The Origins of [C ii] Emission in Local Star-forming Galaxies

Abstract

The [C II] 158 μm fine-structure line is the brightest emission line observed in local star-forming galaxies. As a major coolant of the gas-phase interstellar medium, [C II] balances the heating, including that due to far-ultraviolet photons, which heat the gas via the photoelectric effect. However, the origin of [C II] emission remains unclear because C + can be found in multiple phases of the interstellar medium. Here we measure the fractions of [C II] emission originating in the ionized and neutral gas phases of a sample of nearby galaxies. We use the [N II] 205 μm fine-structure line to trace the ionized medium, thereby eliminating the strong density dependence that exists in the ratio of emission originates from neutral gas. We find that the fraction of [C II] originating in the neutral medium has a weak dependence on dust temperature and the surface density of star formation, and has a stronger dependence on the gas-phase metallicity. In metal-rich environments, the relatively cooler ionized gas makes substantially larger contributions to total [C II] emission than at low abundance, contrary to prior expectations. Approximate calibrations of this metallicity trend are provided.

Central molecular zones in galaxies:12CO-to-13CO ratios, carbon budget, andXfactors

“…CO and [CI] emission can only originate in a neutral gas, but [CII] emission can also come from an ionized gas. The fraction of the [CII] emission from the neutral gas f CII relevant to our analysis is estimated from the Herschel intensities of the [NII] 122µm (PACS) and 205µm (SPIRE) lines in the usual way; for a detailed description of this procedure and mapping results on many of the sample galaxies, see Croxall et al (2017). Typically, 20% of the [CII] emission comes from ionized gas, and the neutral gas fractions of interest to us range from 0.57 to 0.92 (mean 0.81, average 0.75, with a standard deviation 0.21).…”

“…The newly determined multi-transition 12 COto-13 CO isotopologue ratios allow us to determine more accurately the CO gas column densities and their relation to the much more abundant H 2 gas, including the values of X in a large number of galaxy central regions. The analysis of the 12 CO and 13 CO spectral lines is of particular importance in the interpretation of the Herschel Space Observatory (2009-2013) observations of galaxies in the two submillimeter [CI] lines and the farinfrared [CII] line (Israel et al 2015;Kamenetzky et al 2016;Fernández-Ontiveros et al 2016;Lu et al 2017;Croxall et al 2017;Díaz-Santos et al 2017;and Herrera-Camus et al 2018). With it, we will place significant constraints on the relation between molecular and atomic carbon and determine the carbon budget in the observed galaxy centers.…”

Full of Orions: a 200-pc mapping of the interstellar medium in the redshift-3 lensed dusty star-forming galaxy SDP.81

“…ionized gas). The contribution of ionized gas to [C ii] emission can be estimated using [N ii] 122/205-µm lines (Croxall et al 2017;Herrera-Camus et al 2018b Second, the PDRToolbox predictions need to be adjusted in case a ratio of an optically thin and an optically thick tracer is considered. Namely, the emergent line intensities are predicted for a semi-infinite slab of gas illuminated only from the face side; for the extended starburst environment in SDP.81, the clouds are likely exposed to FUV radiation from multiple sides.…”

A dense, solar metallicity ISM in the z = 4.2 dusty star-forming galaxy SPT 0418−47

“…This technique has been used on individual galaxies at high redshift (e.g. Decarli et al 2014), as well as on samples of nearby galaxies (Kamenetzky et al 2014;Díaz-Santos et al 2017;Croxall et al 2017), yielding [CII] PDR fractions of ∼75%. The latter authors did find a clear dependence on metallicity in this PDR/HII fraction.…”