the obscured star formation) and low-$J$ CO rotational transitions[^2] (which trace the cold, molecular gas that fuels the star formation, @Carilli2013).

Besides the FIR continuum and CO emission, the third bright rest-frame FIR tracer of the star-forming interstellar medium (ISM) is the \[\] 157.74 $\mu$m line, a fine-structure transition of C$^+$ ions. Due to its low ionization energy (11.3 eV) and a relatively low critical density, \[\] traces of a wide range of ISM phases – from the ionized regions to warm molecular clouds to diffuse gas. Depending on the environmental conditions, the upper fine-structure level is populated predominantly by collisions with H, H$_2$ or electrons [@Goldsmith2012].

Starting in the early 1990’s, systematic studies of \[\] emission in local galaxies were enabled by the *Infrared Space Observatory* and the Kuiper Airborne Observatory. These observations revealed a tight correlation between the \[\] line and FIR continuum emission from the heated dust at low SFR surface densities (e.g.,@Stacey1991). However, this correlation breaks at larger FIR surface brightness $\Sigma_\mathrm{FIR}$ – the so-called “*\[CII\]/FIR deficit*” (e.g.,@Malhotra1997 [@Luhman1998; @Malhotra2001; @Luhman2003]) – with the \[\]/FIR ratio decreasing with increasing $\Sigma_\mathrm{FIR}$.

In the last decade, the study of \[\] emission in the nearby Universe was revolutionized by 