open access

https://hdl.handle.net/2115/84713

CO Multi-line Imaging of Nearby Galaxies (COMING). IX. (CO)-C-12(J=2-1)/(CO)-C-12(J=1-0) line ratio on kiloparsec scales

英語

radio lines: ISM

2021-04

Oxford University Press

Publications of the astronomical society of Japan

73(2)

257-285

While molecular gas mass is usually derived from (CO)-C-12(J = 1-0)-the most fundamental line for exploring molecular gas-it is often derived from (CO)-C-12(J = 2-1) assuming a constant (CO)-C-12(J = 2-1)/(CO)-C-12(J = 1-0) line ratio (R-2/1). We present variations of R-2/ 1 and effects of the assumption that R-2/1 is a constant in 24 nearby galaxies using (CO)-C-12 data obtained with the Nobeyama 45mradio telescope and IRAM 30mtelescope. The median of R-2/1 for all galaxies is 0.61, and the weighted mean of R-2/1 by (CO)-C-12(J = 1-0) integrated intensity is 0.66 with a standard deviation of 0.19. The radial variation of R-2/1 shows that it is high (similar to 0.8) in the inner similar to 1 kpc while its median in disks is nearly constant at 0.60 when all galaxies are compiled. In the case that the constant R-2/1 of 0.7 is adopted, we found that the total molecular gas mass derived from (CO)-C-12(J = 2-1) is underestimated/ overestimated by similar to 20%, and at most by 35%. The scatter of molecular gas surface density within each galaxy becomes larger by similar to 30%, and at most by 120%. Indices of the spatially resolved Kennicutt-Schmidt relation by (CO)-C-12(J = 2-1) are underestimated by 10%-20%, at most 39%, in 17 out of 24 galaxies. R-2/1 has good positive correlations with star-formation rate and infrared color, and a negative correlation with molecular gas depletion time. There is a clear tendency of increasing R-2/1 with increasing kinetic temperature (T-kin). Further, we found that not only T-kin but also pressure of molecular gas is important in understanding variations of R-2/1. Special considerations should be made when discussing molecular gas mass and molecular gas properties inferred from (CO)-C-12(J = 2-1) instead of (CO)-C-12(J = 1-0).

学術雑誌論文

Accepted Manuscript

This is a pre-copyedited, author-produced version of an article accepted for publication in Publications of the astronomical society of Japan following peer review. The version of record Volume 73, Issue 2, April 2021, Pages 257–285 is available online at:https://doi.org/10.1093/pasj/psaa119.

0004-6264