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 NGC1300, where star formation activity strongly depends on galactic structure, on $\sim 100$ pc scale. CO images were obtained from ALMA and Nobeyama 45-m telescope. The resultant typical $R_{21}$ in NGC1300 is $0.57\pm 0.06$. We find environmental variations in $R_{21}$; it is the highest in the bar-end region ($0.72\pm 0.08$), followed by arm ($0.60\pm 0.07$) and bar regions ($0.50\pm 0.06$). GMCs with H$\alpha$ emission show a systematically higher ratio ($0.67\pm 0.07$) than those without H$\alpha$ ($0.47\pm 0.05$). In the bar region, where massive star formation is suppressed, H$\alpha$ 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 NGC1300.

arXiv comment18 pages, 14 figures, accepted for publication in ApJ

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