The CO J = 21 / J = 10 Ratio in the Large Magellanic Cloud

Abstract

We observed 34 positions throughout the disk of the Large Magellanic Cloud in the CO J = 21 emission line with the Tokyo-Onsala-ESO-Calán 60 cm radio telescope. Comparing the spectra with those of the J = 10 line at the same angular resolution (9, or 130 pc at 50 kpc), we found that the CO J = 21/J = 10 intensity ratio (R21/10) scatters in a range of 0.51.3. The luminosity ratio averaged for all observed points is 0.92 ± 0.05. The ratio R21/10 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 R21/10 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 R21/10, 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 R21/10 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.