Gravitational interactions and mergers affect the morphologies and dynamics of galaxies from our Local Group to the limits of the observable universe. Observations of interacting galaxies at low redshifts (z ≲ 0.2) yield detailed information about many of the processes at work. I briefly review these processes and the growing evidence that mergers play a major role in the delayed formation of elliptical and early-type disk galaxies both in the field and in clusters. Low-z observations clearly contradict the notion of a single epoch of E formation at z ≳ 2; instead, E and S0 galaxies continue forming to the present. The different rates of E and S0 formation inferred from observations of distant and nearby clusters may partially reflect the dependence of dynamical friction on mass: Major, E-forming mergers may tend to occur earlier than minor, S0-forming mergers because the dynamical friction is strongest for equal-mass galaxies.

Evolution in the merger rate as a function of redshift is in principle the key observable testing hierarchical models for the formation and evolution of galaxies. However, in practice, direct measurement of this quantity has proven difficult. In this opening review I outline the current best estimates for the merger rate as a function of cosmic epoch, focusing mostly upon recent advances made possible by deep ground-based redshift surveys and morphological studies undertaken with HST. I argue that a marriage of these techniques, in an attempt to determine the space density of mergers amongst the abundant morphologically peculiar population at high redshifts, is probably the most promising currently-available avenue for determining the prevalence of mergers at high redshifts. However, resolved kinematical studies, which seem set to become available in the next few years, are probably the best hope for a definitive determination of the space density of mergers at high redshifts.

This talk is mainly about the halos and bulges of Local Group galaxies, in particular those of the Milky Way and M31, and what they can tell us about the role of interactions in galaxy formation. Bulges are potentially of particular interest for galaxy formation because they are widely suspected to be the seeds for galaxy formation. Sites of active star formation at high redshift are often regarded as spheroids in the process of formation. In this talk, I will say little about disks because their formation is conceptually understood, although many details remain uncertain. To form such flat systems, a fairly undisturbed dissipative process is needed. This process occurred fairly early: the disks appear to be in place by a redshift z = 1 (Ellis 1997), with a distribution of scalelengths that is similar to the distribution at z = 0.

A tidal model has been introduced to the triple system of the Galaxy, Large and Small Magellanic Clouds (the LMC and SMC hereafter) and successfully reproduced the Magellanic Stream (Murai and Fujimoto 1980; Lin and Lynden-Bell 1982; Gardiner et al. 1994; Gardiner and Noguchi 1995; Lin et al. 1995), a narrow band of diffuse atomic hydrogen gas emerging from the SMC region, passing by the South Galactic Pole along an overhead great circle spanning over 100° (Wannier and Wrixon 1972; Mathewson et al. 1974). The LMC and SMC have a hydrogen bridge and common envelope (Hindman 1964; McGee and Milton 1966) and, therefore, we can consider that they have been in a binary state for the Hubble time, revolving together around the Galaxy with a halo whose mass is larger than 1012M⊙ if the flat rotation curve extends up to more than 100 kpc. The strong gravitational force due to this heavy halo attracts the Magellanic Stream and produces the high negative radial velocities (Murai and Fujimoto 1980).

The Sagittarius dwarf galaxy, located at a distance of ~ 15 kpc behind the Galactic bulge, is in the process of being tidally disrupted and assimilated into the Milky Way. This unique event allows the physics of galactic merging to be probed in unprecedented detail, and may shed light on the hypothesized population of primordial galaxies that merged to form the Milky Way.

A new method to search for moving groups, specifically tailored for the HIPPARCOS database, is used to search for moving groups among the high velocity stars in this database. Although the sampled volume is small, the high quality of the astrometry makes this search interesting. No moving groups are detected, but limits in the velocity and number of members of possible non-detected groups are given.

Observations at high redshifts are revealing numerous interactions and ongoing mergers. Our own Milky Way is currently merging with the Sagittarius dwarf spheroidal (dSph) galaxy. Past mergers with dwarf galaxies may have contributed significantly to the Galactic halo and possibly to the thick disk. The properties of Local Group dSphs and halo globular clusters impose constraints on the merger history of the Milky Way.

We have investigated 4 Shapley constellations (1996, A&A, 308, 40) to define the associated stellar complexes and their properties. From observed star counts and recent spectral classification we derive an age estimate and the upper mass limit of individual stars in these regions.

There is still a controversy on the mass of the Galaxy in the deep halo, some advocate a conservative view that the rotation velocity ultimately decays in accordance with the Keplerian law at the distance of 50 kpc, while others have come to consider that the rotation curve of the Milky Way, essentially stays flat or is still increasing at the distance of the Magellanic Clouds and the Magellanic Stream.

When the Sagittarius dwarf galaxy (SDG) crosses the galactic plane star formation might occur in its tidal tail. Carbon stars, due to their brightness, are ideal tracers for such an event. Ng & Schultheis (1997) suggested that the Bulge carbon stars (Azzopardi et al. 1991; ALRW) might be at SDG's distance. In the galactic Bulge these stars are completely different from carbon stars found in the LMC, SMC and dwarf spheroidal galaxies. If one keeps these stars in the Bulge and tries to explain their properties through a binary evolution scenario, one has to explain why they are not found up to the red giant branch tip: there is a discrepancy. Ng (1997) argues that part of the mystery concerning the luminosity of the ALRW stars would be solved if they are related to the SDG. It is demonstrated that the carbon star sequence is in that case similar to the observed SMC sequence. The carbon stars are in that case not metal-rich as previously thought, but they have a metallicity comparable to the LMC, with an age between 0.1–1 Gyr. A significant fraction of the carbon stars still have luminosities fainter than the lower LMC limit of . A similar trend is present among carbon stars found in other dwarf spheroidals. A scenario through binary evolution is assumed for these stars, because TP-AGB models cannot explain their origin through a single-star evolution scenario, even if they form immediately after entering the TP-AGB phase.

