Oral Presentation

Significant factor in molecular gas properties and star formation by decomposition of CO emission in Maffei 2

Author(s): Yoshiyuki Yajima (Hokkaido University), Kazuo Sorai (Hokkaido University, University of Tsukuba)

Presenter: Yoshiyuki Yajima (Hokkaido University)

Molecular gas exists in the diffuse component in addition to cloud-like structures. The cloud-phase molecular gas is traced by 13CO line because of its low opacity, while 12CO line traces all molecular gas. To reveal how significantly the cloud/diffuse-phase molecular gas fraction changes properties of molecular gas compared to properties of the cloud component themselves, we conducted 12CO(J=1-0) and 13CO(J=1-0) observations toward the barred spiral galaxies Maffei 2 with the Nobeyama 45 m telescope. Since Galactic emission is seen at specific velocities, we developed a new algorithm to separate emission of Maffei 2 and the Galaxy, and the latter is successfully extracted. 12CO is three-dimensionally decomposed into the two components based on 13CO detection at the same voxel. The luminosity of 12CO without 13CO accounts for 38% of the total luminosity. The integrated-intensity ratio of 13CO-detected 12CO to total 12CO, R_12(13det)/12, which reflects fraction of the cloud (diffuse) component shows significant variation (from 0 to 1) as well as the conventional 12CO/13CO line ratio, R_12/13 (from 8 to 23) in the galaxy. In contrast, 13CO-detected-12CO/13CO line ratio, R_12(13det)/13, which implies properties of the cloud component is rather constant of ~9 over the galaxy. The Kolmogorov-Smirnov test for line ratios between galactic structures shows that p-values systematically decrease in many regions for R_12(13det)/12 than those in R_12/13, and increase in only disk regions for R_12(13det)/13 than R_12/13. Moreover, we found that differences in star-formation efficiency between galactic structures are lowered by ~40%, when molecular gas mass is estimated from 13CO-detected 12CO and 13CO instead of 12CO alone. Our results suggest that proportion of the cloud/diffuse-component molecular gas is a significant factor to characterize properties of molecular gas and star formation, and effects of galactic structures are not strong enough to change clouds' properties, whereas they promote transition of the cloud/diffuse component.