NRO legacy project: M 33 all disk survey of Giant Molecular Clouds (GMCs) with NRO-45m and ASTE-10m telescopes

Abstract

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 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, f mol = ΣH2 /(ΣHi + ΣH2 ), 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 , increased scale height toward the outer disk.