Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium

Abstract

H${}_{\mathrm{I}}$ 21-cm and ${}^{12}$CO 2.6-mm line emissions respectively trace the atomic and molecular gas phases, but they miss most of the opaque H${}_{\mathrm{I}}$ and diffuse H${}_2$ present in the Dark Neutral Medium (DNM) at the transition between the H${}_{\mathrm{I}}$-bright and CO-bright regions. Jointly probing H${}_{\mathrm{I}}$, CO, and DNM gas, we aim to constrain the threshold of the H${}_{\mathrm{I}}$-H${}_2$ transition. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties. We have used dust optical depth measurements at 353 GHz, $\gamma$-ray maps at GeV energies, and H${}_{\mathrm{I}}$ and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6$\times 10^{21}$ cm${}^{-2}$ and 2.5$\times 10^{21}$ cm${}^{-2}$ in total gas column density. We observe larger H${}_2$ column densities than linearly inferred from the CO intensities at $A_{\mathrm{V}}>3$ magnitudes because of the large CO optical thickness: the additional H${}_2$ mass in this regime represents on average 20$\%$ of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H${}_2$ in the molecular column densities decreases with increasing $A_{\mathrm{V}}$ in a cloud. For a half molecular DNM, the fraction decreases from more than 80$\%$ at 0.4 magnitude to less than 20$\%$ beyond 2 magnitudes. In mass, the DNM fraction varies with the cloud properties. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as $M_{\mathrm{H}}^{\mathrm{DNM}}=(62\pm 7){M_{{\mathrm{H}}_2}^{\mathrm{CO}}}^{0.51\pm 0.02}$ across two decades in mass. The phase transitions in these clouds show both common trends and environmental differences.

References in corpus (55)

• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• The Milky Way in Molecular Clouds: A New Complete CO Survey
• The Neutral Atomic Phases of the ISM in the Galaxy
• Planck 2013 results. XI. All-sky model of thermal dust emission
• On the Star Formation Rates in Molecular Clouds
• The Dark Molecular Gas
• The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks
• A Three-Dimensional Map of Milky-Way Dust
• 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
• A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• The COMPLETE Survey of Star-Forming Regions: Phase I Data
• The relation between gas and dust in the Taurus Molecular Cloud
• On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
• Planck Early Results: Thermal dust in Nearby Molecular Clouds
• Planck 2013 results. XIII. Galactic CO emission
• The GALFA-HI Survey: Data Release 1
• CO-dark gas and molecular filaments in Milky Way type galaxies
• GASS: The Parkes Galactic All-Sky Survey. II. Stray-Radiation Correction and Second Data Release
• The Effelsberg-Bonn HI Survey: Milky Way gas. First data release
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey
• A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
• GASS: The Parkes Galactic All-Sky Survey. Update: improved correction for instrumental effects and new data release
• A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGN
• The Spin Temperature of Warm Interstellar H I
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?
• The Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus Molecular Cloud
• Larson's third law and the universality of molecular cloud structure
• Evolution of OH and CO-dark Molecular Gas Fraction Across a Molecular Cloud Boundary In Taurus
• Testing the universality of the star formation efficiency in dense molecular gas
• Evidence for Environmental Changes in the Submillimeter Dust Opacity
• Variation in dust properties in a dense filament of the Taurus molecular complex (L1506)
• The Structure of Molecular Clouds: I - All Sky Near Infrared Extinction Maps
• Dark gas in the solar neighnorhood from extinction data
• Chemical probes of turbulence in the diffuse medium: the TDR model
• A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• Cosmic rays, gas and dust in nearby anticentre clouds : I -- CO-to-H2 conversion factors and dust opacities
• H2 distribution during formation of multiphase molecular clouds
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• E(B-V), N(H I) and N(H_2)
• Imaging diffuse clouds: Bright and dark gas mapped in CO
• UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code
• [CII] observations of H${}_2$ molecular layers in transition clouds
• The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l\approx 105^{°},b\approx +1^{°}$
• HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
• 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
• Allsky NICER and NICEST extinction maps based on the 2MASS near-infrared survey
• Synthetic observations of molecular clouds in a galactic center environment: I. Studying maps of column density and integrated intensity
• A dynamical transition from atomic to molecular intermediate-velocity clouds
• Quantifying the Interstellar Medium and Cosmic Rays in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop using Fermi-LAT Gamma-ray Observations

Cited by in corpus (18)

• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Standing in the shadow of dark gas: ALMA observations of absorption from dark CO in the molecular DNM of Chamaeleon
• Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon
• Quantitative inference of the $H_2$ column densities from 3 mm molecular emission: A case study towards Orion B
• Cosmic-rays, gas, and dust in nearby anti-centre clouds : III -- Dust extinction, emission, and grain properties
• Molecular hydrogen and its proxies HCO${}^{+}$ and CO in the diffuse interstellar medium
• Fermi-LAT gamma-ray study of the interstellar medium and cosmic rays in the Chamaeleon Molecular-Cloud Complex: A look at the dark gas as optically thick HI
• SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data Description & Release
• The hydroxyl satellite-line `flip' as a tracer of expanding HII regions
• Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI
• Evidence for large-scale excesses associated with low HI column densities in the sky I. Dust excess
• The Interstellar Medium: The Key Component in Galactic Evolution and Modern Cosmology
• The Dark Neutral Medium is (Mostly) Molecular Hydrogen
• AMOEBA: Automated Molecular Excitation Bayesian Line-Fitting Algorithm
• Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies
• Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $\gamma$-Ray Sky
• Probing local cosmic rays using Fermi-LAT observation of a mid-latitude region in the third Galactic quadrant
• Gas and Cosmic-Ray Properties in the MBM 53, 54, and 55 Molecular Clouds and the Pegasus Loop as Revealed by HI Line Profiles, Dust, and Gamma-Ray Data