The CO 2-1/1-0 ratio in the southern Milky Way and the Large Magellanic Cloud

First results of the wide-angle survey of the CO J=2-1 line emission from the southern sky are reported. The observations were made with a dedicated 60-cm radio telescope at La Silla, Chile, and the data are directly compared with the Columbia survey of the CO J=1-0 emission. In the fourth quadrant of the Milky Way, the Norma and Centaurus arms show higher 2-1/1-0 line intensity ratios, while the 3-kpc expanding arm shows markedly lower ratios. The overall distribution of the ratio in the Galactic disk can be expressed as a superposition of these irregularities on the galactic-scale gradient of the ratio decreasing with the galactocentric radius. Giant molecular clouds in the Large Magellanic Cloud (LMC) exhibit CO 2-1/1-0 ratios ranging from 0.5 to 1.3, similar to those in the Milky Way. The ratio does not correlate well with the indicators of massive star formation in the recent past. Combined with the concentration of high ratio gas in spiral arms noted in the Milky Way, this supports the hypothesis that the high CO 2-1/1-0 ratio highlights dense concentrations of molecular gas ready for massive star formation. The 30 Dor molecular cloud complex contains a large amount of high ratio gas. Among the other clouds, we found a tendency that higher ratios are observed for clouds closer to the kinetic center of the LMC. Some global mechanism may control the physical conditions of molecular gas there.