The CO J = 2-1/j= 1-0 ratio in the large Magellanic Cloud

K. Sorai, T. Hasegawa, R. S. Booth, M. Rubio, J. I. Morino, L. Bronfman, T. Handa, M. Hayashi, L. A. Nyman, T. Oka, S. Sakamoto, M. Seta, K. S. Usuda

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Abstract

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-Calá 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 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 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.

Original languageEnglish
Pages (from-to)794-802
Number of pages9
JournalAstrophysical Journal
Volume551
Issue number2 PART 1
DOIs
Publication statusPublished - 2001 Apr 20

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Keywords

  • Galaxies: ISM
  • Galaxies: irregular
  • ISM: clouds
  • ISM: molecules
  • Magellanic clouds
  • Radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Sorai, K., Hasegawa, T., Booth, R. S., Rubio, M., Morino, J. I., Bronfman, L., Handa, T., Hayashi, M., Nyman, L. A., Oka, T., Sakamoto, S., Seta, M., & Usuda, K. S. (2001). The CO J = 2-1/j= 1-0 ratio in the large Magellanic Cloud. Astrophysical Journal, 551(2 PART 1), 794-802. https://doi.org/10.1086/320212