We have combined observations of the Antennae galaxies from the radio interferometer ALMA ( Atacama Large Millimeter/submillimeter Array ) and from the optical interferometer GH \alpha FaS ( Galaxy H \alpha Fabry-Perot System ) .
The two sets of observations have comparable angular and spectral resolutions , enabling us to identify 142 giant molecular clouds ( GMCs ) and 303 H ii regions .
We have measured , and compared , their basic physical properties ( radius , velocity dispersion , luminosity ) .
For the H ii regions , we find two physical regimes , one for masses > 10 ^ { 5.4 } \mathrm { M _ { \odot } } of ionized gas , where the gas density increases with gas mass , the other for masses < 10 ^ { 5.4 } \mathrm { M _ { \odot } } of ionized gas , where the gas density decreases with gas mass .
For the GMCs , we find , in contrast to previous studies in other galaxies over a generally lower mass range of clouds , that the gas surface density increases with the radius , hinting at two regimes for these clouds if we consider both sources of data .
We also find that the GMC mass function has a break at 10 ^ { 6.7 } \mathrm { M _ { \odot } } .
Using the velocity dispersion measurements , we claim that the difference between the regimes is the nature of the dominant binding force .
For the regions in the lower mass range , the dominant force is the external pressure , while in the higher mass range it is the internal gravity of the clouds .
In the regime where gravity is dominant , the star formation rate , derived from the dust-corrected H \alpha luminosity , increases super-linearly with the velocity dispersion , and the gas density increases with the gas mass .