C+/H2 Gas in Star-Forming Clouds and Galaxies

Abstract

We present analytic theory for the total column density of singly ionized carbon (C+) in the optically thick photon dominated regions (PDRs) of far-UV irradiated (star-forming) molecular clouds. We derive a simple formula for the C+ column as a function of the cloud (hydrogen) density, the far-UV field intensity, and metallicity, encompassing the wide range of galaxy conditions. When assuming the typical relation between UV and density in the cold neutral medium, the C+ column becomes a function of the metallicity alone. We verify our analysis with detailed numerical PDR models. For optically thick gas, most of the C+ column is mixed with hydrogen that is primarily molecular (H2), and this "C+/H2" gas layer accounts for almost all of the `CO-dark' molecular gas in PDRs. The C+/H2 column density is limited by dust shielding and is inversely proportional to the metallicity down to ~0.1 solar. At lower metallicities, H2 line blocking dominates and the C+/H2 column saturates. Applying our theory to CO surveys in low redshift spirals we estimate the fraction of C+/H2 gas out of the total molecular gas to be typically ~0.4. At redshifts 1<z<3 in massive disc galaxies the C+/H2 gas represents a very small fraction of the total molecular gas (<0.16). This small fraction at high redshifts is due to the high gas surface densities when compared to local galaxies.

15 pages, 14 figures. Accepted to MNRAS. v3 - proofs and minor text modifications

References in corpus (11)

• The Mass-Metallicity Relation at z~2
• COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies: I. Relations between H2, HI, stellar content and structural properties
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Photoprocesses in protoplanetary disks
• Radiative recombination data for modelling dynamic finite-density plasmas
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Chemical Rates on Small Grains and PAHs: C^+ Recombination and H_2 Formation
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• Analytic HI-to-H2 Photodissociation Transition Profiles
• The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18

Cited by in corpus (14)

• The Evolution of the Star-forming Interstellar Medium across Cosmic Time
• The [C II] emission as a molecular gas mass tracer in galaxies at low and high redshift
• Photodissociation and X-Ray Dominated Regions
• Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
• The ALPINE-ALMA [CII] survey. Little to no evolution in the [CII]-SFR relation over the last 13 Gyr
• Dynamical properties of z $\sim 4.5$ dusty star-forming galaxies and their connection with local early type galaxies
• SHINING, A Survey of Far Infrared Lines in Nearby Galaxies. I: Survey Description, Observational Trends, and Line Diagnostics
• 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
• Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• Probing the baryon cycle of galaxies with SPICA mid- and far-infrared observations
• ColdSIM predictions of [C II] emission in primordial galaxies
• Disentangling emission from star-forming regions in the Magellanic Clouds: Linking [OIII]88 micron and 24 micron
• Estimating the baryonic masses of face-on spiral galaxies from stellar kinematics