  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 3x10^8 Msun 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.
