The dominant galaxy of a rich cluster is often an object whose formation and evolution is closely connected to the dynamics of the cluster itself. Hoessel (1980) and Schneider et al. (1983) estimate that 50 % of the dominant galaxies are either of the dumb-bell type or have companions at projected distances less than 20 kpc, which is far in excess of the number expected from chance projection (see also Rood and Leir 1979).

Presently there is no complete sample of these objects, with the exception of the listing of dumb-bell galaxies in BM type I and I-II clusters in the Abell statistical sample of Rood and Leir (1979).

Several panelists stressed the need for care in defining observational criteria for considering a system interacting, beyond some kind of morphological peculiarity. Inevitably, some theoretical guidance is needed in this kind of interpretation. For example, narrow bridges and tails are well understood as results of certain kinds of interactions involving disk systems. Heckman stressed that such features should be seen in the old stellar population (the “Crimson Tide” approach) to avoid confusion with line emission or modest amounts of recent star formation that might have been influenced by hydrodynamical processes. Systems of these kinds will generally have an identifiable companion, perhaps tidally stripped or with its own tidal distortions, allowing further consistency checks on an interaction.

Recent observations of the evolutionary properties of paired and interacting galaxies are reviewed, with special emphasis on their global emission properties and star formation rates. Data at several wavelengths provide strong confirmation of the hypothesis, proposed originally by Larson and Tinsley, that interactions trigger global bursts of star formation in galaxies. The nature and properties of the starbursts, and their overall role in galactic evolution will also be discussed.

Galaxy-galaxy collisions are known to produce drastic changes in morphology and, in many cases, enhance the level of star formation activity in galaxies. In order to better quantify the effects that interactions have on the star formation characteristics of galaxies we have undertaken a multiwavelength survey of a large sample of interacting disk-type galaxies. The sample is optically-selected, the inclusion of systems having been based upon the presence of unusual morphological features—such as tidal tails, plumes, rings, warped disks—suggestive of tidal interaction. The sample is composed of about 115 systems, most of which are spiral-spiral pairs, with a few spiral-elliptical pairs and a few merging systems (see Bushouse 1986 for more details of the sample selection). This sample has now been studied in the optical, infrared, and radio regimes, including optical spectra and Ha images, near-infrared photometry and imaging, far-infrared photometry, HI 21cm emission-line measurements, VLA 20cm maps, and CO emission-line measurements. This paper presents an overview and comparison of the results of the optical, infrared and CO surveys. With these data we can compare the far-infrared and CO properties of the galaxies with the classic optical and radio indicators of star formation activity and thereby determine what, if any, relationships exist between star formation activity and the far-infrared and CO properties of the galaxies.

Much progress has been made in understanding the effects of interaction on galaxies (see reviews in this volume by Heckman and Kennicutt). Evidence for enhanced emission from galaxies in pairs first emerged in the radio (Sulentic 1976) and optical (Larson and Tinsley 1978) domains. Results in the FIR lagged behind until the advent of the IRAS satellite. The last five years have seen numerous FIR studies of optical and IR selected samples of interacting galaxies (e.g., Cutri and McAlary 1985; Joseph and Wright 1985; Kennicutt et al. 1987; Haynes and Herter 1988). Despite all of this work, there are still contradictory ideas about the level and, even, the reality of an FIR enhancement in interacting galaxies. Much of the confusion originates in differences between the galaxy samples that were studied (i.e., optical morphology and redshift coverage). We report on a study of the FIR detection properties for a large sample of interacting galaxies and a matching control sample. We focus on the distance independent detection fraction (DF) statistics of the sample. The results prove useful in interpreting the previously published work. A clarification of the phenomenology provides valuable clues about the physics of the FIR enhancement in galaxies.

We study the “Catalogue of Isolated Pairs of Galaxies in the Northern Hemisphere” (CPG hereafter) by Karachentsev (1972), and a well matched comparison sample taken from the “Catalog of Isolated Galaxies” (Karachentseva 1973, CIG hereafter), in order to quantify the enhanced FIR emission properties of interacting galaxies (see Sulentic 1989 for details of the samples). The isolation criterion used in compiling the CPG and the CIG means that both the pairs and single galaxies have been Httle influenced by their environment for a long time (t ≳ 109 yrs: Stocke et al 1978).

Short wavelength IUE spectra of Arp 248b and UGC 8315N are combined with optical spectra and interpreted using a combination of spectrum synthesis and spectral diagnostics to place constraints on the massive star populations of the central regions of these galaxies and to deduce information about the star formation histories in the last 108 years. We find that both galaxies have substantial fractions of their optical light coming from massive stars and that Arp 248b may be dominated in the UV by WR stars. The UV spectra are dominated by radiation from evolved massive stars and we place an age on the burst in Arp 248b of a few tens of millions of years.

We present the first results of a multiwavelength study of IRAS galaxies with excess radio emission. The sample was selected by cross-correlating the IRAS Faint Source Survey (for |b| ≥ 50°) and the Point Source Catalogue (for 10° < |b| < 50°) with the Texas radio survey. Recent optical (imaging and spectroscopic) and radio (VLA) observations are discussed. These observations will be used to investigate possible connections between radio galaxy activity, star formation and galaxy interactions.

There are three types of supernovae: two subtypes SNIa and lb; and SNII. Late type galaxies produce all types of SN, whereas early types (E, SO, and non-Magellanic irregulars 10) have hosted only SNIa. The recently identified SNIb, like SNII, have massive stars as their progenitors.

