Ultra-high-energy (UHE) γ-rays have been detected from several X-ray binaries. UHE γ-rays from sources in the LMC will interact in the microwave background producing electrons which emit synchrotron radiation in the X-ray and γ-ray bands. This radiation might be observable.

We present CaII and NaI spectra for a sample of stars in a 30'×30' field connecting 30 Dor with SN 1987A. Observations were taken with the CES spectrograph linked via fibres with the 3.6m ESO telescope at a resolution of ˜ 6 km s−1. The LMC gas shows a complex structure with both single and multiple components spanning 250 to 300 km s−1. Results can be interpreted using the multi-layer structure suggested by 21 cm observations. Comparing our optical absorptions with 21 cm emissions yields evidence that layers with increasing radial velocity are located one behind the other. The intermediate velocity components are numerous in CaII, but rare in NaI. Towards Sk-69 211, for which we have the highest S/N CaII spectrum, we have detected a number of components comparable to that found in SN 1987A. The 64 km s−1 cloud is detected in a well-confined region of the field, showing remarkable constancy in the radial velocity, which we interpret as evidence that this cloud is located close to the Galaxy.

Shapley Constellation III in the Large Magellanic Cloud (LMC) is surrounded by a ring of HII regions, called Supershell LMC 4 (Meaburn 1980). In the direction of the centre of this spatial strucutre very little neutral interstellar gas is found (McGee & Milton 1964). The spatial arrangement suggests that a hole exists in the gas layer of the LMC, one which may be blown out by the effect of the winds of the stars of Constellation III or by supernova blasts from the same area. Shock waves moving into surrounding neutral gas triggered star formation in the outer shell. Left inside would be a volume of diluted, and probably hot, ionized gas.

To probe the spatial structure of the gas we have collected high dispersion IUE spectra of some stars inside LMC 4. We have analysed the interstellar lines from the various elements and ions available and are able to derive the velocity structure of the lines of sight observed (Bomans & de Boer 1989). We see a well-defined absorption component near 230 km s−1 and a broader structure around 280 km s−1. The column density of neutral gas is small, as confirmed from the HI 21-cm profiles of Rohlfs et al. (1984). We have redetermined the spectral type of the stars from the IUE spectra in order to estimate the strength of the stellar Lyman-Alpha (Lα) line. Two stars are of early enough type to allow the determination of N(H) from Lα.

The four oxygen-sequence WR stars, Sand 1 in the Small Magellanic Cloud (SMC), Sand 2 in the Large Magellanic Cloud (LMC), and WR 102 and WR 142 in the Galaxy represent the latest stage of the evolution of massive stars (Sanduleak 1971, Barlow and Hummer 1982, Moffat et al. 1985). We have shown WR 102 to be a stripped CO core of a supermassive star (Dopita et al. 1990), probably seen only several thousand years before a SN explosion. The four stars are characterized by extremely energetic stellar winds –Vw from 4500 to 7400 km/s (Barlow and Hummer 1982, Dopita et al. 1990, Torres et al. 1986). Examination of the environments of WO stars leads to the conclusion that the four objects appear to be associated with optical and/or IR shell-like structures, although the short WO-superwind does not prevail in the shell's formation.

Star formation in interacting galaxies is usually strongly enhanced. The star formation rate in the Milky Way is substantially greater than, for example, in the Andromeda Nebula. A plausible cause for this difference may be the interaction of the Milky Way with the Large Magellanic Cloud. We suggest that one of the possibilities for this may be the enhanced formation of cold gas clouds as the gas flows through the gravitational potential well of a tidal wave caused by the interaction; another contribution may come from compression of pre-existing clouds when they pass this way. This scenario is obviously quite similar to that envisioned in “frames” of the density-wave theory.

The Magellanic System is viewed focusing on the global interactions in the System. These give insight into its history and structure. The past orbits of the Magellanic Clouds (MCs) are examined. A tidal encounter between the Large and Small Magellanic Clouds (LMC, SMC) has almost certainly occurred within the last 109 yrs. This hypothesis is supported by the observed structure of the Magellanic System, and so is accepted. The Magellanic Stream is an indirect result of the tidal encounter which is crucial to understanding the Magellanic System. It is a complex interacting gas feature, bifurcated along its entire length with many anomalous velocity H I clouds alongside. The possible models for the Magellanic Stream are examined and here I propose that its origin is due to the collision of a multi-phase halo with the vast region of gas between the LMC and the SMC. In this respect the polar subsystem around our Galaxy is seen to be particularly important. The popular tidal model for the origin of the Magellanic Stream fails to satisfy key observational features, and is thus rejected.

The HI envelope of the Magellanic System extends well beyond the obvious visible regions of each of the Clouds. Modern optical surveys based on UK Schmidt telescope plates and fast measuring machines are now starting to reveal the complex optical counterparts of some of these HI features.

High velocity clouds (HVCs) of neutral atomic hydrogen close to the position of the Magellanic Clouds (MCs) and the Magellanic Stream (Stream) are reviewed. The gas observed at velocities of +70 km/s and +130 km/s in front of the LMC is probably associated with ordinary HVCs in the galactic halo. This is not the case for the gas observed between +150 and +170 km/s which is more likely associated with the MCs. The HVCs observed superimposed onto the Stream are possibly remnants of collisions between a gaseous polar ring around our Galaxy and the bridge region between the MCs. The HVCs found close to and “behind” the tip of the Stream may be regarded as shreds of the Stream precipitating toward the galactic disk. The chemical composition, the radiation field and the temperature in the Stream is discussed in the context of the first spin temperature determination by Wakker (1990), of an HVC.

