Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of NGC 4039

TL;DR

This study characterizes a newly discovered tidal molecular arm in NGC 4039, revealing its structure, molecular cloud properties, and elevated star formation efficiency compared to typical galactic environments.

Contribution

It provides detailed observational data on a tidal molecular arm, highlighting its morphology, cloud properties, and star formation activity, which differ from predictions and other galactic regions.

Findings

The arm extends 3.4 kpc with ~200 pc wide structures.

Star formation efficiency is about 10 times higher than in typical disk galaxies.

Molecular clouds show bead-like morphology with characteristic separation of 350 pc.

Abstract

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.