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

We have conducted 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 [1]. The observations are part of the Nobeyama Radio Observatory M33 All-disk survey of Giant Molecular Clouds project (NRO MAGiC [2]). The spatial resolution is 100 pc. We detect 12CO(J = 3–2) emission of 65 GMCs successfully. The 12CO(J = 1–0) and 12CO(J = 3–2) integrated intensity ratio R3–2/1–0 is spread over a wide range from less than 0.1 to 0.74, having a weighted mean of R3–2/1–0 = 0.26. This weighted mean is slightly smaller than that of the quiescent disk region of the Milky Way. Furthermore, we find that the correlation between the surface density of the star formation rate (ΣSFR), which is derived from a linear combination of Hα and 24 μm emissions, and the 12CO(J = 3–2) integrated intensity still holds at the scale of 100 pc (Figure 1), although the correlation between ICO(1–0) and ΣSFR is not obvious, as we have already shown in the preceding paper [3]. 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 value of 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 (Figure 2). 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. NRO M33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC): II. Dense Gas Formation within Giant Molecular Clouds in M33