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

We thank the anonymous referee for their helpful comments, which significantly improved the manuscript. This paper makes use of the following ALMA data: [ADS/JAO.ALMA#2017.1.00461.S], [ADS/JAO.ALMA#2018.A.00058.S], [ADS/JAO.ALMA#2017.1.00901.S], and [ADS/JAO.ALMA#2019.1.01182.S]. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Data analysis was in part carried out on the Multi-wavelength Data Analysis System operated by the Astronomy Data Center (ADC), National Astronomical Observatory of Japan. K.M. was supported by the ALMA Japan Research Grant of NAOJ ALMA Project, NAOJ-ALMA-289. This work was supported by NAOJ ALMA Scientific Research grant Nos. 2022-22B and JSPS KAKENHI (grant Nos. JP18H05440, JP19H05075, JP21H00049, JP21H01136, and JP21K13962).We present the results of ALMA-ACA 7 m array observations in (CO)-C-12(J = 2-1), (CO)-C-13(J = 2-1), and (CO)-O-18(J = 2-1) line emission toward the molecular-gas disk in the Local Group spiral galaxy M33 at an angular resolution of 7.& DPRIME;31 x 6.& DPRIME;50 (30 x 26 pc). We combined the ACA 7 m array (CO)-C-12(J = 2-1) data with the IRAM 30 m data to compensate for emission from diffuse molecular-gas components. The ACA+IRAM combined (CO)-C-12(J = 2-1) map clearly depicts the cloud-scale molecular-gas structure over the M33 disk. Based on the ACA+IRAM (CO)-C-12(J = 2-1) cube data, we cataloged 848 molecular clouds with a mass range from 10(3)-10(6) M (& ODOT;). We found that high-mass clouds (& GE;10(5) M (& ODOT;)) tend to associate with the 8 & mu;m bright sources in the spiral arm region, while low-mass clouds (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), 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.ALMA: NAOJ-ALMA-289ALMA Japan Research Grant of NAOJ ALMA Project NAOJ-ALMA-289NAOJ ALMA Scientific Research 2022-22BMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (KAKENHI) JP18H05440, JP19H05075, JP21H00049, JP21H01136, JP21K1396

An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog

The nature of molecular clouds and their statistical behavior in sub-solar metallicity environments are not fully explored yet. We analyzed an unbiased CO($J$ = 2-1) survey data at a spatial resolution of $\sim$2 pc in the northern region of the Small Magellanic Cloud (SMC) with the Atacama Compact Array to characterize the CO cloud properties. A cloud decomposition analysis identified 426 spatially/velocity-independent CO clouds and their substructures. Based on the cross-matching with known infrared catalogs by Spitzer and Herschel, more than 90% CO clouds show spatial correlations with point sources. We investigated the basic properties of the CO clouds and found that the radius-velocity linewidth ($R$-$\sigma_{v}$) relation follows the Milky Way (MW) like power-low exponent, but the intercept is $\sim$1.5 times lower than that in the MW. The mass functions ($dN/dM$) of the CO luminosity and virial mass are characterized by an exponent of $\sim$1.7, which is consistent with previously reported values in the Large Magellanic Cloud and MW.Comment: 18 pages, 9 figures. Accepted for publication in The Astrophysical Journa

An ALMA study of the massive molecular clump N159W-North in the Large Magellanic Cloud: A possible gas flow penetrating one of the most massive protocluster systems in the Local Group

Massive dense clumps in the Large Magellanic Cloud can be an important laboratory to explore the formation of populous clusters. We report multiscale ALMA observations of the N159W-North clump, which is the most CO-intense region in the galaxy. High-resolution CO isotope and 1.3 mm continuum observations with an angular resolution of $\sim$0."25($\sim$0.07 pc) revealed more than five protostellar sources with CO outflows within the main ridge clump. One of the thermal continuum sources, MMS-2, shows especially massive/dense nature whose total H$_2$ mass and peak column density are $\sim$10$^{4}$ $M_{\odot}$ and $\sim$10$^{24}$ cm$^{-2}$, respectively, and harbors massive ($\sim$100 $M_{\odot}$) starless core candidates identified as its internal substructures. The main ridge containing this source can be categorized as one of the most massive protocluster systems in the Local Group. The CO high-resolution observations found several distinct filamentary clouds extending southward from the star-forming spots. The CO (1-0) data set with a larger field of view reveals a conical-shaped, $\sim$30 pc long complex extending toward the northern direction. These features indicate that a large-scale gas compression event may have produced the massive star-forming complex. Based on the striking similarity between the N159W-North complex and the previously reported other two high-mass star-forming clouds in the nearby regions, we propose a $"$teardrops inflow model$"$ that explains the synchronized, extreme star formation across $>$50 pc, including one of the most massive protocluster clumps in the Local Group.Comment: 25 pages, 14 figures, 6 tables, Accepted for publication in Ap

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