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 $\times$ 6".50 (30 pc $\times$ 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 $10^3$ $M_{\odot}$ to $10^6$ $M_{\odot}$. We found that high-mass clouds ($\geq 10^5 M_{\odot}$) tend to associate with the $8 \mu$m-bright sources in the spiral arm region, while low-mass clouds ($< 10^5 M_{\odot}$) tend to be apart from such $8 \mu$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: Corbelli et al. 2017), 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.