A Herschel [C ii] Galactic plane survey - I. The global distribution of ISM gas components

By J. Pineda, W. Langer, Thirukumaran Velusamy, Paul F. Goldsmith. 2013

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Context. The [C ii] 158 μm line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions. Aims. Velocity-resolved observations of [C ii] are the most powerful technique available to disentangle the emission produced by these components. These observations can also be used to trace CO-dark H2 gas and determine the total mass of the ISM. Methods. The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [C ii] 158 μm line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [C ii] emission for Galactic latitudes b = 0 ◦ , ±0.5 ◦ ,a nd±1.0 ◦ . We combine these maps with those of H i, 12 CO, and 13 CO to separate the different phases of the ISM and study their properties and distribution in the Galactic plane. Results. [C ii] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10kpc. It traces the envelopes of evolved clouds as well as clouds that are in the transition between atomic and molecular. We estimate that most of the observed [C ii] emission is produced by dense photon dominated regions (∼47%), with smaller contributions from COdark H2 gas (∼28%), cold atomic gas (∼21%), and ionized gas (∼4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is ∼43%. We find that the warm and diffuse CO-dark H2 is distributed over a larger range of Galactocentric distances (4−11kpc) than the cold and dense H2 gas traced by 12 CO and 13 CO (4−8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from ∼20% at 4kpc to ∼80% at 10kpc. On average, CO-dark H2 accounts for ∼30% of the molecular mass of the Milky Way. When the CO-dark H2 component is included, the radial distribution of the CO-to-H2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [C ii] emission emerging from dense photon dominated regions is associated with modest far-ultraviolet fields in the range χ0 � 1−30.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/1328e8bafbccbaa6d763b8d135938b01ef6beb0b

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Citations in the corpus

The ALPINE-ALMA [CII] survey (2020, background): “There is also evidence that [CII] is often emitted from the diffuse CNM or HII regions in addition to PDRs (Herrera-Camus et al. 2017; Pineda et al. 2013; Vallini et al. 2015), and it also traces the difuse ionized gas; see (Pavesi et al. 2016, 2019).”
A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies (2017, methodology): “Observations at significantly higher spectral resolution than our PACS data, such as those obtained for the Milky-Way with Herschel/HIFI, would be needed in order to correct for this effect (Pineda et al. 2013; Gerin et al. 2015; Langer et al. 2016).”
Physical Processes in the Interstellar Medium (2014, methodology): “…gas with Galactocentric radius can be measured by combining data from CO observations, which trace clouds with high concentrations of both H2 and CO, and C+ observations, which trace so-called “dark molecular gas”, i.e. clouds with high H2 fractions but little CO (see e.g. Pineda et al., 2013).”
DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA (2016, background): “The DNM gas at the H I–H2 interface, however, is more abundant than CO-bright H2 beyond the solar circle (Abergel et al. 2011; Pineda et al. 2013) and it should better correlate with H I spatially.”
THREE-DIMENSIONAL HYDRODYNAMIC SIMULATIONS OF MULTIPHASE GALACTIC DISKS WITH STAR FORMATION FEEDBACK. I. REGULATION OF STAR FORMATION RATES (2013, result): “…of Paper I that both warm and cold atomic gas are expected to be present in the ISM for a wide range of conditions, consistent with observations of both the solar-neighborhood (Heiles & Troland 2003) and distant outer disk regions (Dickey et al. 2009; Pineda et al. 2013) of the Milky Way.”
The Herschel Dwarf Galaxy Survey - I. Properties of the low-metallicity ISM from PACS spectroscopy (2015, background): “The [C ii] 157 µm line has been studied in a variety of objects, such as Galactic PDRs (e.g., Bennett et al. 1994; Pineda et al. 2013), normal spiral galaxies (Stacey et al. 1991; Malhotra et al. 1997; Negishi et al. 2001; Brauher et al. 2008), lowmetallicity galaxies (Poglitsch et al. 1995; Madden…”
The Nine Lives of Cosmic Rays in Galaxies (2015, methodology): “The abundance of missing gas exceeds that of the CO-bright H2 beyond the solar circle (Pineda et al. 2013, Planck Collaboration et al. 2011a).”
HERSCHEL SURVEY OF GALACTIC OH+, H2O+, AND H3O+: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE (2014, background): “Metallicities tend to increase at smaller Galactocentric radii (Rolleston et al. 2000), and a typical assumed gradient in carbon abundance results in a factor of 2–3 increase in x(C) at Rgal = 3 kpc (e.g., Wolfire et al. 2003; Pineda et al. 2013; Langer et al. 2014).”
Very extended cold gas, star formation and outflows in the halo of a bright QSO at z>6 (2014, background): “This behaviour of [C ii] has been observed in low metallicity systems such as Haro 11 and in the outskirts of the Milky Way and other local galaxies (Kramer et al. 2013; Parkin et al. 2013; Pineda et al. 2013; Langer et al. 2014).”
CO-dark gas and molecular filaments in Milky Way-type galaxies (2014, methodology): “Averaged over their whole survey, Pineda et al. (2013) find a dark gas fraction fDG 0.30.”