Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations

Abstract

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the CO(J=1-0) emission line, using a CO-to-H2 conversion factor, X. Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different to those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.

9 pages, 8 figures, accepted for publication in MNRAS

Cited by in corpus (48)

• The Resolved Properties of Extragalactic Giant Molecular Clouds
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• Photodissociation and X-Ray Dominated Regions
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• Effective destruction of CO by cosmic rays: implications for tracing H${}_2$ gas in the Universe
• A complete 12CO 2-1 map of M51 with HERA: I. Radial averages of CO, HI, and radio continuum
• ALMA spectroscopic survey in the Hubble Ultra Deep Field: CO luminosity functions and the evolution of the cosmic density of molecular gas
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• The EDGE-CALIFA Survey: Using Optical Extinction to Probe the Spatially-Resolved Distribution of Gas in Nearby Galaxies
• The Molecular Gas Reservoirs of $z\sim 2$ Galaxies: A comparison of CO(1-0) and dust-based molecular gas masses
• Central molecular zones in galaxies: 12CO-to-13CO ratios, carbon budget, and X factors
• Photodissociation Region Diagnostics Across Galactic Environments
• Grand challenges in protoplanetary disc modelling
• The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18
• The Astrochemical Impact of Cosmic Rays in Protoclusters I: Molecular Cloud Chemistry
• Molecular line intensities as measures of cloud masses - II. Conversion factors for specific galaxy types
• Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
• ALMA imaging of C2H emission in the disk of NGC1068
• The CO(3-2)/CO(1-0) luminosity line ratio in nearby star-forming galaxies and AGN from xCOLD GASS, BASS and SLUGS
• The CO-dark molecular gas mass in 30 Doradus
• Radiative transfer meets Bayesian statistics: where does a galaxy's [CII] emission come from?
• Photochemical-dynamical models of externally FUV irradiated protoplanetary discs
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• Modelling the ArH${}^{+}$ emission from the Crab Nebula
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters
• Detection of warm molecular hydrogen in the circumstellar disk around the Herbig Ae star HD97048
• Astrochemical Correlations in Molecular Clouds
• The Parsec-Scale Relationship Between I_CO and A_V in Local Molecular Clouds
• Rethinking the N(H2)/I(CO) Conversion Factor
• Water Formation During the Epoch of First Metal Enrichment
• Revealing The CO X-factor In Dark Molecular Gas through Sensitive ALMA Absorption Observations
• A Photodissociation Region study of NGC 4038
• Exploring the Possibility of Identifying Hydride and Hydroxyl Cations of Noble Gas Species in the Crab Nebula Filament
• Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI
• Synthetic CO emission and the $X_{\mathrm{CO}}$ factor of young molecular clouds: a convergence study
• Molecular dark matter in galaxies
• Metallicity dependence of the CO-to-H${}_2$ and the [CI]-to-H${}_2$ conversion factors in galaxies
• A PDR model for the FIR mid-$J$ CO ladder with universal rotational temperature in star forming region
• Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts
• Dense and diffuse gas in dynamically active clouds
• Herschel 158$\mu$m [CII] Observations of "CO-dark" Gas in the Perseus Giant Molecular Cloud
• A high-resolution extinction mapping technique for face-on disc galaxies
• Problems of Star Formation Theory and Prospects of Submillimeter Observations
• Star formation in the circumgalactic high-velocity cloud Complex H
• Cosmic rays, gas and dust in the Central Molecular Zone I -- $X_{CO}$ factors, cosmic-ray densities and dust opacities