ALMA Observations of the Antennae Galaxies. I. A New Window on a Prototypical Merger

We present the highest spatial resolution (≈0.''5) CO (3-2) observations to date of the "overlap" region in the merging Antennae galaxies (NGC 4038/39), taken with the Atacama Large Millimeter/submillimeter Array. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament of CO gas that breaks up into roughly 10 individual knots. Each individual knot has a low internal velocity dispersion (≈10 km s^-1) the dispersion of the ensemble of knots in the filament is also low (≈10 km s^-1). At the other extreme, we find that the individual clouds in the supergiant molecular cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80 km s^-1), and a large dispersion among the ensemble (≈80 km s^-1). Other large-scale features observed in CO emission, and their correspondence with historical counterparts using observations in other wavelengths, are also discussed. We compare the locations of small-scale CO features with a variety of multi-wavelength observations, in particular broad- (BVIJH) and narrow-band data (H_α and Pa_β) taken with the Hubble Space Telescope, and radio (3.6 cm) continuum observations taken with the Karl G. Jansky Very Large Array. This comparison leads to the development of an evolutionary classification system that provides a framework for studying the sequence of star cluster formation and evolution—from diffuse supergiant molecular clouds (SGMCs) to proto, embedded, emerging, young, intermediate/old clusters. The relative timescales have been assessed by determining the fractional population of sources at each evolutionary stage. The main uncertainty in this estimate is the identification of four regions as candidate protoclusters (i.e., strong compact CO emission but no clearly associated radio emission). Using the evolutionary framework, we estimate that the maximum age range of clusters in a single GMC is ≈10 Myr, which suggests that the molecular gas is removed over this timescale, resulting in the cessation of star formation and the destruction of the GMC within a radius of about 200 pc.