2012A&A...538L...9H

We present an analysis of super-giant molecular complexes (SGMCs) in the overlap region of the Antennae galaxy merger, based on ALMA CO(3-2) interferometry and VLT/SINFONI imaging spectroscopy of H₂ 1-0 S(1) at angular resolutions of 0.9" and 0.7", respectively. All but one SGMC have multiple velocity components offset from each other by up to 150 km/s. H₂ line emission is found in all SGMCs and the kinematics of H₂ and CO are well matched. H₂/CO line ratios vary by up to a factor of 10 among SGMCs and different velocity components of the same SGMCs. We also identify the CO counterpart of a bright, compact source of near-IR H₂ line emission, which shows no Brγ, and was first identified with SINFONI. This source has the highest H₂/CO line ratio, and coincides with the steepest CO velocity gradient of the entire overlap region. With a size of 50 pc and a virial mass of a few 10⁷M_☉ it is perhaps a pre-cluster cloud that has not yet formed significant numbers of massive stars. We present observational evidence that the H₂ emission is powered by shocks, and demonstrate how the H₂ 1-0 S(1) and the CO(3-2) lines can be used as tracers of energy dissipation and gas mass, respectively. The variations in the H₂/CO line ratio may indicate that the SGMCs are dissipating their turbulent kinetic energy at different rates. The compact source could represent a short (∼1Myr) evolutionary stage in the early formation of super-star clusters.