The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Abstract

Aims: We wish to separate and quantify the CO luminosity and CO-H2 conversion factor applicable to diffuse but partially-molecular ISM when H2 and CO are present but C+ is the dominant form of gas-phase carbon. Methods: We discuss galactic lines of sight observed in \HI, HCO+ and CO where CO emission is present but the intervening clouds are diffuse (locally \AV\ $\la 1$ mag) with relatively small CO column densities $\NCO \la 2\times10^{16}\pcc$. We separate the atomic and molecular fractions statistically using \EBV\ as a gauge of the total gas column density and compare NH2 to the observed CO brightness. Results: Although there are H2-bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightness to NH2 for diffuse ISM does not differ from the usual value of 1\K km/s of integrated CO brightness per $2\times 10^{20}$ H2 $\pcc$ . Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius in surveys of CO emission near the galactic plane. Conclusions: Commonality of the CO-H2 conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and H2 that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas.

Accepted to A&A 8 May 2010

References in corpus (10)

• Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
• On the Interstellar Medium and Star Formation Demographics of Galaxies in the Local Universe
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
• The relation between column densities of interstellar OH and CH molecules
• Time-dependent H2 formation and protonation
• Imaging galactic diffuse gas: Bright, turbulent CO surrounding the line of sight to NRAO150

Cited by in corpus (95)

