Oral Presentation

CO(2–1)/CO(1–0) line ratio on a ∼100 parsec scale in the nearby barred galaxy NGC 1300

Author(s): Fumiya Maeda, Fumi Egusa (Univ. of Tokyo), Kouji Ohta (Kyoto Univ.), Yusuke Fujimoto (Univ. of Aizu), Asao Habe(Hokkaido Univ.), Yoshihisa Asada(Kyoto Univ.)

Presenter: Fumiya Maeda (Univ. of Tokyo)

CO(2–1) emission is often used as a tracer of giant molecular clouds (GMCs) as an alternative to CO(1–0) emission in recent years. Therefore, understanding the environmental dependence of the line ratio of CO(2–1)/CO (1–0), R21, on the GMC scale is important to accurately estimate the mass of GMCs. We thus measured R21 in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on a ∼100 pc scale. CO images were obtained from the Atacama Large Millimeter/submillimeter Array and the Nobeyama 45 m telescope. The resultant typical R21 in NGC 1300 is 0.57 ± 0.06. We find environmental variations in R21: it is the highest in the bar-end region (0.72 ± 0.08), followed by arm (0.60 ± 0.07) and bar regions (0.50 ± 0.06). GMCs with Hα emission show a systematically higher ratio (0.67 ± 0.07) than those without Hα (0.47 ± 0.05). In the bar region, where massive star formation is suppressed, Hα emission is not associated with most GMCs, resulting in the lowest R21. These results raise a possibility that properties of GMCs derived from CO(2–1) observations with the assumption of a constant R21 are different from those derived from CO(1–0) observations. Furthermore, we find the R21 measured on the kiloparsec scale tends to be lower than that of the GMCs, probably due to the presence of an extended diffuse molecular gas in NGC 1300.