ALMA Observations of Giant Molecular Clouds in M33. I. Resolving Star Formation Activities in the Giant Molecular Filaments Possibly Formed by a Spiral Shock

We report molecular line and continuum observations toward one of the most massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an angular resolution of 0"44 × 0"27 (∼2 pc × 1 pc). We have found that the GMC is composed of several filamentary structures in ¹²CO and ¹³CO(J = 2-1). The typical length, width, and total mass are ∼50-70 pc, ∼5-6 pc, and ∼10⁵ M_⊙, respectively, which are consistent with those of giant molecular filaments (GMFs) as seen in the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the direction of the galaxy's rotation, and several H II regions are located at the downstream side relative to the filaments with an offset of ∼10-20 pc. These observational results indicate that the GMFs are considered to be produced by a galactic spiral shock. The 1.3 mm continuum and C¹⁸O(J = 2-1) observations detected a dense clump with the size of ∼2 pc at the intersection of several filamentary clouds, which is referred to as the "hub filament," possibly formed by a cloud-cloud collision. A strong candidate for protostellar outflow in M33 has also been identified at the center of the clump. We have successfully resolved the parsec-scale local star formation activity in which the galactic scale kinematics may induce the formation of the parental filamentary clouds.