[Paper Review] A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components

J. L. Pineda, W. D. Langer+2 more|arXiv (Cornell University)|Apr 29, 2013

Astrophysics and Star Formation StudiesPhysics and Astronomy78 references140 citations

TL;DR

This study presents the first velocity-resolved longitude-velocity maps of [C II] 158 μm emission across the Galactic plane using Herschel/HIFI, enabling separation of interstellar medium (ISM) components. It reveals that 47% of [C II] emission arises from dense photon-dominated regions (PDRs), with CO-dark H₂ contributing 30% of the Milky Way's molecular mass and increasing in dominance with Galactocentric distance.

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.

Motivation & Objective

• To map the global distribution of interstellar gas components using velocity-resolved [C II] 158 μm emission across the Galactic plane.
• To disentangle contributions from different ISM phases—ionized gas, cold/warm atomic gas, CO-dark H₂, and molecular gas—along the line of sight.
• To quantify the radial distribution of CO-dark H₂ and its impact on the CO-to-H₂ conversion factor.
• To estimate the far-UV radiation field strength (χ₀) in regions traced by [C II] and CO using observed line ratios.
• To improve the accuracy of molecular gas mass estimates by including CO-dark H₂ in the total H₂ budget.

Proposed method

• Acquired velocity-resolved [C II] 158 μm emission maps across Galactic latitudes b = 0°, ±0.5°, and ±1.0° using the Herschel/HIFI instrument.
• Combined [C II] data with H i 21 cm, 12 CO, and 13 CO line data to separate ISM components via velocity and spatial decomposition.
• Used the [C II]/12 CO intensity ratio to estimate the far-UV radiation field (χ₀) in dense PDRs, comparing with PDR model predictions.
• Applied a multi-component decomposition to isolate contributions from cold neutral medium (CNM), warm neutral medium (WNM), and CO-dark H₂.
• Calculated the CO-to-H₂ conversion factor (X_CO) both with and without inclusion of CO-dark H₂ to assess its radial dependence.
• Used PDR models (Kaufman et al. 1999) to interpret observed [C II]/12 CO ratios in terms of H₂ volume density and χ₀.

Experimental results

Research questions

• RQ1 What is the spatial and velocity distribution of [C II] emission across the Galactic plane, and how does it trace different ISM components?
• RQ2 What fraction of the total [C II] emission originates from dense PDRs, CO-dark H₂, cold atomic gas, and ionized gas?
• RQ3 How does the distribution of CO-dark H₂ vary with Galactocentric distance, and what fraction of the Milky Way’s molecular mass does it represent?
• RQ4 How does the inclusion of CO-dark H₂ affect the radial trend of the CO-to-H₂ conversion factor?
• RQ5 What is the typical far-UV radiation field (χ₀) associated with [C II] emission in dense PDRs?

Key findings

• Most [C II] emission (47%) originates from dense photon-dominated regions (PDRs), with significant contributions from CO-dark H₂ (28%), cold atomic gas (21%), and ionized gas (4%).
• CO-dark H₂ accounts for approximately 30% of the total molecular gas mass in the Milky Way, increasing from ~20% at 4 kpc to ~80% at 10 kpc in Galactocentric distance.
• The warm and diffuse CO-dark H₂ extends over a larger radial range (4–11 kpc) than cold, dense H₂ traced by 12 CO and 13 CO (4–8 kpc).
• The CO-to-H₂ conversion factor increases with Galactocentric distance, and the slope becomes steeper when CO-dark H₂ is included, reflecting combined effects of metallicity and thermal pressure gradients.
• The [C II]/12 CO ratio suggests that most [C II] emission from dense PDRs arises under modest far-UV fields, with χ₀ ≈ 1–30, independent of Galactocentric distance.
• Inside the Solar radius (R < 8 kpc), cold neutral medium (CNM) dominates atomic gas (43% of total atomic gas), while warm neutral medium (WNM) dominates in the outer Galaxy.