NRO M33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC): II. Dense Gas Formation within Giant Molecular Clouds in M33

We report the results of our observations of the 12CO (J=1-0) and 12CO (J=3-2) line emission of 74 major giant molecular clouds (GMCs) within the galactocentric distance of 5.1 kpc in the Local Group galaxy M33. The observations have been conducted as part of the Nobeyama Radio Observatory M33 All-disk survey of Giant Molecular Clouds project (NRO MAGiC). The spatial resolutions are 80 pc for 12CO (J=1-0) and 100 pc for 12CO (J=3-2). We detect 12CO (J=3-2) emission of 65 GMCs successfully. Furthermore, we find that the correlation between the surface density of the star formation rate, which is derived from a linear combination of Halpha and 24um emissions, and the 12CO (J=3-2) integrated intensity still holds at this scale. This result show that the star-forming activity is closely associated with warm and dense gases that are traced with the 12CO (J=3-2) line, even in the scale of GMCs. We also find that the GMCs with a high star-forming activity tend to show a high integrated intensity ratio (R3-2/1-0). Moreover, we also observe a mass-dependent trend of R3-2/1-0 for the GMCs with a low star-forming activity. From these results, we speculate that the R3-2/1-0 values of the GMCs with a low star-forming activity mainly depend on the dense gas fraction and not on the temperature, and therefore, the dense gas fraction increases with the mass of GMCs, at least in the GMCs with a low star-forming activity.
Comment: 17 pages, 5 figures, Accepted for publication in PASJ, 2012, Vol. 64, No. 6