CO(2-1)/CO(1-0) line ratio on $\sim$100 parsec scale in the nearby barred galaxy NGC1300

Abstract

CO(2-1) emission is often used as a tracer of the 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), R 21 , on GMC scale is important to accurately estimate the mass of the GMCs. We thus measured the R 21 in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on ∼ 100 pc scale. CO images were obtained from ALMA and Nobeyama 45-m telescope. The resultant typical R 21 in NGC 1300 is 0.57 ± 0.06. We find environmental variations in R 21 ; 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 R 21 . These results raise a possibility that properties of GMCs derived from CO(2-1) observations with the assumption of a constant R 21 are different from those derived from CO(1-0) observations. Furthermore, we find the R 21 measured on kpc scale tends to be lower than that of the GMCs probably due to the presence of an extended diffuse molecular gas in NGC 1300.