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

Abstract

We report a Fermi-LAT $\gamma$-ray analysis for the Chamaeleon molecular-cloud complex using a total column density (NH) model based on the dust optical depth at 353 GHz (${\tau }_{353}$) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the NH model as a function of ${\tau }_{353}$, NH $\propto$ ${\tau }_{353}^{1/\alpha }$ ($\alpha$ $\ge$ 1.0), to explicitly take into account a possible nonlinear ${\tau }_{353}$/NH ratio. We found that a nonlinear relation, $\alpha$$\sim$1.4, gives the best fit to the $\gamma$-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit NH model, we derived the CO-to-H2 conversion factor (XCO) and gas mass, taking into account uncertainties of the NH model. The value of XCO is found to be (0.63-0.76) $\times$10${}^{20}$ cm${}^{-2}$ K${}^{-1}$ km${}^{-1}$ s, which is consistent with that of a recent $\gamma$-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) $\times$ 10${}^4$ Msun, of which the mass of additional gas not traced by standard HI or CO line surveys is 20-40%. The additional gas amounts to 30-60% of the gas mass estimated in the case of optically thin HI and has 5-7 times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick HI and CO-dark H2. We also derived the $\gamma$-ray emissivity spectrum, which is consistent with the local HI emissivity derived from LAT data within the systematic uncertainty of $\sim$20%

arXiv comment32 pages, 22 figures, accepted for ApJ

References in corpus (35)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• HEALPix -- a Framework for High Resolution Discretization, and Fast Analysis of Data Distributed on the Sphere
• The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Cosmic-ray propagation and interactions in the Galaxy
• Propagation of cosmic-ray nucleons in the Galaxy
• Planck 2013 results. XI. All-sky model of thermal dust emission
• The Dark Molecular Gas
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• The Fermi Large Area Telescope On Orbit: Event Classification, Instrument Response Functions, and Calibration
• Planck Early Results: All sky temperature and dust optical depth from Planck and IRAS: Constraints on the "dark gas" in our galaxy
• A New $\Sigma -D$ Relation and Its Application to the Galactic Supernova Remnant Distribution
• Inverse Compton Origin of the Hard X-Ray and Soft Gamma-Ray Emission from the Galactic Ridge
• Planck 2013 results. XIII. Galactic CO emission
• CO-dark gas and molecular filaments in Milky Way type galaxies
• Modeling CO Emission: I. CO as a Column Density Tracer and the X-Factor in Molecular Clouds
• Planck intermediate results. XVII. Emission of dust in the diffuse interstellar medium from the far-infrared to microwave frequencies
• Formation of Turbulent and Magnetized Molecular Clouds via Accretion Flows of HI Clouds
• GASS: The Parkes Galactic All-Sky Survey. Update: improved correction for instrumental effects and new data release
• Changes Of Dust Opacity With Density in the Orion A Molecular Cloud
• Nuclear enhancement factor in calculation of Galactic diffuse gamma-rays: A new estimate with DPMJET-3
• Evidence for Environmental Changes in the Submillimeter Dust Opacity
• Constraints on the Cosmic-Ray Density Gradient beyond the Solar Circle from Fermi gamma-ray Observations of the Third Galactic Quadrant
• 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
• Fermi LAT study of cosmic-rays and the interstellar medium in nearby molecular clouds
• HI, CO, and Planck/IRAS dust properties in the high-latitude-cloud complex, MBM 53, 54, 55 and HLCG 92-35; Possible evidence for an optically thick HI envelope around the CO clouds
• Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
• Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
• Optically thick HI does not dominate dark gas in the local ISM
• HI, CO, and Dust in the Perseus Cloud
• EGRET Observations of the Diffuse Gamma-Ray Emission in Orion: Analysis Through Cycle 6
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• Synthetic Observations of 21cm HI Line Profiles from Inhomogeneous Turbulent Interstellar HI Gas with Magnetic Field
• 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 (12)

• Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
• SOFIA/FIFI-LS Full-disk [CII] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions
• CO-to-H${}_2$ Conversion and Spectral Column Density in Molecular Clouds: The Vriability of $X_{\mathrm{CO}}$ Factor
• The interstellar medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma$-rays
• An Expanding Shell of Neutral Hydrogen Associated with SN 1006: Hints for the Single-Degenerate Origin and Faint Hadronic Gamma-Rays
• Element Stratification in the Middle-Aged Type Ia Supernova Remnant G344.7-0.1
• Census of High- and Medium-mass Protostars V. CO Abundance and the Galactic $X_{\text{CO}}$ Factor
• Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $\gamma$-Ray Sky
• 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
• Modelling of dust emission of a filament in the Taurus molecular cloud
• Associated molecular and atomic clouds with X-ray shell of superbubble 30 Doradus C in the LMC