The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds

Abstract

To test the theoretical understanding that finding bright CO emission depends primarily on dust shielding, we investigate the relationship between CO emission ($I_{\mathrm{CO}}$) and the amount of dust (estimated from IR emission and expressed as "$A_V$") across the Large Magellanic Cloud, the Small Magellanic Cloud, and the Milky Way. We show that at our common resolution of 10 pc scales, $I_{\mathrm{CO}}$ given a fixed line-of-sight $A_V$ is similar across all three systems despite the difference in metallicity. We find some evidence for a secondary dependence of $I_{\mathrm{CO}}$ on radiation field; in the LMC, $I_{\mathrm{CO}}$ at a given $A_V$ is smaller in regions of high $T_{\mathrm{dust}}$, perhaps because of an increased photodissociating radiation field. We suggest a simple but useful picture in which the CO-to-H${}_2$ conversion factor (\xco) depends on two separable factors: (1) the distribution of gas column densities, which maps to an extinction distribution via a dust-to-gas ratio; and (2) the dependence of $I_{\mathrm{CO}}$ on $A_V$. Assuming that the probability distribution function (PDF) of local Milky Way clouds is universal, this approach predicts a dependence of \xco\ on $Z$ between $Z^{-1}$ and $Z^{-2}$ above about a third solar metallicity. Below this metallicity, CO emerges from only the high column density parts of the cloud and so depends very sensitively on the adopted PDF and the H${}_2$/{\sc Hi} prescription. The PDF of low metallicity clouds is thus of considerable interest and the uncertainty associated with even an ideal prescription for \xco\ at very low metallicity will be large.

20 pages, 11 figures, submitted to MNRAS

References in corpus (60)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Star Formation in the Milky Way and Nearby Galaxies
• The CO-to-H2 Conversion Factor
• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies
• Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
• Planck 2013 results. XI. All-sky model of thermal dust emission
• The Dark Molecular Gas
• Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results
• Gas-to-Dust mass ratios in local galaxies over a 2 dex metallicity range
• The CO-to-H2 Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies
• The CO-to-H2 Conversion Factor From Infrared Dust Emission Across the Local Group
• A Molecular Star Formation Law in the Atomic Gas Dominated Regime in Nearby Galaxies
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• The Star Formation Rate and Gas Surface Density Relation in the Milky Way: Implications for Extragalactic Studies
• Common-Resolution Convolution Kernels for Space- and Ground-Based Telescopes
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Planck Early Results: All sky temperature and dust optical depth from Planck and IRAS: Constraints on the "dark gas" in our galaxy
• Probing the evolution of molecular cloud structure: From quiescence to birth
• The Magellanic Clouds Photometric Survey: The Large Magellanic Cloud Stellar Catalog and Extinction Map
• A General Model for the CO-H2 Conversion Factor in Galaxies with Applications to the Star Formation Law
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• The Metallicity Dependence of the CO-to-H${}_2$ Conversion Factor from Observations of Local Group Galaxies
• The metallicity dependence of the CO {\rightarrow} H_2 conversion factor in z>1 star forming galaxies
• The relation between gas and dust in the Taurus Molecular Cloud
• The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN I: Catalog of Molecular Clouds
• The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud
• Low CO Luminosities in Dwarf Galaxies
• On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
• Planck Early Results: Thermal dust in Nearby Molecular Clouds
• Bump Cepheids in the Magellanic Clouds: Metallicities, the Distances to the LMC and SMC, and the Pulsation-Evolution Mass Discrepancy
• 2MASS wide field extinction maps - I. The Pipe nebula
• Planck 2013 results. XIII. Galactic CO emission
• 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
• Giant Molecular Clouds in M33 -- II. High Resolution Observations
• Inverse temperature dependence of the dust submillimeter spectral index
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• The Atomic-to-Molecular Transition in Galaxies. III. A New Method for Determining the Molecular Content of Primordial and Dusty Clouds
• Planck Early Results: Origin of the submm excess dust emission in the Magellanic Clouds
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• Molecular clouds have power-law probability distribution functions
• Molecular Gas in the Low Metallicity, Star Forming Dwarf IC 10
• Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud
• Herschel Inventory of The Agents of Galaxy Evolution (HERITAGE): the Large Magellanic Cloud dust
• ALMA Resolves 30 Doradus: Sub-parsec Molecular Cloud Structure Near the Closest Super-Star Cluster
• Resolved Giant Molecular Clouds in Nearby Spiral Galaxies: Insights from the CANON CO (1-0) Survey
• Determining dust temperatures and masses in the Herschel era: the importance of observations longward of 200 micron
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
• Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data
• Unusual CO Line Ratios and kinematics in the N83/N84 Region of the Small Magellanic Cloud
• A COMPLETE Look at The Use of IRAS Emission Maps to Estimate Extinction and Dust Temperature
• Dust Emission from the Perseus Molecular Cloud
• The Spectrum of Integrated Millimeter Flux of the Magellanic Clouds and 30-Doradus from TopHat and DIRBE Data
• The Spatial Distribution of Dust and Stellar Emission of the Magellanic Clouds
• The VIRUS-P Exploration of Nearby Galaxies (VENGA): The Xco Gradient in NGC 628
• The Influence of Far-Ultraviolet Radiation on the Properties of Molecular Clouds in the 30 Dor Region of the Large Magellanic Cloud
• The Effect of Noise in Dust Emission Maps on the Derivation of Column Density, Temperature and Emissivity Spectral Index
• Estimating Column Density in Molecular Clouds with FIR and Sub-mm Emission Maps

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• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• The Relationship Between Molecular Gas, HI, and Star Formation in the Low-Mass, Low-Metallicity Magellanic Clouds
• The EDGE-CALIFA Survey: Using Optical Extinction to Probe the Spatially-Resolved Distribution of Gas in Nearby Galaxies
• First Results from the $Herschel$ and ALMA Spectroscopic Surveys of the SMC: The Relationship Between [CII]-bright Gas and CO-bright Gas at Low Metallicity
• Dependence of $X_{\mathrm{CO}}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium
• Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO
• Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters
• The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies
• Chemical evolution of giant molecular clouds in simulations of galaxies
• The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds
• Over-sized gas clumps in an extremely-metal-poor molecular cloud revealed by ALMA's pc-scale maps
• The 1.1 mm Continuum Survey of the Small Magellanic Cloud: Physical Properties and Evolution of the Dust-selected Clouds
• Extinction and dust/gas ratio in the H I ridge region of the LMC based on the IRSF/SIRIUS near-infrared survey
• Molecular Gas and Star Formation in the Cartwheel
• Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts
• Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA
• Gathering dust: A galaxy-wide study of dust emission from cloud complexes in NGC 300
• The unusual ISM in Blue and Dusty Gas Rich Galaxies (BADGRS)
• Using CO to Measure Molecular Masses
• A Catalog of Stellar and Dust Properties for 500,000 Stars in the Southwest Bar of the Small Magellanic Cloud
• Weak CS Emission in an Extremely Metal-poor Galaxy DDO 70
• The Interstellar Medium and Star Formation in Dwarf Galaxies