ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex

We present the results of ALMA observations in ¹²CO(J = 2 − 1), ¹³CO(J = 2 − 1), and C¹⁸O(J = 2 − 1) lines and 1.3 mm continuum emission toward a massive (∼10⁶ M _⊙) giant molecular cloud associated with the giant H ii region NGC 604 in one of the nearest spiral galaxies, M33, at an angular resolution of 0.″44 × 0.″27 (1.8 pc × 1.1 pc). The ¹²CO and ¹³CO images show highly complicated molecular structures composed of a lot of filaments and shells whose lengths are 5–20 pc. We found three 1.3 mm continuum sources to be dense clumps at the edges of two shells and also at the intersection of several filaments. We examined the velocity structures of the ¹²CO(J = 2 − 1) emission in the shells and filaments containing dense clumps, and concluded that expansion of the H ii regions cannot explain the formation of such dense cores. Alternatively, we suggest that cloud–cloud collisions induced by an external H i gas flow and the galaxy's rotation compressed the molecular material into dense filaments/shells which are ongoing high-mass star formation sites. We propose that multiple gas converging/colliding events with a velocity of a few tens of kilometers per second are necessary to build up NGC 604, the most significant cluster-forming complex in the Local Group of galaxies.