Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of
  NGC 4039
  The properties of tidally induced arms provide a means to study molecular
cloud formation and the subsequent star formation under environmental
conditions which in principle are different from quasi stationary spiral arms.
We report the properties of a newly discovered molecular gas arm of likely
tidal origin at the south of NGC 4039 and the overlap region in the Antennae
galaxies, with a resolution of 1"68 x 0"85, using the Atacama Large
Millimeter/submillimeter Array science verification CO(2-1) data. The arm
extends 3.4 kpc (34") and is characterized by widths of ~ 200 pc (2") and
velocity widths of typically \DeltaV ~ 10-20 km/s . About 10 clumps are strung
out along this structure, most of them unresolved, with average surface
densities of \Sigma_gas ~ 10-100 Msun pc^{-2}, and masses of (1-8) x 10^6 Msun.
These structures resemble the morphology of beads on a string, with an almost
equidistant separation between the beads of about 350 pc, which may represent a
characteristic separation scale for giant molecular associations. We find that
the star formation efficiency at a resolution of 6" (600 pc) is in general a
factor of 10 higher than in disk galaxies and other tidal arms and bridges.
This arm is linked, based on the distribution and kinematics, to the base of
the western spiral arm of NGC 4039, but its morphology is different to that
predicted by high-resolution simulations of the Antennae galaxies.
