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

¹ Joetsu University of Education, Japan
² Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Japan
³ Department of Astronomy, School of Science, The University of Tokyo, Japan
⁴ Department of Physical Science, Osaka Prefecture University, Japan
⁵ National Astronomical Observatory of Japan, Japan
⁶ Institute of Astronomy, School of Science, The University of Tokyo, Japan
⁷ Graduate School of Science and Engineering, Kagoshima University, Japan

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 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, f_mol = Σ_H2/(Σ_Hi + Σ_H2), 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.