ALMA Observations of Giant Molecular Clouds in M33. III. Spatially Resolved Features of the Star formation Inactive Million-solar-mass Cloud

We present ¹²CO (J = 2-1), ¹³CO (J = 2-1), and C¹⁸O (J = 2-1) observations toward GMC-8, one of the most massive giant molecular clouds (GMCs) in M33 using ALMA with an angular resolution of 0"44 × 0"27 (∼2 pc × 1 pc). The earlier studies revealed that its high-mass star formation is inactive in spite of a sufficient molecular reservoir with a total mass of ∼10⁶ M_⊙. The high-angular resolution data enable us to resolve this peculiar source down to a molecular clump scale. One of the GMC's remarkable features is that a round-shaped gas structure (the "Main cloud") extends over the ∼50 pc scale, which is quite different from the other two active star-forming GMCs dominated by remarkable filaments/shells obtained by our series of studies in M33. The fraction of the relatively dense gas traced by the ¹³CO data with respect to the total molecular mass is only ∼2%, suggesting that their spatial structure and the density are not well developed to reach an active star formation. The CO velocity analysis shows that the GMC is composed of a single component as a whole, but we found some local velocity fluctuations in the Main cloud and extra blueshifted components at the outer regions. Comparing the CO with previously published large-scale H I data, we suggest that an external atomic gas flow supplied a sufficient amount of material to grow the GMC up to ∼10⁶ M_⊙.