Molecular Gas Dynamics in NGC 6946: A Bar-driven Nuclear Starburst ``Caught in the Act''

Abstract

We present high angular resolution (~1" and 0.6") millimeter interferometric observations of the 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), allow the first detection of a molecular gas spiral in the inner ~10" (270 pc) with a large concentration of molecular gas (M_H2~1.6×10⁷ M_solar) within the inner 60 pc. This nuclear clump shows evidence for a ringlike geometry with a radius of ~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.

Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).