Abstract. The region of M 17 has been imaged at 10.5 and 20.0 µm with the groundbased infrared camera MANIAC. In addition to a prominent diffuse emission bar (4. 5×0. 3) extending southeast to northwest at the interface between the H ii region and the southwestern molecular cloud, the mosaic of 133 single frames at each wavelength revealed 22 compact sources. One of these sources is the Kleinmann-Wright-Object and another was previously identified as the ultra-compact H ii region M 17-UC1. Combining the N-band and Q-band data with near infrared data yielded spectral energy distributions that classifies all sources to be of Class I. The observed luminosities were between 55 and 4775 L , which suggests that these sources represent the youngest generation of massive early type stars in M 17 and are surrounded by relics of their protostellar clouds. The morphology of the 10.5 and 20.0 µm emission towards some of the sources reveals flattened structures and may be the first evidence of the presence of circumstellar disks around massive stars.

Context. NGC 4441 is a candidate for a merger between a spiral and an elliptical galaxy (S+E merger), because it shows typical tidal structures such as an optical tail and two shells. With a far-infrared luminosity of ∼5 × 10 9 L this galaxy belongs to the class of moderate luminosity mergers, in which the merging process induces (if at all) only a moderate starburst. Aims. The study of the atomic gas content allows us to investigate the merger history and the impact on the star formation. In particular, it is not clear from simulations whether an S+E merger leads to a gas concentration in the nucleus, resulting in a starburst, or whether the gas is spread out and therefore too diffuse for new star forming regions.Methods. We used the Westerbork Radio Synthesis Telescope to observe the properties of the H i. By using this interferometer, we are able to study the large-scale H i distribution and kinematics with high spatial and velocity resolution. Results. We found two H i tails extending out to more than 40 kpc. In a central disc, the gas shows a fairly regular rotation pattern indicating that the gas started to settle after the merger. The total H i mass adds up to 1.5 × 10 9 M . By comparing the high resolution H i maps with deep optical images, differences between the stellar and gaseous tidal features are apparent, which indicates an S+E merger origin.

The Medusa (NGC 4194) is a well-studied nearby galaxy with the disturbed appearance of a merger and evidence for ongoing star formation. In order to test whether it could be the result of an interaction between a gas-rich disk-like galaxy and a larger elliptical, we have carried out optical and radio observations of the stars and the gas in the Medusa, and performed N-body numerical simulations of the evolution of such a system. We used the Nordic Optical Telescope to obtain a deep V-band image and the Westerbork Radio Synthesis Telescope to map the large-scale distribution and kinematics of atomic hydrogen. A single Hi tail was found to the South of the Medusa with a projected length of ∼56 kpc (∼5 ) and a gas mass of 7 × 10 8 M , thus harbouring about one third of the total Hi mass of the system. Hi was also detected in absorption toward the continuum in the center. Hi was detected in a small nearby galaxy to the North-West of the Medusa at a projected distance of 91 kpc. It is, however, unlikely that this galaxy has had a significant influence on the evolution of the Medusa. The simulations of the slightly prograde infall of a gas-rich disk galaxy on an larger, four time more massive elliptical (spherical) galaxy reproduce most of the observed features of the Medusa. Thus, the Medusa is an ideal object to study the merger-induced star formation contribution from the small galaxy of a minor merger.

Interaction and merging are the two most important driving forces of galaxy evolution. In recent years, much research activity has been focused on ULIRGs, which are the result of a disk-disk merger, since they are among the most spectacular objects in the universe. However, ULIRGs are rare objects in the local universe. Thus, a study of a sample of merging galaxies of moderate FIR luminosity (¡ 10 10 ¢ 10 11 L£) may teach us as much as or even more about galaxy formation and evolution than investigations of the most extreme objects. Mergers having a moderate FIR luminosity can also be produced by merging two gas-rich galaxies with unequal mass or a spiral and an elliptical (S+E). Alternatively, they can be a result of a faded major merger. We have obtained multicolour optical and NIR imaging data, optical spectroscopy, HI and CO (i.e. molecular gas) data for a large part of a sample of moderate luminosity merger candidates. A cross section of the results will be presented, and some preliminary conclusions will be drawn. These include an assessment of the crucial question of the extent (and existence) of the starburst resulting from the merger-a dominating young stellar population and significant dust obscuration is indeed suggested for most of the sample galaxies.