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

TL;DR

This study investigates the origins of [CII] emission in local star-forming galaxies, revealing that most emission comes from neutral gas and that metallicity significantly influences its source, with implications for understanding galaxy interstellar media.

Contribution

It provides the first quantitative analysis of [CII] emission origins using [NII] lines to distinguish gas phases, highlighting metallicity's role in emission contributions.

Findings

60-80% of [CII] emission from neutral gas

Weak dependence of [CII] origin on dust temperature and star formation surface density

Stronger metallicity dependence, with metal-rich environments showing more ionized gas contribution

Abstract

The [CII] 158um fine-structure line is the brightest emission line observed in local star-forming galaxies. As a major coolant of the gas-phase interstellar medium, [CII] balances the heating, including that due to far-ultraviolet photons, which heat the gas via the photoelectric effect. However, the origin of [CII] emission remains unclear, because C+ can be found in multiple phases of the interstellar medium. Here we measure the fractions of [CII] emission originating in the ionized and neutral gas phases of a sample of nearby galaxies. We use the [NII] 205um fine-structure line to trace the ionized medium, thereby eliminating the strong density dependence that exists in the ratio of [CII]/[NII] 122um. Using the FIR [CII] and [NII] emission detected by the KINGFISH and Beyond the Peak Herschel programs, we show that 60-80% of [CII] emission originates from neutral gas. We find that the fraction of [CII] originating in the neutral medium has a weak dependence on dust temperature and the surface density of star formation, and a stronger dependence on the gas-phase metallicity. In metal-rich environments, the relatively cooler ionized gas makes substantially larger contributions to total [CII] emission than at low abundance, contrary to prior expectations. Approximate calibrations of this metallicity trend are provided.