What is our problem? Observed figures of the Galactic globular clusters are somewhat elliptic in many cases. For not a few clusters (e.g. 23 of 72), the observed ellipticities (Shawl & White 1987) are ≥ 0.1 within ~ 2rh. By a series of numerical experiments, we study whether or not tidal force from the Galactic disk can produce the observed ellipticities in such an inner region (Shimada 1996)

The M81 group has great advantages for dwarf galaxy studies: it has about three times the dwarf content of the Local Group but is at only about a quarter of the Virgo cluster distance. We searched for HI in 23 optically selected dwarf members and possible members of the M81 group with the Nançay decimetric radio telescope in the velocity range of −529 to 1826 km s−1 with considerably better sensitivity (2–4 mJy rms.) than previous surveys. Half the objects observed are irregular dwarfs, expected to be HI-rich and detectable at Nançay if at the distance of the M81 group (4 Mpc).

Nature and origin of the high-velocity clouds (HVCs) remain enigmatic after thirty years (Wakker & van Woerden 1997, ARA&A, 35, 217), owing to lack of distance information. Hypotheses range from supernova shells at 100 pc to intergalactic clouds at 1 Mpc. On statistical grounds, Blitz et al. (1996, BAAS, 28, 1349) claim that the HVCs are “remnants of Local Group formation, best explained as members of the Local Group of galaxies”. Reliable distances must come from the presence or absence of absorption at the HVC's velocity in spectra of stars at different distances. For Complex A, MgII absorption is seen in HST spectra of the Seyfert galaxy Mrk 106, but not in the star PG0859+593 at 4 kpc (Wakker et al. 1996, ApJ, 473, 834). La Palma spectra of the RR Lyr star AD UMa at 11 kpc distance show CaII absorption by Complex A at both K and H, which is lacking at 4 kpc. These absorptions are not confused with stellar metal lines. Our distance bracket 4 < d < 11 kpc places Complex A in the Galactic Halo, at 2.5 < z < 7.5 kpc above the plane; a distance similar to Local Group galaxies is excluded. The HI mass implied lies between 0.15 and 1.2 times 106M⊙. Our result precludes local origins for this HVC. It allows an origin in a Galactic Fountain, or in interaction of infalling intergalactic material (from the Magellanic System or the Local Group) with the Galactic Halo.

We surveyed the central 3° × 6° region of the Large Magellanic Cloud (LMC) with angular resolution of 10″.0 at the J(1.25μm), H(1.65μm) and K′(2.15μm) bands. The observations were performed from Oct to Nov, 1994 at the Siding Spring Observatory. We used a 25 cm/F3.5 Newtonian telescope equipped with a 512×512 PtSi-Camera (Ueno et al. 1992). The plate scale was 4.″6 × 6.″0/pixel and the total field of view was 40.′2 × 52.′3. The system was attached onto the Automated Patrol Telescope of the University of New South Wales. The limiting magnitudes of the survey were 13.6, 11.9, and 10.0 magnitude (3σ) at the J, H, and K′ bands, respectively. The positions and the J, H, and K′ magnitudes were derived for 1599 point sources whose S/N ratio were more than four at the K′-band. The dominant components of the sources are red super giants (RSG), luminous M-type giants and AGB stars in the LMC. The distributions of RSG, luminous giants and CO line flux are spatially different from each other. This supports the idea that the position of the active star forming region has changed during the past 108 years. The detected sources are cross-identified with the IRAS Point Source Catalog. Sixty-two of 680 IRAS sources in the region spatially coincide with the detected sources and the spectrum energy distributions from 1.25μm to 100μm were derived for these 62 sources. Half of them are well described by a black body spectrum, while the others show a flat or redder spectrum. These latter sources are supposed to be stars with dust envelopes.

The system of the Magellanic Clouds is considered to be dynamically interacting among themselves and with our Galaxy. This interaction is thought to be the cause of many complicated features seen in the Magellanic Clouds and the Magellanic Stream (see Westerlund 1990, A&AR, 2, 27). In order to better understand the formation and evolution of the Magellanic System, we carry out realistic N-body simulations of the tidal distortion of the Small Magellanic Cloud (SMC) due to our Galaxy and the Large Magellanic Cloud (LMC).

We review the observational evidences for tidal dwarf galaxies, a class of small galaxies formed out of the tidal debris of collisions between massive galaxies. Tidal dwarfs are found far from the interacting parent galaxies, associated with massive clouds of atomic hydrogen located at the tip of long tidal tails. These newly formed galaxies are among the best cases for the study of galaxy formation in the nearby Universe.

HI observations are an excellent tool for investigating tidal interactions. Ongoing major and minor interactions which can lead to traumatic mergers or to accretion and the triggering of star formation, show distinct HI signatures. Interactions and mergers in the recent past can also be recognized in the HI structure and kinematics. Recent 21cm line surveys of large samples of galaxies indicate that at least one out of every four galaxies shows signs of a present interaction or merger/accretion events in the recent past.