A statistical criterion has been proposed to reveal the random and physical clusterings among stars, galaxies and other objects. This criterion has been applied to the galaxy triples of the list by Karachentseva, Karaschentsev and Scherbanovsky, and the double galaxies of the list by Dahari where the primary components are the Seyfert galaxies. The confident physical, probable physical, probable optical and confident optical groups have been identified. The limit difference of radial velocities of components for the confident physical multiple galaxies has also been estimated.

The purpose of this work is a verification of the virial mass estimations for small galaxy groups. The dynamical evolution of triple and quintuple galaxies has been studied by the numerical simulations. The dependence of the virial coefficient k(t) versus time was derived. Initially k(0) = 0. The function k(t) has some strong oscillations from 0.02 to 0.99. Generally, these oscillations are quasiperiodical ones. Such a behavior of k(t) is caused by formation in a system of close isolated temporary double subsystems. A strong correlation between the virial coefficient and the least mutual distance in the system is observed.

Large-scale structures of the universe have been variously described as sheetlike, filamentary, cellular, bubbles or spongelike. Recently cosmic strings become one of viable candidates for galaxy formation scenario, and some of the large-scale structures seem to be simply explained by the open cosmic strings).

In addition to the well established global HI-deficiency seen in Virgo cluster spirals, many of them are found to be HI-deficient in their inner disks. We show that collisions between galaxies in a cluster can lead to the preferential removal of HI from these galaxies. This follows directly from the application of the Spitzer-Baade collisional gas removal mechanism to galaxies consisting of stars and a two-component ISM of HI and H2, with HI having the largest filling factor. This can account for both the observed Hl-deficiency in the inner disks of these galaxies as well as their normal H2 contents. The frequency of galaxy collisions in the Virgo cluster is shown to be large enough to make collisional gas removal a viable mechanism.

The problem of the origin of starburst and nuclear (nonstellar) activity in galaxies is reviewed. A physical understanding of mechanism(s) that induce both types of activity requires one to address the following issues: 1) what is the source of fuel that powers starbursts and active galactic nuclei? and 2) how it is channeled towards the central regions of host galaxies? As a possible clue, we examine the role of non-axisymmetric perturbations of galactic disks and analyze their potential triggers. Global gravitational instabilities in the gas on scales ~ 100 pc appear to be crucial for fueling the active galactic nuclei.

The satellite of an S-galaxy will experience opposing dynamical-friction forces from the stars of the disk and the halo. If these forces are in balance, the satellite may travel in a stable, near-circular orbit whose radius, for a wide range of physical parameters, should be limited to a zone 1.2 to 1.4 times the disk radius, much as is observed.

Galaxy-galaxy interactions have long attracted many extragalactic astronomers in various aspects. A number of computer simulations performed in the 1970s have successfully reproduced the peculiar morphologies observed in interacting disk galaxies and clarified that tidal deformation explains most of the observed global peculiarities. However, most of these simulations have used test particles in modelling the disk component. Tidal response of a self-gravitating disk remains to be further clarified.

Another topic which is intensely discussed at present is the relation between galaxy-galaxy interactoins and activity. Many observations suggest that interactions trigger strong starbursts and possibly active galactic nuclei (AGN). However, the detailed mechanism of triggering is not yet clear. It is vital here to understand the dynamics of interstellar gas.

Tidal interaction between the galaxies in binary systems leads to important consequences. Some peculiarities in galactic morphology as well as the transfer of matter from one galaxy to another may be due to this factor. In particular, gas flows in intergalactic space may be formed. Such flows enriching one component with gas from the other may play a substantial role in the evolution of mixed (S-E) pairs. One can mention several facts corroborating the possibility of the gas transfer from the spiral to the elliptical galaxy.

Multi-color surface photometry (BVri) is presented for the tidal features in a sample of interacting galaxies. Large color variations are found between the morphological components and within the individual components. The blue colors in the primary and the tidal features are most dramatic in B-V, and not in V- i indicating that star formation instead of metallicity or age dominates the colors. Color variations between components is larger in systems shortly after interaction begins and diminishes to a very low level in systems which are merged. Photometric models for interacting systems are presented which suggest that a weak burst of star formation in the tidal features could cause the observed color distributions. Dynamical models indicate that compression occurs during the development of tidal features causing an increase in the local density by a factor of between 1.5 and 5. Assuming this density increase can be related to the star formation rate by a Schmidt law, the density increases observed in the dynamical models may be responsible for the variations in color seen in some of the interacting systems. Limitations of the dynamical models are also discussed.

Starbursts have been a puzzling field of research for more than a decade. It is evident that they played a significant rôle in the evolution of many galaxies but still quite little is known about the starburst mechanisms. A way towards a better interpretation of the available data is the comparison with evolution models of starbursts. The modelling of starbursts and the fitting of such model starbursts to observed data will be discussed in this paper. The models have been applied to a subset of starburst and post-starburst galaxies in a sample of 30 interacting systems. These galaxies are not ultraluminous FIR galaxies but rather ordinary starburst galaxies with FIR luminosities of a few 1010 to a few 1011L⊙.

