The reliability of [CII] as a star formation rate indicator
  The [CII] 157.74 \mu m line is an important coolant for the neutral
interstellar gas. Since [CII] 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 as a function of the [CII]
luminosity for a sample of 24 star-forming galaxies in the nearby universe.
This sample includes objects classified as HII regions or LINERs, but omits all
Seyfert galaxies with a significant AGN contribution to the mid-infrared
photometry. In order to calibrate the SFR against the line luminosity, we rely
on both GALEX FUV data, which is an ideal tracer of the unobscured star
formation, and MIPS 24 \mu m, to probe the dust-enshrouded fraction of star
formation. In case of normal star-forming galaxies, the [CII] luminosity
correlates well with the star formation rate. However, the extension of this
relation to more quiescent (Halpha EW <= 10 A) or ultraluminous galaxies should
be handled with caution, since these objects show a non-linearity in the
L([CII])-to-L(FIR) ratio as a function of L(FIR) (and thus, their star
formation activity).
  We provide two possible explanations for the origin of the tight correlation
between the [CII] emission and the star formation activity on a global
galaxy-scale. A first interpretation could be that the [CII] emission from PDRs
arises from the immediate surroundings of star-forming regions. Since PDRs are
neutral regions of warm dense gas at the boundaries between HII regions and
molecular clouds and they provide the bulk of [CII] emission in most galaxies,
we believe that a more or less constant contribution from these outer layers of
photon-dominated molecular clumps to the [CII] emission provides a
straightforward explanation for this close link between the [CII] luminosity
and SFR. Alternatively,... (abridged)
