Molecular Gas around the Double Nucleus in M 83

The center of M83, a barred starburst galaxy with a double nucleus, has been observed in the CO(J=2–1) and CO(J=3–2) lines with the Submillimeter Array. The molecular gas shows a distribution and kinematics typical for barred galaxies at ∼kpc radii, but reveals unusual kinematics around the double nucleus in the central ∼300 pc. Our CO velocity data show that the visible nucleus in M83 is at least 3 (65 pc) away from the galaxy’s dynamical center, which most likely coincides with the center of symmetry previously determined in K band and is suggested to host another nucleus. We discovered high-velocity molecular gas associated with the visible off-center nucleus, and also found a steep velocity gradient across it. We attribute these features to a gas disk rotating around the off-center nucleus, which may be the remnant of a small galaxy accreted by M83. The dynamical mass of this component is estimated to be 3 × 108M⊙ within a radius of 40 pc. The dynamical perturbation from the off-center nucleus may have played a key role in shaping the lopsided starburst. Subject headings: galaxies: dynamics and kinematics — galaxies: ISM — galaxies: starburst — galaxies: nuclei — galaxies: individual (M83, NGC 5236)