A multitransition CO study of the antennae galaxies NGC 4038/9

For the Antennae interacting galaxy pair, we have obtained high-quality, fully sampled (CO)-C-12 J = 1-0 and 3-2 maps of the regions surrounding the nuclei and the area of overlap between the two galaxies. The maps possess an angular resolution of 15 or 1.5 kpc, so far the highest resolution maps available at both the J = 1-0 and 3-2 transitions. In addition, (CO)-C-12 J = 2-1 data have been obtained for the positions of the two nuclei, as well as in part of the overlap region with 20 angular resolution. The (CO)-C-12 J = 1-0, 2-1, and 3-2 emissions all peak in an off-nucleus region adjacent to where the two disks overlap. Use of the conventional X factor yields similar to4 x 10(9) M-circle dot molecular gas mass in the overlap region. It is difficult to understand how such a large amount of molecular gas can be accumulated in this region given the relatively short lifetime of molecular clouds and the limited period of time for this region to form. Line emission at (CO)-C-13 J = 2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the (CO)-C-12/(CO)-C-13 J = 2-1 and 3-2 integrated intensity ratios are remarkably high in the overlap region. This is the first published case in which such high (CO)-C-12/(CO)-C-13 J = 2-1 and 3-2 ratios are found outside a galactic nucleus. Detailed large velocity gradient (LVG) modeling indicates that the (CO)-C-12 and (CO)-C-13 emissions originate in different spatial components. The (CO)-C-12 emission may originate within a nonvirialized low-density gas component with a large gradient in velocity. Assuming a CO-to-H-2 abundance ratio of 10(-4), the X factor given by the LVG model is an order of magnitude lower than the conventional value for molecular clouds in the Milky Way, but it scales inversely as the assumed value for the CO-to-H2 abundance ratio. Accordingly, we suggest the possibility that the strong CO emission in the overlap region of the Antennae galaxies is associated with increased radiative efficiency, possibly caused by a large velocity dispersion within the individual molecular clouds. A comparison of the CO J = 3-2 emission with the SCUBA 850 mum continuum in the Antennae galaxies shows that the CO line emission contributes globally 46% of the 850 mum continuum flux and that the ratio of (CO)-C-12 J = 3-2 to SCUBA 850 mum flux varies by a factor of 2 across the system. After correcting for the (CO)-C-12 J = 3-2 contamination, the dust emission at 850 mum detected by SCUBA is consistent with the thermal emission from a single warm dust component with a mass of 1.7 x 10(7) M-circle dot.