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

Abstract

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 ($\ge 10^5{M}_{\odot }$) 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.

23 pages, 17 figures, accepted for publication in the Astrophysical Journal

References in corpus (26)

• Structural Analysis of Molecular Clouds: Dendrograms
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
• Physical properties of molecular clouds for the entire Milky Way disk
• Bias-free Measurement of Giant Molecular Cloud Properties
• A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
• Giant Molecular Cloud Catalogues for PHANGS-ALMA: Methods and Initial Results
• New Constraints on the ${}^{12}$CO(2-1)/(1-0) Line Ratio Across Nearby Disc Galaxies
• Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA
• AUTO-MULTITHRESH: A General Purpose Automasking Algorithm
• Full-disc ${}^{13}$CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H${}_2$ conversion factor
• Dynamical signatures of a LCDM-halo and the distribution of the baryons in M33
• CO Multi-line Imaging of Nearby Galaxies (COMING). IX. ${}^{12}$CO($J$=2-1)/${}^{12}$CO($J$=1-0) line ratio on kiloparsec scales
• Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks
• ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud
• CO Multi-line Imaging of Nearby Galaxies (COMING): I. Physical properties of molecular gas in the barred spiral galaxy NGC 2903
• The molecular gas mass of M33
• A CO isotopologue Line Atlas within the Whirlpool galaxy Survey (CLAWS)
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions
• CARMA Survey toward Infrared-bright Nearby Galaxies (STING). IV. Spatially Resolved 13CO in Spiral Galaxies
• Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions
• ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog
• ALMA Observations of Giant Molecular Clouds in M33 III: Spatially Resolved Features of the Star-Formation Inactive Million-solar-mass Cloud
• CO Multi-line Imaging of Nearby Galaxies (COMING). X. Physical conditions of molecular gas and the local SFR-Mass relation
• Molecular Line Ratio Diagnostics Along the Radial Cut and Dusty UV-bright Clumps in a Spiral Galaxy NGC 0628

Cited by in corpus (5)

• The Karl G. Jansky Very Large Array Local Group L-band Survey (LGLBS)
• ACA CO(J=2-1) Mapping of the Nearest Spiral Galaxy M33. II. Exploring the Evolution of Giant Molecular Clouds
• Sub-kpc scale gas density histogram of the Galactic molecular gas: a new statistical method to characterise galactic-scale gas structures
• Molecular cloud matching in CO and dust in M33 II. Physical properties of giant molecular clouds
• M^3D: Mosaicking M33 with MUSE datacubes I. Unveiling the diversity of HII regions in M33 with MUSE