Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts

Abstract

The factor relating CO emission to molecular hydrogen column density, XCO, is still subject to uncertainty, in particular at low metallicity. Here, to quantify XCO at two different spatial resolutions, we exploit a dust-based method together with ALMA 12-m and ACA data and HI maps of three nearby metal-poor starbursts, NGC625, NGC1705, and NGC5253. Dust opacity at 250pc resolution is derived based on dust temperatures estimated by fitting two-temperature modified blackbodies to Herschel PACS data. By using the HI maps, we are then able to estimate dust-to-gas ratios in the atomic-gas dominated regions, and infer total gas column densities and H2 column densities as the difference with HI. Finally, from the ACA CO(1-0) maps, we derive XCO. We use a similar technique with 40 pc ALMA 12-m data for the three galaxies, but instead derive dust attenuation at 40 pc resolution from reddening maps based on VLT/MUSE data. At 250 pc resolution, XCO $\sim$ 10^22 - 10^23 cm^-2 / K.km/s, 5-1000 times the Milky Way value, with much larger values than would be expected from a simple metallicity dependence. Instead at 40 pc resolution, XCO again shows large variation, but is roughly consistent with a power-law metallicity dependence, given the Z $\sim$ 1/3 Zsun metal abundances of our targets. The large scatter in both estimations could imply additional parameter dependence, that we have investigated by comparing XCO with the observed velocity-integrated brightness temperatures, ICO, as predicted by recent simulations. Indeed, larger XCO is significantly correlated with smaller ICO, but with slightly different slopes and normalizations than predicted by theory. Such behavior can be attributed to the increasing fraction of CO-faint H2 gas with lower spatial resolution. This confirms the idea the XCO is multi-variate, depending not only on metallicity but also on CO brightness temperature and beam size.

A&A, in press (16 figures, 1 appendix)

References in corpus (27)

• Updated Nearby Galaxy Catalog
• UV-extended E-MILES stellar population models: young components in massive early-type galaxies
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• The Data Processing Pipeline for the MUSE Instrument
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• The PHANGS-MUSE survey -- Probing the chemo-dynamical evolution of disc galaxies
• 2MASS wide field extinction maps - I. The Pipe nebula
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• The triggering of starbursts in low-mass galaxies
• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• Highly efficient star formation in NGC 5253 possibly from stream-fed accretion
• Cool dust heating and temperature mixing in nearby star-forming galaxies
• Shaken, not blown: the gentle baryonic feedback of nearby starburst dwarf galaxies
• Extreme dust disks in Arp 220 as revealed by ALMA
• Photodissociation Region Diagnostics Across Galactic Environments
• Dependence of $X_{\mathrm{CO}}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium
• Inflows and Outflows in the Dwarf Starburst Galaxy NGC 5253: High-Resolution HI Observations
• Untangling the Nature of Spatial Variations of Cold Dust Properties in Star Forming Galaxies
• Star formation histories of the LEGUS dwarf galaxies (III): the non-bursty nature of 23 star forming dwarf galaxies
• The resolved scaling relations in DustPedia: Zooming in on the local Universe
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• 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
• HI synthesis observations of the blue compact dwarf NGC 1705

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• Metallicity dependence of the CO-to-H${}_2$ and the [CI]-to-H${}_2$ conversion factors in galaxies
• ALMA FACTS. II. Large Scale Variations in the 12CO(J=2-1) to 12CO(J=1-0) Line Ratio in Nearby Galaxies
• 12CO and 13CO observation of the low-metallicity dwarf galaxy DDO 154
• Predicting the resolved CO emission of $z=1-3$ star-forming galaxies
• Molecular Clouds at the Edge of the Galaxy II. Physical properties and scaling relations