J an 2 00 3 A Multi-transition CO Study of The “ Antennae ” Galaxies NGC 4038 / 9

For the Antennae interacting galaxy pair, we have obtained high quality, fully sampled CO 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 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 J=1–0, 2–1, 3–2 emission all peak in an off-nucleus region adjacent to where the two disks overlap. Use of the conventional X factor yields ∼ 4 × 10 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 CO J=2–1 and 3–2 is detected at selected points in the two nuclei and the overlap region. Both the CO/CO 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/CO J=2–1 and 3–2 ratios are found outside a galactic nucleus. Detailed LVG modeling indicates that the CO and CO emission originate in different spatial components. The CO emission may originate within a non-virialized low density gas component with a large velocity gradient. Assuming a CO-to-H2 abundance ratio of 10 , 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 this 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. Dept. of Astronomy, U. of Toronto, 60 St. George St. Toronto, ON M5S−3H8, Canada Nobeyama Radio Observatory, Minamimakimura, Nagano 384-1305, Japan