The Structure, Kinematics, and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253

We present 5.″2 × 2.″6 resolution interferometry of CO J = 1 → 0 emission from the starburst galaxy NGC 253. The high spatial resolution of these new data, in combination with recent high-resolution maps of 13CO, HCN, and near-infrared emission, allow us for the first time to link unambiguously the gas properties in the central starburst of NGC 253 with its bar dynamics. We confirm that the star formation results from bar-driven gas flows as seen in "twin peaks" galaxies. Two distinct kinematic features are evident from the CO map and position-velocity diagram: a group of clouds rotating as a solid body about the kinematic center of the galaxy and a more extended gas component associated with the near-infrared bar. We model the line intensities of CO, HCN, and 13CO to infer the physical conditions of the gas in the nucleus of NGC 253. The results indicate increased volume densities around the radio nucleus in a twin peaks morphology. Compared with the CO kinematics, the gas densities appear highest near the radius of a likely inner Linblad resonance and slightly lead the bar minor axis. This result is similar to observations of the face-on, twin peaks galaxy NGC 6951 and is consistent with models of starburst generation due to gas inflow along a bar.