Collisions between galaxies can induce large morphological changes in the participants and, in the case of colliding disk galaxies, bridges and tails are often formed. Observations of such systems indicate a wide variation in color (see Larson and Tinsley, 1978) and that some of the participants are experiencing enhanced rates of star formation, especially in their central regions (Bushouse 1986, 1987; Kennicutt et al., 1987, Bushouse, Lamb, and Werner, 1988). In this paper we describe progress we have made in understanding some of the dynamics of interacting galaxies using N-body stellar dynamical computer experiments, with the goal of extending these models to include a hydrodynamical treatment of the gas so that a better understanding of globally enhanced star formation will eventually be forthcoming.

At this meeting much attention has been focussed on interactions and mergers between roughly equal mass galaxies. On the contrary, I will begin by mentioning a few justifications for studying collisions with relatively low mass companions, specifically, less than about one third of the mass of the target galaxy. The first is simply that such collisions are likely to be common, given that the galaxy luminosity function is broad. The second reason is that such collisions have evidently been less well studied than collisions between nearly equal partners. However, there are a few important exceptions to this generalization, including the sinking satellite problem (e.g. Quinn and Goodman 1986), and the collisional model for the formation of shell galaxies in which a companion of negligible mass is completely disrupted(e.g. Dupraz and Combes 1986, Hernquist and Quinn 1988). The third, and potentially most important reason, is that the effects of a collision with a low-mass companion are less extreme (at least from the big galaxy’s point of view!). Thus, these effects are closer to the theorist’s ideal of a “small perturbation”. This is important for both conceptually understanding the effects of the collision, and for justifying the use of approximate numerical techniques (e.g. restricted three-body) to study them.

A companion can induce a varitey of morphological changes in a galaxy. I use N-body simulations to study the effects of different kinds of perturbations on the dynamics of a disk galaxy. The model is two-dimensional, with a disk consisting of about 60,000 particles. Most of the particles (80 %) represent the old stellar population with a high velocity dispersion, while the rest (20 %) represent gas clouds with a low velocity dispersion. Initially, the velocity dispersion corresponds to Q = 1 for the “star” particles, and Q = 0 for the “gas” particles, where Q is Toomre’s (1964) stability parameter. The gas clouds can collide inelastically. The disk is stabilized by a rigid halo potential, and by the random motions of the old “star” particles. To simulate the effect of an encounter on the disk, a companion galaxy, modelled as a point mass, can move in a co-planar orbit around the disk. A complete description of the N-body code is found in Thomasson (1989).

We discuss new evidence which supports the existence of thick disks in elliptical/S0 galaxies. Numerical simulations of weak interactions with thick disk systems produce shell structures very similar in appearance to those observed in many shell galaxies. We think this model presents a more plausible explanation for the formation of shell structures in elliptical/S0 galaxies than does the merger model and, if correct, supports the existence of thick disks in elliptical/S0 galaxies.

In order to contribute to an understanding of collision-induced staxburst activities, we present a detailed case study on the staxburst galaxy Maxkaxian 297 (= NGC 6052 = Arp 209; hereafter Mrk 297). This galaxy is classified as a clumpy irregular galaxy (hereafter CIGs) according to its morphological properties (cf. Heidmann, 1987). Two major clumps and many small clumps are observed in the entire region of Mrk 297 (Hecquet, Coupinot, and Maucherat 1987). A typical major clump of CIGs has a diameter of a few hundred pc and its dynamical mass is estimated as an order of 108M⊙ (Taniguchi and Tamura 1987).

This has been an excellent conference with a very international component and a rich and wide ranging diversity of views on the topical subject of paired and interacting galaxies. The southern hospitality shown to us by our hosts Jack Sulentic and Bill Keel has been most gracious and the general growth of the astronomy group at the University of Alabama is most impressive.

The conference began with the presentation of the basic data sets on pairs, groups, and interacting galaxies with the latter being further discussed with respect to both global properties and properties of the galactic nuclei. Then followed the theory, modelling and interpretation using analytic techniques, simulations and general modelling for spirals and ellipticals, starbursts and active galactic nuclei. Before the conference I had written down the three questions concerning pairs, groups and interacting galaxies that I hoped would be answered at the meeting: (1) How do they form? including the role of initial conditions, the importance of subclustering, the evolution of groups to compact groups, and the fate of compact groups; (2) How do they evolve? including issues such as relevant timescales, the role of halos and the problem of overmerging, the triggering and enhancement of star formation and activity in the galactic nuclei, and the relative importance of dwarf versus giant encounters; and (3) Are they important? including the frequency of pairs and interactions, whether merging and interactions are very important aspects of the life of a normal galaxy at formation, during its evolution, in forming bars, shells, rings, bulges etc., and in the formation and evolution of active galaxies. In what follows I shall summarize the meeting and where possible focus on these three central issues. Since this is a conference summary my references are all to papers presented at this meeting.