Ionized and neutral gas in the peculiar star/cluster complex in NGC 6946

Abstract

The characteristics of ionized and H i gas in the peculiar star/cluster complex in NGC 6946, obtained with the 6-m telescope (BTA) Special Astrophysical Observatory Russian Academy of Sciences (RAS), the Gemini North telescope, and the Westerbork Synthesis Radio Telescope, are presented. The complex is unusual as hosting a super star cluster, the most massive known in an apparently non-interacting giant galaxy. It contains a number of smaller clusters and is bordered by a sharp C-shaped rim. We found that the complex is additionally unusual in having peculiar gas kinematics. The velocity field of the ionized gas reveals a deep oval minimum, ∼300 pc in size, centred 7 arcsec east of the supercluster. The Vr of the ionized gas in the dip centre is 100 km s-1 lower than in its surroundings, and emission lines within the dip appear to be shock-excited. This dip is near the centre of an H i hole and a semi-ring of H ii regions. The H i (and less certainly, H ii) velocity fields reveal expansion, with the velocity reaching ∼30 km s-1 at a distance about 300 pc from the centre of expansion, which is near the deep minimum position. The superstar cluster is at the western rim of the minimum. The sharp western rim of the whole complex is plausibly a manifestation of a regular dust arc along the complex edge. Different hypotheses about the complex and the Vr depression's origins are discussed, including a high-velocity H i cloud/dark minihalo impact, a blue compact dwarf galaxy merging, and a gas outflow due to release of energy from the supercluster stars. © 2007 The Authors.