We present the results of the Nobeyama Radio Observatory (NRO) M 33 All Disk (30′ × 30′,
or 7.3 kpc × 7.3 kpc) Survey of Giant Molecular Clouds (NRO MAGiC) based on
12CO(J = 1–0) observations using the NRO 45-m telescope and
12CO(J = 3–2) observations using the ASTE 10-m telescope.
The spatial resolution of the resultant 12CO(J = 1–0) map is
193, corresponding to 81 pc, which is sufficient to identify each Giant Molecular Cloud
(GMC) in the disk. We found clumpy structures with a typical spatial scale of
~100 pc, corresponding to GMCs, and no diffuse, smoothly distributed component
of molecular gas at this sensitivity.
We obtained a map of the molecular fraction,
fmol = ΣH2/(ΣHi + ΣH2),
at a 100-pc resolution. This is the first fmol map covering an
entire galaxy with a GMC-scale resolution. The correlation between
fmol and gas surface density shows two distinct sequences.
The presence of two correlation sequences can be explained by differences in metallicity,
i.e., higher (~2-fold) metallicity in the central region
(r < 1.5 kpc) than in the outer parts. Alternatively,
differences in scale height can also account for the two sequences, i.e.,
increased scale height toward the outer disk.