The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC 4945 (original) (raw)

We have mapped the central region of NGC 4945 in the J = 2 → 1 transition of 12 CO, 13 CO, and C 18 O, as well as the continuum at 1.3 mm, at an angular resolution of 5 ′′ × 3 ′′ with the Submillimeter Array. The relative proximity of NGC 4945 (distance of only 3.8 Mpc) permits a detailed study of the circumnuclear molecular gas and dust in a galaxy exhibiting both an AGN (classified as a Seyfert 2) and a circumnuclear starburst in an inclined ring with radius ∼2. ′′ 5 (∼50 pc). We infer the systemic velocity ∼585 km s −1 from channel maps and PV-diagrams. We find that all three molecular lines trace an inclined rotating disk with major axis aligned with that of the starburst ring and large-scale galactic disk, and which exhibits solid-body rotation within a radius of ∼5 ′′ (∼95 pc). The rotation curve flattens beyond this radius, and the isovelocity contours exhibit an S-shaped asymmetry suggestive of a highly inclined bar as has been invoked to produce a similar asymmetry observed on larger scales. We infer an inclination for the nuclear disk of 62 • ±2 • , somewhat smaller than the inclination of the large-scale galactic disk of ∼78 •. The continuum emission at 1.3 mm also extends beyond the starburst ring, and is dominated by thermal emission from dust. If it traces the same dust emitting in the far-infrared, then the bulk of this dust must be heated by star-formation activity rather than the AGN. We discover a kinematically-decoupled component at the center of the disk with a radius smaller than 1. ′′ 4 (27 pc), but which spans approximately the same range of velocities as the surrounding disk. This component has a higher density than its surroundings, and is a promising candidate for the circumnuclear molecular torus invoked by AGN unification models.