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.

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.…”

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.…”

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.…”