Two Populations of Molecular Clouds in the Antennae Galaxies
  Super star clusters --- extremely massive clusters found predominately in
starburst environments --- are essential building blocks in the formation of
galaxies and thought to dominate star formation in the high-redshift universe.
However, the transformation from molecular gas into these ultra-compact star
clusters is not well understood. To study this process, we used the
Submillimeter Array and the Plateau de Bure Interferometer to obtain high
angular resolution (~1.5" or 160 pc) images of the Antennae overlap region in
CO(2--1) to search for the molecular progenitors of the super star clusters. We
resolve the molecular gas distribution into a large number of clouds, extending
the differential cloud mass function down to a 5\sigma completeness limit of
3.8x10^5 M_sun. We identify a distinct break in the mass function around log
M_mol/M_sun ~ 6.5, which separates the molecular clouds into two distinct
populations. The smaller, less massive clouds reside in more quiescent areas in
the region, while the larger, more massive clouds cluster around regions of
intense star formation. A broken power-law fit to the mass function yields
slopes of \alpha = -1.39+/-0.10 and \alpha = -1.44+/-0.14 for the low- and
high-mass cloud population, well-matched to the mass function found for super
star clusters in the Antennae galaxies. We find large velocity gradients and
velocity dispersions at the locations of intense star formation, suggestive of
compressive shocks. It is likely that these environmental factors contribute to
the formation of the observed massive molecular clouds and super star clusters
in the Antennae galaxies.
