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.
