CO(2-1)/CO(1-0) Line Ratio on a similar to 100 Parsec Scale in the Nearby Barred Galaxy NGC 1300

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), R (21), on the GMC scale is important to accurately estimate the mass of GMCs. We thus measured R (21) in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on a similar to 100 pc scale. CO images were obtained from the Atacama Large Millimeter/submillimeter Array and the 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 H alpha emission show a systematically higher ratio (0.67 +/- 0.07) than those without H alpha (0.47 +/- 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 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.