2001ApJ...551..794S

We observed 34 positions throughout the disk of the Large Magellanic Cloud in the CO J=2-1 emission line with the Tokyo-Onsala-ESO-Calán 60 cm radio telescope. Comparing the spectra with those of the J=1-0 line at the same angular resolution (≃9', or 130 pc at 50 kpc), we found that the CO J=2-1/J=1-0 intensity ratio (R_2–1/1–0) scatters in a range of 0.5-1.3. The luminosity ratio averaged for all observed points is 0.92±0.05. The ratio R_2–1/1–0 is approximately unity (0.95±0.06) in 30 Dor, consistent with optically thick and thermalized emission, even in the southern part where massive star formation does not occur yet. This suggests that the high R_2–1/1–0 is not primarily due to the UV radiation from young stars but rather to the intrinsic nature of the molecular gas that is relatively dense (≳10³ cm^–3) and may be ready to form stars. In addition to a cloud-to-cloud difference of R_2–1/1–0, there exists a radial gradient of the ratio of 0.94±0.11 in the inner region (≲2 kpc from the kinematic center) and 0.69±0.11 in the outer region (≳2 kpc from the center, excluding the 30 Doradus complex). The higher R_2–1/1–0 in the inner galaxy might be due to relatively higher gas densities within CO clumps in molecular clouds and/or higher external heating in that region.