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 ¹²CO(J = 2-1), ¹³CO(J = 2-1), and C¹⁸O(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 ¹²CO(J = 2-1) data with the IRAM 30 m data to compensate for emission from diffuse molecular-gas components. The ACA+IRAM combined ¹²CO(J = 2-1) map clearly depicts the cloud-scale molecular-gas structure over the M33 disk. Based on the ACA+IRAM ¹²CO(J = 2-1) cube data, we cataloged 848 molecular clouds with a mass range from 10³-10⁶ M _⊙. We found that high-mass clouds (≥10⁵ M _⊙) tend to associate with the 8 μm bright sources in the spiral arm region, while low-mass clouds (<10⁵ M _⊙) tend to be apart from such 8 μm 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.