Building up mass in the centers of late type galaxies

Abstract

AbstractWe present highest angular resolution (~ 1″ and 0.35″) mm-interferometric observations of the HCN(1-0), 12CO(1-0) and 12CO(2-1) line emission in the central 300 pc of the late-type spiral galaxy NGC 6946. The data, obtained with the IRAM Plateau de Bure Interferometer (PdBI) shows for the first time a molecular gas spiral in the inner ~ 10″ (270 pc) with a large concentration of molecular gas ($M_{H_2} \sim\,1.6\times10^7\,M_{\odot}$) within the inner 60 pc, The gas distribution in the central 50 pc has been resolved and is consistent with a gas ring or spiral driven by a bar. Both the distribution of the molecular gas as well as its kinematics can be well explained by the influence of an inner stellar bar of about 400 pc length as tested via a qualitative model for the gas flow. NGC 6946 is a prime example of molecular gas kinematics being driven by a small-scale, secondary 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 Myrs, to have occurred in an extended (albeit small) volume around the nucleus, and to be closely related to the presence of an inner bar.