Bar-driven mass build-up within the central 50 pc of NGC 6946

We have used the new extended A configuration of the IRAM Plateau de Bure interferometer to study the dense molecular gas in the nucleus of the nearby spiral galaxy NGC 6946 at unprecedented spatial resolution in the HCN(1–0) and 12CO(2–1) lines. The gas distribution in the central 50 pc has been resolved and is consistent with a gas ring or spiral driven by the inner 400 pc long stellar bar. For the first time, it is possible to directly compare the location of (dense) giant molecular clouds with that of (optically) visible HII regions in space-based images. We use the 3 mm continuum and the HCN emission to estimate in the central 50 pc the star formation rates in young clusters that are still embedded in their parent clouds and hence are missed in optical and near-IR surveys of star formation. The amount of embedded star formation is about 1.6 times as high as that measured from HII regions alone, and appears roughly evenly split between ongoing dust-obscured star formation and very young giant molecular cloud cores that are just beginning to form stars. The build-up of central mass seems to have continued over the past ≥10 Myr, to have occurred in an extended (albeit small) volume around the nucleus, and to be closely related to the presence of an inner bar