Dependence of Molecular Cloud Samples on Angular Resolution, Sensitivity, and Algorithms

Abstract

In this work, we investigate the observational and algorithmic effects on molecular cloud samples identified from position-position-velocity (PPV) space. By smoothing and cutting off the high quality data of the Milky Way Imaging Scroll Painting (MWISP) survey, we extract various molecular cloud samples from those altered data with the DBSCAN (density-based spatial clustering of applications with noise) algorithm. Those molecular cloud samples are subsequently used to gauge the significance of sensitivity, angular/velocity resolution, and DBSCAN parameters. Two additional surveys, the FCRAO Outer Galaxy Survey (OGS) and the CfA-Chile 1.2 m complete CO (CfA-Chile) survey, are used to verify the MWISP results. We found that molecular cloud catalogs are not unique and the boundary and therefore the number shows strong variation with angular resolution and sensitivity. At low angular resolution (large beam sizes), molecular clouds merge together in PPV space, while low sensitivity (high cutoffs) misses small faint molecular clouds and takes bright parts of large molecular clouds as single ones. At high angular resolution and sensitivity, giant molecular clouds (GMCs) are resolved into individual clouds, and their diffuse components are also revealed. Consequently, GMCs are more appropriately interpreted as clusters or aggregates of molecular clouds, i.e., GMCs represent molecular cloud samples themselves.

25 pages, 21 figures. Accepted for publication in AJ

References in corpus (31)

• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• Structural Analysis of Molecular Clouds: Dendrograms
• Physical properties of molecular clouds for the entire Milky Way disk
• The Milky Way Imaging Scroll Painting (MWISP): Project Details and Initial Results from the Galactic Longitude of 25.8deg to 49.7deg
• Clustered and Triggered Star Formation in W5: Observations with Spitzer
• Giant Molecular Cloud Catalogues for PHANGS-ALMA: Methods and Initial Results
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• The Perils of Clumpfind: The Mass Spectrum of Sub-structures in Molecular Clouds
• New Constraints on the ${}^{12}$CO(2-1)/(1-0) Line Ratio Across Nearby Disc Galaxies
• The molecular cloud lifecycle
• HCN Observations of Dense Star-Forming Gas in High Redshift Galaxies
• A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant
• The integrated properties of the molecular clouds from the JCMT CO(3-2) High Resolution Survey
• The SEDIGISM survey: Molecular clouds in the inner Galaxy
• The Per-Tau Shell: A Giant Star-Forming Spherical Shell Revealed by 3D Dust Observations
• The Mass-Size Relation and the Constancy of GMC Surface Densities in the Milky Way
• Evolution of giant molecular clouds across cosmic time
• The evolution of Giant Molecular Filaments
• The width of Herschel filaments varies with distance
• Distances and statistics of local molecular clouds in the first Galactic quadrant
• The SEDIGISM survey: a search for molecular outflows
• Quantifying the effects of spatial resolution and noise on galaxy metallicity gradients
• Strong dependence of the physical properties of cores on spatial resolution in observations and simulations
• Searching for Molecular Outflows with Support Vector Machines: Dark Cloud Complex in Cygnus
• High Resolution LAsMA ${}^{12}$CO and ${}^{13}$CO Observation of the G305 Giant Molecular Cloud Complex : I. Feedback on the Molecular Gas
• CO Outflow Candidates Toward the W3/4/5 Complex I: The Sample and its Spatial Distribution
• On the Beam Filling Factors of Molecular Clouds
• CO(2-1)/CO(1-0) line ratio on $\sim$100 parsec scale in the nearby barred galaxy NGC1300
• Which Molecular Cloud Structures Are Bound?
• Examinations of CO completeness based on three independent CO surveys
• Burton's Curse: The Impact of Bulk Flows on the Galactic Longitude-Velocity Diagram and the Illusion of a Continuous Perseus Arm

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