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10/02/14 SO/NOAO Joint Colloquium Series: Paul Goldsmith, JPL

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Paul Goldsmith, Jet Propulsion Laboratory, California Institute of Technology

Tracing the Interstellar Medium and Star Formation with [CII] and CO

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 has surveyed the [CII] line over the entire Galactic disk with velocityresolved observations using the Herschel HIFI instrument. We present the first longitudevelocity maps of the [CII] emission. [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 in the disk is produced by dense PDRs (47%), with smaller contributions from CO-dark H2 gas (28%), cold atomic gas (21%), and ionized gas (4%). 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 relative 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. We discuss recent calculations of the excitation rates of the [CII] line by collisions with H, and use those with a radiative transfer analysis to derive simple expressions for analyzing [CII] emission. Recent Herschel measurements of [CII] and [NII] absorption yield valuable information about densities, and also suggest that spectrally unresolved observations towards strong continuum sources, can drastically underestimate the [CII] emission, of interest to both Galactic and extragalactic astronomers in the context of the “[CII] Deficit”.

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