Seyfert Activity and Nuclear Star Formation in the Circinus Galaxy

We present high angular resolution (015-05) near-infrared images and spectroscopy of the Circinus galaxy, the closest Seyfert 2 galaxy known. The data reveal a nonstellar nuclear source at 2.2 μm whose radius is smaller than 1.5 pc. The coronal line region and the hot molecular gas emission extend for 20-50 pc in the ionization cone. The data do not show evidence for a pointlike concentration of dark mass; we set an upper limit of 4 × 10⁶ M_☉ to the mass of a putative black hole.

We find evidence for a young, nuclear stellar population, with typical ages between 4 × 10⁷ and 1.5 × 10⁸ yr. The luminosity of the starburst inside a few hundred parsecs is comparable to the intrinsic luminosity of the Seyfert nucleus, and together the two account for most of the observed bolometric luminosity of the galaxy. Within the central 12 pc, the starburst has an age of about 7 × 10⁷ yr and radiates ~2% of the luminosity of the active nucleus. We discuss the implications of these results for models that have been proposed for the starburst-active galactic nucleus connection.