The reliability of [C ii] as an indicator of the star formation rate

The [C ii] 157.74 μm line is an important coolant for the neutral interstellar gas. Since [C ii] is the brightest spectral line for most galaxies, it is a potentially powerful tracer of star formation activity. In this paper, we present a calibration of the star formation rate (SFR) as a function of the [C ii] luminosity for a sample of 24 star-forming galaxies in the nearby Universe. This sample includes objects classified as H ii regions or low-ionization nuclear emission-line regions, but omits all Seyfert galaxies with a significant contribution from the active galactic nucleus to the mid-infrared photometry. In order to calibrate the SFR against the line luminosity, we rely on both Galaxy Evolution Explorer far-ultraviolet data, which is an ideal tracer of the unobscured star formation, and MIPS 24 μm, to probe the dust-enshrouded fraction of star formation. In the case of normal star-forming galaxies, the [C ii] luminosity correlates well with the SFR. However, the extension of this relation to more quiescent (Hα EW ≤ 10 Å) or ultraluminous galaxies should be handled with caution, since these objects show a non-linearity in the -to-LFIR ratio as a function of LFIR (and thus, their star formation activity). We provide two possible explanations for the origin of the tight correlation between the [C ii] emission and the star formation activity on a global galaxy-scale. A first interpretation could be that the [C ii] emission from photodissociation regions (PDRs) arises from the immediate surroundings of star-forming regions. Since PDRs are neutral regions of warm dense gas at the boundaries between H ii regions and molecular clouds and they provide the bulk of [C ii] emission in most galaxies, we believe that a more or less constant contribution from these outer layers of photon-dominated molecular clumps to the [C ii] emission provides a straightforward explanation for this close link between the [C ii] luminosity and SFR. Alternatively, we consider the possibility that the [C ii] emission is associated with the cold interstellar medium, which advocates an indirect link with the star formation activity in a galaxy through the Schmidt law.