• Star Formation in the Milky Way and Nearby Galaxies
• The CO-to-H2 Conversion Factor
• Molecular Gas and Star Formation in Nearby Disk Galaxies
• 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 survey of molecular gas in luminous sub-millimetre galaxies
• Physical properties of molecular clouds for the entire Milky Way disk
• Turbulent molecular clouds
• The Dawes Review 9: The role of cold gas stripping on the star formation quenching of satellite galaxies
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• Cold gas properties of the Herschel Reference Survey. II. Molecular and total gas scaling relations
• The Plateau de Bure + 30m Arcsecond Whirlpool Survey reveals a thick disk of diffuse molecular gas in the M51 galaxy
• The EDGE-CALIFA Survey: Interferometric Observations of 126 Galaxies with CARMA
• The CO-H2 Conversion Factor in Disc Galaxies and Mergers
• Modeling CO Emission: II. The Physical Characteristics that Determine the X factor in Galactic Molecular Clouds
• Cold gas properties of the Herschel Reference Survey. I. 12CO(1-0) and HI data
• A high-dispersion molecular gas component in nearby galaxies
• Molecular Gas Dominated 50 kpc Ram Pressure Stripped Tail of the Coma Galaxy D100
• Chemical probes of turbulence in the diffuse medium: the TDR model
• Short GMC lifetimes: an observational estimate with the PdBI Arcsecond Whirlpool Survey (PAWS)
• Distribution and mass of diffuse and dense CO gas in the Milky Way
• Probability Distribution Functions OF 12CO(J = 1-0) Brightness and Integrated Intensity in M51: The PAWS View
• Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data
• N(H I)/E(B-V)
• Cold and Warm Atomic Gas around the Perseus Molecular Cloud. II. The Impact of High Optical Depth on the HI Column Density Distribution and Its Implication for the HI-to-H2 Transition
• Full-disc ${}^{13}$CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H${}_2$ conversion factor
• Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations
• Cosmic rays, gas and dust in nearby anticentre clouds : I -- CO-to-H2 conversion factors and dust opacities
• E(B-V), N(H I) and N(H_2)
• Spitzer IRS Detection of Molecular Hydrogen Rotational Emission Towards Translucent Clouds
• Imaging diffuse clouds: Bright and dark gas mapped in CO
• The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium
• The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• Indications of a sub-linear and non-universal Kennicutt-Schmidt relationship
• How well does CO emission measure the H${}_2$ mass of MCs?
• Formaldehyde Densitometry of Starburst Galaxies: Density-Independent Global Star Formation
• A diversity of starburst-triggering mechanisms in interacting galaxies and their signatures in CO emission
• Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas
• Optically thick HI does not dominate dark gas in the local ISM
• Dust-to-gas ratio, $X_{\mathrm{CO}}$ factor and CO-dark gas in the Galactic anticentre: an observational study
• Planck intermediate results. XXVIII. Interstellar gas and dust in the Chamaeleon clouds as seen by Fermi LAT and Planck
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Chemical complexity in local diffuse and translucent clouds: ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN
• Physical properties of CO-dark molecular gas traced by C${}^{+}$
• Probing The Local Environment of the Supernova Remnant HESS J1731-347 with CO and CS Observations
• A Massive Molecular Torus inside a Gas-Poor Cirnumnuclear Disk in the Radio Galaxy NGC 1052 Discovered with ALMA
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• CO and H2 Absorption in the AA Tauri Circumstellar Disk
• Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations
• Residual Cooling and Persistent Star Formation amid AGN Feedback in Abell 2597
• Synthetic observations of molecular clouds in a galactic center environment: I. Studying maps of column density and integrated intensity
• Small-scale physical and chemical structure of diffuse and translucent molecular clouds along the line of sight to Sgr B2
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. I. Observational constraints from ALMA and NOEMA
• Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon
• CO-to-H${}_2$ Conversion and Spectral Column Density in Molecular Clouds: The Vriability of $X_{\mathrm{CO}}$ Factor
• Standing in the shadow of dark gas: ALMA observations of absorption from dark CO in the molecular DNM of Chamaeleon
• Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy
• Widespread Galactic CF+ absorption: detection toward W49 with the Plateau de Bure Interferometer
• A High-Resolution Study of the CO-H2 Conversion Factor in the Diffuse Cloud MBM 40
• Interstellar gas towards CTB 37A and the TeV gamma-ray source HESS J1714-385
• Formation and Fractionation of CO (carbon monoxide) in diffuse clouds observed at optical and radio wavelengths
• Molecular hydrogen and its proxies HCO${}^{+}$ and CO in the diffuse interstellar medium
• Revealing The CO X-factor In Dark Molecular Gas through Sensitive ALMA Absorption Observations
• The Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of "CO-Dark" Gas in the Perseus Arm
• Rotational Excitation of polar molecules by H2 and electrons in diffuse clouds
• CARMA Survey toward Infrared-bright Nearby Galaxies (STING). IV. Spatially Resolved 13CO in Spiral Galaxies
• DeGaS-MC: Dense Gas Survey in the Magellanic Clouds I -- An APEX survey of HCO+ and HCN(2-1) toward the LMC and SMC
• Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC
• Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38
• Role of Electon Excitation and Nature of Molecular Gas in Cluster Central Elliptical Galaxies
• High-resolution HI and CO observations of high-latitude intermediate-velocity clouds
• CO+ as a probe of the origin of CO in diffuse interstellar clouds
• Theoretical modelling of two-component molecular discs in spiral galaxies
• The SCUBA-2 Web Survey: I. Observations of CO(3-2) in hyper-luminous QSO fields
• A broad HCO+ absorption line associated with the circumnuclear torus of NGC 1052
• First Detection of HCO${}^{+}$ Absorption in the Magellanic System
• Sensitive CO(1-0) Survey in Pegasus-Pisces Reduces CO-Dark Gas Inventory by Factor of Two
• CO Multi-line Imaging of Nearby Galaxies (COMING). X. Physical conditions of molecular gas and the local SFR-Mass relation
• On estimating the atomic hydrogen column density from the H I 21 cm emission spectra
• The extragalactic $\gamma$-ray background: imprints from the physical properties and evolution of star-forming galaxy populations
• Reconciling X-ray and 21cm HI absorption gas column densities toward obscured AGN
• CO emission and CO hotspots in diffuse molecular gas
• Atomic and Molecular Gas Components in Spiral Galaxies of the Virgo Cluster
• The Interstellar Medium: The Key Component in Galactic Evolution and Modern Cosmology
• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules II. The absorption line survey with the IRAM 30 m telescope
• H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium
• Shaken or stirred: the diffuse interstellar medium with exceptionally high SiO abundance
• Simultaneously modelling far-infrared dust emission and its relation to CO emission in star forming galaxies
• Properties of atomic hydrogen gas in the Galactic plane from THOR 21-cm absorption spectra: a comparison with the high latitude gas
• Optical Reddening, Integrated HI Optical Depth, Total Hydrogen Column Density
• The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also traces diffuse gas
• Galactic HCO+ Absorption toward Compact Extragalactic Radio Sources
• Physical conditions in Centaurus A's northern filaments II: Does the HCO${}^{+}$ emission highlight the presence of shocks?
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. II. Comparison between observations and theoretical models