A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components (1304.7770v3)

Abstract: [Abridged] The [CII] 158um 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, including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). The Galactic Observations of Terahertz C+ (GOTC+) project surveys the [CII] line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [CII] emission for Galactic latitudes b=0deg, +-0.5deg, and +-1.0deg. [CII] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10 kpc. We estimate that most of the observed [CII] emission is produced by dense PDRs (47%), with smaller contributions from CO-dark 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 (WNM) 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 (4-11 kpc) than the cold and dense H2 gas traced by 12CO and 13CO (4-8kpc). The fraction of CO-dark H2 to total H2 increases with Galactocentric distance, ranging from 20% at 4 kpc to 80% at 10 kpc. 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 [CII] emission emerging from dense PDRs is associated with modest far-ultraviolet fields in the range chi0~1-30.

• The paper reveals that dense PDRs contribute 47% of [CII] emission, with CO-dark H2 and atomic gas accounting for 28% and 21% respectively across the Galactic plane.
• It utilizes Herschel/HIFI velocity-resolved observations to clearly distinguish multiple ISM phases and validate CNM and WNM estimates via H I 21 cm data.
• The findings prompt a revision of the CO-to-H2 conversion factor by demonstrating a steep radial gradient in CO-dark H2, advancing models of Galactic evolution.

Analysis of the Herschel [C\,II] Galactic Plane Survey: Insights into ISM Gas Components

The research conducted by Pineda et al., using the Herschel [C\,II] Galactic Plane Survey, significantly advances our understanding of the interstellar medium (ISM) by providing detailed observations of the ionized carbon line [C\,II] at 158 μm. This line is crucial for delineating the lifecycle stages of interstellar matter and offers insights into diverse ISM phases: from diffuse ionized and atomic regions to transitional and photon-dominated regions (PDRs).

The paper presents an extensive survey conducted with the Herschel/HIFI instrument, which resolves the velocity of the [C\,II] emission across the Galactic disk. This permits a separation of the various ISM components and facilitates a comprehensive paper of their distribution throughout the Galactic plane. The findings reveal that the [C\,II] emission predominantly associates with spiral arms, most notably within Galactocentric distances of 4 to 10 kpc. The ability to map these emissions with a high degree of resolution underscores the considerable variability and complexity within these regions.

Pineda et al. quantify the contributions of different ISM components to the observed [C\,II] emission. Dense PDRs account for approximately 47% of the emission, while CO-dark H2 gas comprises around 28%, cold atomic gas 21%, and ionized gas a mere 4%. These fractions are not only indicative of the respective roles each ISM component plays but also illuminate the significance of [C\,II] as a diagnostic tool for molecular clouds, particularly those that are CO-dark due to inadequate FUV shielding for CO formation.

Addressing the H\,I (atomic) gas component, the authors employ the H\,I 21 cm line to discern the cold neutral medium (CNM) and warm neutral medium (WNM). The CNM is shown to dominate within the Solar radius, while the WNM prevails in the outer Galaxy. Notably, the paper estimates that the CNM constitutes about 43% of the total atomic gas, aligning well with local ISM observations.

A pivotal outcome of the survey is the identification and quantification of CO-dark H2 gas, which, extending across a larger Galactocentric range than its cold, dense counterparts, constitutes roughly 30% of the Milky Way’s molecular mass. This CO-dark H2 fraction grows from 20% at 4 kpc to 80% at 10 kpc. This gradient necessitates a revision of the traditional CO-to-H2 conversion factor, especially when the CO-dark H2 component is accounted for, yielding a steeper radial gradient.

Further, the paper argues that most [C\,II] emission from dense PDRs is linked to modest far-UV field intensities (χ_0 ≈ 1-30). This observation is in line with earlier models predicting that the Galactic [C\,II] emission typically originates from PDRs with moderate FUV fields.

The paper's implication for future ISM studies is profound, offering groundwork for refining our models of galaxy evolution and star formation. Additionally, the Herschel [C\,II] data can serve as a baseline for interpreting similar observations in other galaxies, particularly at high redshifts, where understanding the lifecycle of ISM gas components is crucial.

In conclusion, this paper amplifies our comprehension of Galactic structure and ISM dynamics through its rigorous analysis of [C\,II] emissions, illustrating both the diverse phenotypes of interstellar gas and the critical transitional phases of molecular gas development, thereby enriching the field’s observational foundations and interpretive models. Future explorations building on this survey may well focus on refining the detail and scope of such surveys, prompting new revelations regarding the ISM’s role in galactic ecosystems.