Molecular gas dynamics in NGC 6946:: A bar-driven nuclear starburst caught in the act''

We present high angular resolution (similar to 1 '' and 0.'' 6) millimeter interferometric observations of the (CO)-C-12(1-0) and (CO)-C-12(2-1) line emission in the central 300 pc of the late-type spiral galaxy NGC6946. The data, obtained with the IRAM Plateau de Bure Interferometer (PdBI), allow the first detection of a molecular gas spiral in the inner similar to 10 '' ( 270 pc) with a large concentration of molecular gas (M-H2 similar to 1.6 x 10(7) M-circle dot) within the inner 60 pc. This nuclear clump shows evidence for a ringlike geometry with a radius of similar to 10 pc as inferred from the position-velocity diagrams. Both the distribution of the molecular gas and its kinematics can be well explained by the influence of an inner stellar bar of about 400 pc length. A qualitative model of the expected gas flow shows that streaming motions along the leading sides of this bar are a plausible explanation for the high nuclear gas density. Thus, NGC 6946 is a prime example of molecular gas kinematics being driven by a small-scale, secondary stellar bar.