At a distance of 3.3 Mpc, M81 group is one of the nearest groups of galaxies and thus an ideal place to study various galactic phenomena. Observations at all wavelengths can be made with high sensitivity and excellent spatial resolution (1″ = 15 pc). The three core members M81, M82, and NGC 3077 are known to be closely interacting from the 21cm HI emission studies (van der Hulst 1979, Yun et al. 1993, 1994), and all three galaxies show evidence for an AGN or starburst activity, possibly induced by the on-going tidal interactions. A more distant member NGC 2976 also shows evidence for tidal interaction within the group (Appleton et al., 1981). Therefore M81 group is an excellent test ground for studying tidal interaction and tidally induced activities.

HI observations have revealed large gaseous extensions in interacting and merging systems. The interstellar gas is obviously dragged out in tidal tails during an encounter, and the percentage of HI in the tails increases with the merging stage. However, the opposite is true for the molecular gas, which is observed highly concentrated towards the nuclei of interacting galaxies, amounting to a significant fraction of the dynamical mass. Statistically, there appears to be more gas observed in interacting galaxies than in normal, isolated ones. As N-body simulations show, the gas is driven inwards in the interaction process by the strong gravity torques, before being consumed through star formation in the triggered starbursts. We review here all observations that could bring more knowledge about the state of the gas in the outer parts of galaxies, and about accretion processes. The link with the observations of the Lyα absorbers at low and high redshifts is discussed.

Ring galaxies are believed to represent a special case of a collision between two galaxies, in which one of the galaxies impacts and passes through the center of another disk system (e.g. Lynds & Toomre 1976). Although rare, this kind of low orbital-angular-momentum collision leads to a recognizable structure, namely a luminous blue star-forming ring (Appleton & Marston 1997), which should be easily identifiable even at moderate redshift. Indeed, Lavery et al. (1996) have used this fact, and their relative rarity at low-redshift, to conclude that rings (and therefore presumably all collisions) are over-represented in deep HST fields.

No conference on the interaction of galaxies would be complete without a contribution on M51 (NGC5194) and its companion NGC5195. Much observational and theoretical work has been carried out to try to understand this interacting pair, and to elucidate the morphological features and weak AGN which are thought to be the result of the interaction.

Imaging of a large number of faint galaxies with the Hubble Space Telescope reveals a significant excess of interacting and peculiar objects among them (Griffiths et al., 1994, Burkey et al., 1994). Being expected in general, this result needs, however, to be expressed quantitatively. To estimate evolutionary changing of the relative number of interacting galaxies, δint(z), from their redshift z, one must determine reliably the local value, δint(0). But due to a subjective manner of definition of interacting galaxy system, the published values of δint(0) differ from one author to another several times. The most appropriate sample to estimate δint(0) would be quite a representative sample of very nearby galaxies, which is restricted by their distance or radial velocity. Below we consider data on such a fair sample, named the Local Volume (=LV), which is the most complete one at the present moment.

Although current observations and theoretical models indicate that galaxy mergers and interactions are catalysts in the process of galaxy evolution, we have only a limited quantitative understanding of some basic aspects of the process. For example, the rate at which galaxies merge is poorly constrained. We can simplify the problem by considering only disk galaxies, which because of the fragility of their disks (cf. Tóth and Ostriker 1992) have presumably not suffered a major merger. Even so, these galaxies have almost certainly experienced the infall of small companion galaxies at some time. The Milky Way is currently experiencing the accretion of the Sagittarius dwarf (Ibata, Gilmore, & Irwin 1994) and will eventually accrete the Magellanic Clouds (Tremaine 1976). To understand how galaxies evolve, we need to have quantitative knowledge of the accretion rate as a function of mass for all types of galaxies. Here we consider only the accretion of companion galaxies (~ 10% by mass) onto large spiral galaxies.

One important approach to the study of galaxy evolution is to identify those galaxies whose spectral and/or morphological characteristics suggest that they are in transition. For example, “E+A” galaxies, which have strong Balmer absorption lines and no significant [OII] emission, are generally interpreted as post-starburst galaxies in which the star formation ceased within the last ~ Gyr (Fig. 1). This transition between a star forming and non-star forming state is a critical link in any galaxy evolution model in which a blue, star forming disk galaxy evolves into a SO or elliptical. Another possible evolutionary track is that the star formation in an “E+A” resumes at some later time, if enough gas remains in the galaxy after its starburst ends. Given this ambiguity, it is important to investigate (1) the environment's role in “E+A” evolution, (2) the stellar and gas morphologies of “E+A”s, (3) the likely progenitors of “E+A”s, and (4) how common the “E+A” phase is in the evolution of galaxies.

We present new radio continuum images of the supermassive, peculiar galaxy NGC 1961 at 1.5, 4.9, 8.4 and 15 GHz. NGC 1961 (Arp 184) is a very massive (dynamical mass > 1012M⊙) Sb galaxy with a peculiar and asymmetric optical appearance. Furthermore, it exhibits an unusual, asymmetric HI distribution with an extensive wing of gas extending 30 kpc to the north-west and a sharp edge to the south-east (Shostak et al. 1982).

We have simulated a series of N-body models of tidal encounters between a disk galaxy and a perturbing galaxy to investigate the dynamical properties of tidally-induced galactic bars (Noguchi 1987, MNRAS, 228, 635). We have also calculated N-body models on isolated galaxies with bar-unstable disks (Ostriker & Peebles 1973, ApJ, 186, 467) to make a comparison between galactic bars of different origins (Miwa & Noguchi 1997, submitted).

