The CO J=2-1/J=1-0 Ratio in the Large Magellanic Cloud

Details The CO J=2-1/J=1-0 Ratio in the Large Magellanic Cloud The CO J=2-1/J=1-0 Ratio in the Large Magellanic Cloud

Apr 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001apj...551..794s&link_type=abstract

The Astrophysical Journal, Volume 551, Issue 2, pp. 794-802.

Astronomy and Astrophysics

Astronomy

10

Galaxies: Irregular, Galaxies: Ism, Ism: Clouds, Ism: Molecules, Galaxies: Magellanic Clouds, Radio Lines: Ism

Scientific paper

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 (R2-1/1-0) scatters in a range of 0.5-1.3. The luminosity ratio averaged for all observed points is 0.92+/-0.05. The ratio R2-1/1-0 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 R2-1/1-0 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 R2-1/1-0, 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 R2-1/1-0 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.

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