Reviving Oemler & Tinsley’s (1979) suggestion that SNIa also come from short-lived stars, we have asserted that they need not occur in all early-type galaxies. SNIa occur only in those galaxies that have access to gas and can form stars in their main body. (SN in nuclear regions are a different matter altogether). In this model, SNIa are not associated with typical stellar population of E/SOs but with regions of localized star formation. Note that data on SNIa from spirals is already consistent with this model (see Kochhar 1989 for references and details).

Infrared imaging of interacting galaxies is especially interesting because their optical appearance is often so chaotic due to extinction by dust and emission from star formation regions, that it is impossible to locate the nuclei or determine the true stellar distribution. However, at near-infrared wavelengths extinction is considerably reduced, and most of the flux from galaxies originates from red giant stars that comprise the dominant stellar component by mass. Thus near infrared images offer the opportunity to study directly components of galactic structure which are otherwise inaccessible. Such images may ultimately provide the framework in which to understand the activity taking place in many of the mergers with high IRAS luminosities.

Kennicutt & Kent (1983) have shown that the global Ha emission from a spiral galaxy is an indicator of the formation rate of massive stars. Moss, Whittle &: Irwin (1988) have surveyed two clusters (Abell 347 and 1367) for galaxies with Ha emission using a high dispersion objective prism technique. The purpose of the survey is to investigate environmental effects on star formation in spiral galaxies, and in particular to ascertain whether star formation is enhanced in cluster spirals. Approximately 20 % of CGCG galaxies were detected in emission. Two plates of excellent quality were obtained for each of the two clusters, and galaxies were only identified to have emission if this was detected on both plates of a plate pair. In this way, plate flaws and other spurious identifications of emission could be rejected, and weak emission confirmed.

Some pecularities of the circummucleus of the Seyfert galaxy NGC 7469 were revealed, plausibly caused by interaction with the satellite IC 5283 and a starlike detail, situated on the edge of the west spiral branch 14” from the nucleus. Shock excited HII regions were noted in the part of NGC 7469 turned toward the satellite IC 5283. The galaxy’s central radio structure (λ ~ 6 cm) stretches in the direction toward the satellite IC 5283 and the starlike detail. The spectum and color index of the starlike detail suggest that it is a cluster of early type stars (Mv = -19m) and dust clouds (Av = 3m), in NGC 7469.

During the last decades a number of surveys of peculiar galaxies have been carried out and accurate positions become available. Since peculiarities are a possible evidence of radio emission (Wright, 1974; Sulentic, 1976; Stocke et al.,1978), we have selected a sample of 24 peculiar galaxies with optical jet-like features or extensions in different optical catalogues, mainly the Catalogue of Southern Peculiar Galaxies and Associations (Arp and Madore, 1987) and the ESO/Uppsala Survey of the ESO(B) Atlas (Lauberts, 1982) for observation at the radiocontinuum frequency of 22 GHz. The sample is listed in the Table.

The peculiar galaxy NGC 520 (Arp 157) is often interpreted as an interacting pair of galaxies. The identification of the two bulges and overall morphology of the two galaxies has long been a puzzle which we attempt to solve in this paper. New longslit optical spectroscopy and near-infrared images of NGC 520 are presented. These data suggest that the northwest peak is the bulge of one of two galaxies in the system. The other larger bulge is clearly evident in the K band image in the middle of the dust lane. The stellar radial velocity profile in the central 10” of the larger bulge is consistent with counterrotation seen in the molecular gas component. This kinematic subsystem could be the remains of a merged gas-rich irregular.

In the course of expanding the search of Kleinmann et.al. (1988) for distant, infrared-luminous objects, we noticed (as is often remarked) that a large number of infrared-selected galaxies have close neighbors or show merger characteristics (e.g. tidal tails, distorted disks). Because the sample size is large (567 infrared galaxies and 2182 field galaxies), this sample is ideal for statistically examining the importance of interactions among infrared galaxies. In particular, we compare the nearest-neighbor distribution and the two-point correlation function of our sample with that of a control sample of field galaxies.

The idea that interactions between galaxies can lead to enhanced galactic activity has a long and noble lineage. Since the early days of Baade and Minkowski (1954), through the seminal paper of Toomre and Toomre (1977) with its “stoking the furnace” imagery, to the similarly colorful and influential “feeding the monster” paper of Gunn (1979), and the landmark work by Larson and Tinsley (1978), this idea was elaborated upon and generalized. The pace of development of this hypothesis has accelerated greatly on both the observational and theoretical fronts during the 1980’s, due in part to a growing perception that the AGN phenomenon could only be understood when the role of the environment in triggering or nurturing the activity was understood (cf. Balick and Heckman 1982). In recent years, the impact of the extragalactic IRAS database (on the observational side) and of the new generation of supercomputers and innovative software (on the numerical side) have led to an almost explosive growth in the number of papers written about this subject.

We report some results from recent and ongoing work which relate to the connection between nuclear activity and tidal interactions. We suggest that tidal events are in general a necessary but not sufficient condition for nuclear activation. We also suggest that nuclear activity generally develops at a ‘late stage’ of a tidal encounter, following star-formation and dust evaporation, and when the most obvious morphological disturbances may have disappeared.

Extremely Luminous far-infrared galaxies (ELFs) are a class of galaxy discovered independently by several groups. The class is characterised by a quasar-like total luminosity (1011 — 1013 L⊙) which is radiated almost entirely in the far-infrared, and it has been suggested that obscured quasar cores may be responsible for generating this luminosity. Here I demonstrate that ELFs appear in several guises which can be characterised by the number of quasar cores they contain (zero, one or two), and develop a unified model to account for these differences.