Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way

Abstract

We present the ALMA survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0.9 arcsec ~ 2 pc) spatial resolution while covering large area: four 250 pc x 250 pc regions that encompass ~2/3 of NGC 6822's star formation. In these regions, we resolve ~150 compact CO clumps that have small radii (~2-3 pc), narrow line width (~1 km/s), and low filling factor across the galaxy. This is consistent with other recent studies of low metallicity galaxies, but here shown with a 15 times larger sample. At parsec scales, CO emission correlates with 8 micron emission better than with 24 micron emission and anti-correlates with Halpha, so that PAH emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and CO-to-H2 ratio ~1-2 times the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular cloud. Using dust to trace H2 for the entire complex, we find CO-to-H2 to be ~20-25 times the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H2-to-HI ratio is low globally and only mildly above unity within the complexes. The SFR-to-H2 ratio is ~3-5 times higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.

Accepted for publication in The Astrophysical Journal; 22 pages, 10 figures, 7 tables

References in corpus (22)

• Theory of Star Formation
• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The non-universality of the Molecular Gas Depletion Timescale
• An Ultraviolet-to-Radio Broadband Spectral Atlas of Nearby Galaxies
• Bias-free Measurement of Giant Molecular Cloud Properties
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• ALMA Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst
• Inefficient Star Formation In Extremely Metal Poor Galaxies
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Turbulence sets the initial conditions for star formation in high-pressure environments
• ALLSMOG: an APEX Low-redshift Legacy Survey for MOlecular Gas. I - molecular gas scaling relations, and the effect of the CO/H2 conversion factor
• Particularly Efficient Star Formation in M33
• The spatially resolved Kennicutt-Schmidt relation in the HI dominated regions of spiral and dwarf irregular galaxies
• The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer
• The Molecular Interstellar Medium of the Local Group Dwarf NGC6822
• A New Cepheid Distance Measurement and Method for NGC 6822
• The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• The recent star formation in NGC 6822 from HST imaging

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• Molecular Cloud Populations in the Context of Their Host Galaxy Environments: A Multiwavelength Perspective
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• PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions
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• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions
• The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds
• Dusty Stellar Birth and Death in the Metal-Poor Galaxy NGC 6822
• Over-sized gas clumps in an extremely-metal-poor molecular cloud revealed by ALMA's pc-scale maps
• Massive young stellar objects in the Local Group irregular galaxy NGC6822 identified using machine learning
• CO(2-1) Survey at 9 parsec resolution in the SMC
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• Wide-field CO isotopologue emission and the CO-to-H${}_2$ factor across the nearby spiral galaxy M101
• DeGaS-MC: Dense Gas Survey in the Magellanic Clouds I -- An APEX survey of HCO+ and HCN(2-1) toward the LMC and SMC
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• The Lyman Alpha Reference Sample XI: Efficient Turbulence Driven Lyα Escape and the Analysis of IR, CO and [C II]158 μm
• PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies
• The impact of carbon and oxygen abundances on the metal-poor initial mass function
• The Karl G. Jansky Very Large Array Local Group L-band Survey (LGLBS)
• Gas dynamics and star formation in NGC 6822
• Extremely weak CO emission in IZw 18
• Resolved star formation in the metal poor star-forming region Magellanic Bridge C
• Simultaneous Deep Measurements of CO isotopologues and Dust Emission in Giant Molecular Clouds in the Andromeda Galaxy
• Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts
• Metallicity dependence of the CO-to-H${}_2$ and the [CI]-to-H${}_2$ conversion factors in galaxies
• Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA
• Massive young stellar objects in the Local Group spiral galaxy M33 identified using machine learning
• ALMA Observations of the Molecular Clouds in NGC 625
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• Inadequate turbulent support in low-metallicity molecular clouds
• Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship
• Sardinia Radio Telescope observations of Local Group dwarf galaxies -- I. The cases of NGC6822, IC1613, and WLM
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• Tight correlation of star formation with [Ci] and CO lines across cosmic time
• Molecular Clouds at the Edge of the Galaxy II. Physical properties and scaling relations
• A sensitive search for water masers associated with star formation regions in the Local Group galaxy NGC 6822