We present the results of a modest grid of three-dimensional (SPH) simulations of nearly head-on collisions between two model galaxies, each consisting of a rigid halo and a gas-rich disk component. The companion to primary mass ratio used is typically 0.2 - 0.35. Simple models of radiative cooling and heating from young star activity are also included in the simulations. Star formation is assumed to occur when the local gas density exceeds a threshold value. In these collisions the primary gas disk remains largely intact, though a gas bridge is splashed out between the two galaxies. Star formation occurs in asymmetric ring-like waves in the primary. The bridge is predicted to have essentially no ongoing star formation. These results agree with observations of the Cartwheel and VII Zw 466 ring galaxies.

Thomson (1992) suggested that the Cen A group of galaxies may be the result of one interaction. Here we report full N-body simulations made to study this proposal in the M81-M82-NGC 3077 group.

We have observed a total of 74 polar ring galaxies (PRGs), PRG candidates and related objects at 3 radio observatories: Green Bank (Richter et al. 1994, AJ, 107, 1), Effelsberg (Huchtmeier 1997, A&A, 319, 401) and Nançay (van Driel et al. 1997, A&AS, in press; van Driel et al. 1998, A&A, in prep.). Our main aim is to identify systems suitable for HI synthesis mapping, crucial for an understanding of their dynamical state. Most objects were selected using one or more of the following 3 (<8000 km/s), or blue magnitude brighter than 15.5 mag, or kinematically confirmed PRG or good PRG candidate. A total of 62 objects were detected, 39 of which with more than one telescope.

This review focus on some issues which seem relevant to recent discussions: (1) how halo structure influences tail length, (2) the fate of power-law density cusps, (3) the results of unequal-mass disk galaxy mergers, and (4) the behavior of hot and cold gas in merging disk galaxies.

We present some preliminary results on the properties of multiple merger remnants. We discuss their axial ratios, their V/σ ratio as a function of ellipticity, and their circular velocity profiles.

The phenomenon of counterrotation is observed when two galaxy components have their angular momenta projected antiparallel onto the sky. It follows that if the two components rotate around the same axis, the counter-rotation is intrinsic. On the contrary the counterrotation is only apparent if the rotation axes are misaligned and the line-of-sight lies in between the two vectors or their antivectors. In the case of intrinsic counterrotation the two components can be superimposed or radially separated.

A single merger scenario for making galaxies such as NGC 4550 possessing equal coplanar counter-rotating stellar disks is investigated by collisionless N-body technique. The scenario is successful in producing an axisymmetric disk made of two almost equal counter-rotating populations. The final disk shows a clear bimodal line profile in the outer part, which demonstrates that disk-disk mergers do not always produce ellipticals.

We present some preliminary results of an on-going project to investigate the phenomenon of counter-rotation in galaxies. The analysis of two Sb and two barred galaxies shows the presence of stellar counter-rotating components in the central 1 to 3 kpc of each galaxy. This, along with similar structures found in ellipticals and early-type spirals, demonstrate that the counter-rotation extends along the Hubble sequence. The physical origin of these features is briefly discussed stressing the possible relevance of this phenomenon in understanding the general problem of galaxy formation.

It has been known since the early simulations of Wright (1972), and Toomre & Toomre (1972), that interactions between galaxies can give rise to quite spectacular morphological features, including spiral structure in disk galaxies, and extensive tails. It appears that long tidal tails only arise from interactions involving disk galaxies (Toomre & Toomre 1972), and thus the presence of two opposed long tidal tails in a number of disturbed galaxies such as the Antennae (Whitmore & Schweizer 1995), NGC 3921 (Schweizer 1996), and NGC 7252 (Schweizer 1982), has led to the interpretation of these galaxies as disk-disk mergers in progress. These systems can be modeled rather successfully, as the work of Hibbard & Mihos (1995) has shown.

We review the observed properties of globular cluster systems and their implications for models of galaxy formation. Observations show that globular clusters form in gas-rich mergers, and that bimodal metallicity distributions are common in the globular cluster systems of ellipticals, with the metal-poor population more extended than the metal-rich one. These are three of the four predictions of the simple merger model of Ashman & Zepf (1992). The fourth prediction concerns the properties of the globular cluster systems of spirals, and is still to be tested by observation. Adopting Occam's razor, the confirmation of the fundamental predictions of the merger model from both young and old globular cluster systems is strong evidence that typical elliptical galaxies formed from the mergers of spiral galaxies. However, the simplifying assumptions of the Ashman-Zepf merger model limit its applicability to certain complex situations such as the formation of cD galaxies. We conclude this review by introducing new observational and theoretical programs that will further the understanding of the physical mechanisms of globular cluster and galaxy formation.

There exists a relationship between globular cluster mean metallicity and parent galaxy luminosity (e.g. Brodie & Huchra 1991; Forbes et al. 1996), which appears to be similar to that between stellar metallicity and galaxy luminosity. The globular cluster relation has a similar slope but is offset by about 0.5 dex to lower metallicity. The similarity of these relations suggests that both the globular cluster system and their parent galaxy have shared a common chemical enrichment history. If we can understand the formation and evolution of the globulars, we will also learn something about galaxy formation. With this aim in mind we have created the SAGES (Study of the Astrophysics of Globular clusters in Extragalactic Systems) project. Project members include Brodie, Elson, Forbes, Freeman, Grillmair, Huchra, Kissler–Patig and Schroder. We are using HST Imaging and Keck spectroscopy to study extragalactic globular cluster systems. Further details are given at http://www.ucolick.org/~mkissler/Sages/sages.html.