individual associations from the underlying disk light so as to study their evolution and luminosity function.

Many barred spiral galaxies have an outer ring (R) or a pseudoring (R') in their outer disk. R. Buta (e.g. Buta 1995) has developed a detailed classification for different types of outer rings. Two main families of outer rings are recognized. In R1 family the major axis of the ring is perpendicular to the bar and the spiral arms wind 180° before meeting the other arm. In R2 family the major axis of the ring is parallel to the bar and the spiral arms wind 270°.

The outer rings are usually considered to be related to the outer Lindblad resonance (OLR) of a rotating stellar bar. The ring shapes correspond to families of closed orbits in the vicinity of this resonance (e.g. Contopoulos & Grosbøl 1989), and also the sizes of the outer rings relative to the bar component fit to this suggestion. Furthermore, test-particle simulations which use analytical bar potentials can produce different types of outer rings near the OLR (Schwarz 1981; Byrd et al. 1994). We have obtained similar results when we modelled ringed galaxy IC 4214 by constructing the gravitational potential from near-IR observations (Buta et al. 1999; Salo et al. 1999).

The overall morphology of the barred spiral galaxy NGC 7479 was modelled numerically in a simple minor merger scenario. I paid special attention to the morphology and features of the velocity field along the dominant spiral arm. The mass of the merging companion was 1/30 of the total mass of the primary. The companion was started in a circular prograde orbit at six disk scale lengths from the centre of the primary. I followed the evolution of the merger until the companion had reached the nuclear region of the primary. A comparison between the modelled and observed morphologies of the stellar and the ionized and neutral gas distributions and velocity fields suggests that the transient look of NGC 7479 is a result of a minor merger. I also tested the effects of variations in several initial parameters of the merger. See Laine and Heller, 1999, for further details.