The Large and Small Magellanic Clouds (LMC, SMC) have been in a binary state for the last 10 Gyr, and their centre-of-mass revolves around the Galaxy with a massive dark halo of 1012Mo. The dashed line in Fig. 1 is a relative orbit of the SMC, approaching the LMC 0.2 Gyr ago with impact parameter of 0.5 to 4 kpc (Murau & Fujimoto 1980, Fujimoto & Noguchi 1990). At this close approach, their gas clouds collided hydrodynamically to make shock compressions and coalescences in various mass scale. The ribbon is a computed orbit of a huge gas complex thus generated at α=5h40m and δ=-68° and reaching the present position.

We are undertaking empirical tests for the effects of metallicity on the zero-point of the Period-Luminosity relations for Cepheids in M31 and in the Small Magellanic Cloud (SMC), as compared to the Large Magellanic Cloud (LMC). For M31 we have used BVRI data on comparable sets of Cepheids in three radially separated, and chemically different, fields in order to solve simultaneously for the reddenings and the true distance moduli to each of these fields. Recent theory predicts that the reddening-corrected moduli should differ by 0.8 mag given the observed metallicity range in our M31 Cepheid sample; we observe a maximum range of only 0.17 mag. For the SMC BVRIJHK data, treated in a similar fashion, the predicted theoretical effects on the short wavelength (B and V) data may be visible, but with low statistical certainty and at a reduced amplitude with respect to the predictions.

Radial velocities of 20 stars members of young clusters and associations located in the inter-Cloud region within 2h < R.A. < 5h tend to be lower than the HI velocities in the same direction. The stellar velocities do not show a pronounced velocity gradiant as observed for the HI from the SMC to the LMC. This suggests that the stellar components belong to the SMC wing. Our photometry and spectroscopy support the view that one of the stars at 4h20m is a foreground HB star.

We use a simple, spherically symmetric model for the Galaxy based on the rotation curve for the inner parts. We extrapolate beyond the rotation curve boundary, assuming a dark corona whose total mass is a model parameter. For each mass value assume we calculate the allowed angular-momentum interval for which a cloud is neither tidally disrupted, nor escapes. On the basis of this investigation we reach a conclusion about the expected mass of the Galaxy.

We present deep CCD photometry of three associations lying between the Magellanic Clouds and sitting on the HI ridge. ZAMS fittings show that these clusters are located at the distance of the Clouds. Furthermore, a slight distance gradient is observed. Main sequences matched to isochrones reveal that these aggregates are very young, no more than 108 years old. An age difference from east to west is also suggested.

The structure and evolution of massive stars is significantly influenced by effects of chemical composition in a low-metallicity environment (as compared to the solar neighbourhood, SN), such as the Magellanic Clouds. A fundamental ingredient in evolutionary models is the stellar mass-loss rate M. Lower metal content decreases the mass-loss rates derived theoretically, which in turn affects the stellar evolution models. On the other hand, different evolutionary models predict different stellar parameters (especially L), which again influence M so that an iterative procedure is required to achieve self-consistency.

This model was devised in an attempt to resolve the discrepancy between the values derived for the mass of the Galaxy from distant satellites and from the Magellanic Stream. In contrast to most previous models, the orbit of the Magellanic Cloud system is not bound to the Galaxy, and the Stream was developed as the result of a tidal encounter between M31 and the Magellanic Cloud system about 6 Byr ago.

A number of starburst galaxies are included in the Atlas of Peculiar Galaxies (Arp 1966) which is the collection of peculiar-morphology galaxies showing mutual tidal interaction. A scenario of starburst can be summarized as follows (e.g. Sofue 1987, Noguchi 1988). Tidal interaction between galaxies causes an oval disturbance in the gravitational potential of a galaxy. The oval disturbance causes a bar in the innermost region of the galaxy. By this bar a galactic shock wave is excited, which results in rapid accretion of gas toward the central region. The accreted gas produces a dense ring or a core of molecular gas in the central region, from which massive stars are born. Intense UV radiation from the stars heats dust in the dense molecular gas core/ring, and the dust emits strong FIR emission.

The disk of the Milky Way suffered from the tidal effect as the Magellanic Clouds were passing by. Numerical stimulations were performed to study the evolution of the mass distribution in this disk. These simulations were run with the galactic disk initially flat, and different sets of the initial position and velocity of the Magellanic Clouds were considered. One of the most conspicuous observational facts is the warp of the disk of the Milky Way. Results show that the characteristics of this warp are related to the orbit of the Magellanic Clouds.

Backend processors for use in gravitational N-body problems were constructed. They are as cheap as a personal computer, yet their speed is comparable to a supercomputer. As an example we show some results of simulations which were done for the dynamical evolution of galaxies in a compact group.

Currently, we are measuring the absolute proper motions of the Magellanic Clouds relative to background galaxies, using plates taken with the ESO Schmidt Telescope. In spite of the small epoch difference of about 15 years, an accuracy of 0.5-1.0 milliarcsecs (mas) may be achieved using large numbers of stars and galaxies. Measurement and reduction procedures are presented; a preliminary solution for the absolute proper motion of the LMC from the measurements of three plates gives a result similar to the independent study of Jones et al. (1989).