[CII] 158 $\mu$m Emission as a Star Formation Tracer
  The [CII] 157.74 $\mu$m transition is the dominant coolant of the neutral
interstellar gas, and has great potential as a star formation rate (SFR)
tracer. Using the Herschel KINGFISH sample of 46 nearby galaxies, we
investigate the relation of [CII] surface brightness and luminosity with SFR.
We conclude that [CII] can be used for measurements of SFR on both global and
kiloparsec scales in normal star-forming galaxies in the absence of strong
active galactic nuclei (AGN). The uncertainty of the $\Sigma_{\rm
[CII]}-\Sigma_{\rm SFR}$ calibration is $\pm$0.21 dex. The main source of
scatter in the correlation is associated with regions that exhibit warm IR
colors, and we provide an adjustment based on IR color that reduces the
scatter. We show that the color-adjusted $\Sigma_{\rm[CII]}-\Sigma_{\rm SFR}$
correlation is valid over almost 5 orders of magnitude in $\Sigma_{\rm SFR}$,
holding for both normal star-forming galaxies and non-AGN luminous infrared
galaxies. Using [CII] luminosity instead of surface brightness to estimate SFR
suffers from worse systematics, frequently underpredicting SFR in luminous
infrared galaxies even after IR color adjustment (although this depends on the
SFR measure employed). We suspect that surface brightness relations are better
behaved than the luminosity relations because the former are more closely
related to the local far-UV field strength, most likely the main parameter
controlling the efficiency of the conversion of far-UV radiation into gas
heating. A simple model based on Starburst99 population-synthesis code to
connect SFR to [CII] finds that heating efficiencies are $1\%-3\%$ in normal
galaxies.
