ACA CO(J = 2–1) Mapping of the Nearest Spiral Galaxy M33. I. Initial Results and Identification of Molecular Clouds

We present the results of ALMA-ACA 7 m array observations in ^12 CO( J = 2–1), ^13 CO( J = 2–1), and C ^18 O( J = 2–1) line emission toward the molecular-gas disk in the Local Group spiral galaxy M33 at an angular resolution of 7.″31 × 6.″50 (30 × 26 pc). We combined the ACA 7 m array ^12 CO( J = 2–1) data with the IRAM 30 m data to compensate for emission from diffuse molecular-gas components. The ACA+IRAM combined ^12 CO( J = 2–1) map clearly depicts the cloud-scale molecular-gas structure over the M33 disk. Based on the ACA+IRAM ^12 CO( J = 2–1) cube data, we cataloged 848 molecular clouds with a mass range from 10 ^3 –10 ^6 M _⊙ . We found that high-mass clouds (≥10 ^5 M _⊙ ) tend to associate with the 8 μ m bright sources in the spiral arm region, while low-mass clouds (<10 ^5 M _⊙ ) tend to be apart from such 8 μ m bright sources and to exist in the inter-arm region. We compared the cataloged clouds with GMCs observed by the IRAM 30 m telescope at 49 pc resolution (IRAM GMC), and found that a small IRAM GMC is likely to be identified as a single molecular cloud even in ACA+IRAM CO data, while a large IRAM GMC can be resolved into multiple ACA+IRAM clouds. The velocity dispersion of a large IRAM GMC is mainly dominated by the line-of-sight velocity difference between small clouds inside the GMC rather than the internal cloud velocity broadening