A Multitransition CO Study of the Antennae Galaxies NGC 4038/9

Abstract

For the Antennae interacting galaxy pair, we have obtained high-quality, fully sampled 12CO 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, 12CO 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 12CO 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 ~4 × 109 M☉ 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 13CO J = 2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the 12CO/13CO 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 12CO/13CO J = 2-1 and 3-2 ratios are found outside a galactic nucleus. Detailed large velocity gradient (LVG) modeling indicates that the 12CO and 13CO emissions originate in different spatial components. The 12CO emission may originate within a nonvirialized low-density gas component with a large gradient in velocity. Assuming a CO-to-H2 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 μm continuum in the Antennae galaxies shows that the CO line emission contributes globally 46% of the 850 μm continuum flux and that the ratio of 12CO J = 3-2 to SCUBA 850 μm flux varies by a factor of 2 across the system. After correcting for the 12CO J = 3-2 contamination, the dust emission at 850 μm detected by SCUBA is consistent with the thermal emission from a single warm dust component with a mass of 1.7 × 107 M☉.