NRO LEGACY PROJECT: M 33 ALL DISK SURVEY OF GIANT MOLECULAR CLOUDS (GMCS) WITH NRO-45M AND ASTE-10M TELESCOPES

摘要

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 ~(12)CO(J = 1-0) observations using the NRO 45-m telescope and ~(12)CO(J = 3-2) observations using the ASTE 10-m telescope. The spatial resolution of the resultant ~(12)CO(J = 1-0) map is 19''.3, 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, f_(mol) = Σ_(H_2)/(Σ_(Hi) Σ_(H_2)), at a 100-pc resolution. This is the first f_(mol) map covering an entire galaxy with a GMC-scale resolution. The correlation between f_(mol) 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.