[CII] observations of H${}_2$ molecular layers in transition clouds

Abstract

We present the first results on the diffuse transition clouds observed in [CII] line emission at 158 microns (1.9 THz) towards Galactic longitudes near 340deg (5 LOSs) and 20deg (11 LOSs) as part of the GOT C+ survey. Out of the total 146 [CII] velocity components detected by profile fitting we identify 53 as diffuse molecular clouds with associated ${}^{12}$CO emission but without ${}^{13}$CO emission and characterized by A${}_V$ < 5 mag. We estimate the fraction of the [CII] emission in the diffuse HI layer in each cloud and then determine the [CII] emitted from the molecular layers in the cloud. We show that the excess [CII] intensities detected in a few clouds is indicative of a thick H${}_2$ layer around the CO core. The wide range of clouds in our sample with thin to thick H${}_2$ layers suggests that these are at various evolutionary states characterized by the formation of H${}_2$ and CO layers from HI and C${}^{+}$, respectively. In about 30% of the clouds the H${}_2$ column densities (''dark gas'') traced by the [CII] is 50% or more than that traced by ${}^{12}$CO emission. On the average about 25% of the total H${}_2$ in these clouds is in an H${}_2$ layer which is not traced by CO. We use the HI, [CII], and ${}^{12}$CO intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates.

To be published in HIFI A&A special edition

References in corpus (7)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The VLA Galactic Plane Survey
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• C${}^{+}$ detection of warm dark gas in diffuse clouds
• A clumpy-cloud PDR model of the global far-infrared line emission of the Milky Way
• On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
• A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields observed by Herschel

Cited by in corpus (32)

• The CO-to-H2 Conversion Factor
• 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
• Very extended cold gas, star formation and outflows in the halo of a bright QSO at z>6
• Collisional Excitation of the [CII] Fine Structure Transition in Interstellar Clouds
• A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
• C${}^{+}$ detection of warm dark gas in diffuse clouds
• How long does it take to form a molecular cloud?
• UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code
• Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions
• Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales
• Radiative transfer meets Bayesian statistics: where does a galaxy's [CII] emission come from?
• Origin and z-distribution of Galactic diffuse [CII] emission
• [CII] 158 micron line detection of the warm ionized medium in the Scutum--Crux spiral arm tangency
• A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields observed by Herschel
• Carbon gas in SMC low-metallicity star-forming regions
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
• Kinematics and properties of the Central Molecular Zone as probed with [C II]
• Extended Carbon Line Emission in the Galaxy: Searching for Dark Molecular Gas along the G328 Sightline
• Submillimeter Line Emission from LMC 30Dor: The Impact of a Starburst on a Low Metallicity Environment
• The Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of "CO-Dark" Gas in the Perseus Arm
• Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC
• HI-H${}_2$ transition: exploring the role of the magnetic field
• The first results from the Herschel-HIFI mission
• $\text{Herschel}$ Photometric Observations of $\mathrm{L}\mathrm{ITTLE}$ $\mathrm{T}\mathrm{HINGS}$ Dwarf Galaxies
• Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA
• Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission and FUSE UV H2 Absorption
• High Latitude, Translucent Molecular Clouds as Probes of Local Cosmic Rays
• The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics
• Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship
• Observations of ionized carbon towards the $\gamma$-ray-bright supernova remnant RX J1713.7-3946
• CO Observations of the SMC-N66 Hii Region with ALMA: Properties of Clumps along Filamentary Molecular Clouds and Possible Expansion Motion