Full disc [C

Context. As a major cooling line of interstellar gas, the far-infrared 158 μm line from singly ionised carbon [C <ns0:sc>II</ns0:sc>] is an important tracer of various components of the interstellar medium in galaxies across all spatial and morphological scales. Yet, there is still not a strong constraint on the origins of [C <ns0:sc>II</ns0:sc>] emission. Aims. In this work, we derive the resolved [C <ns0:sc>II</ns0:sc>] star formation rate relation and aim to unravel the complexity of the origin of [C <ns0:sc>II</ns0:sc>]. Methods. We used the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy to map [C <ns0:sc>II</ns0:sc>] in three nearby star-forming galaxies at sub-kiloparsec scales, namely, NGC 3627, NGC 4321, and NGC 6946, and we compared these [C <ns0:sc>II</ns0:sc>] observations to the galactic properties derived from complementary data from the literature. Results. We find that the relationship between the [C <ns0:sc>II</ns0:sc>] fine structure line and star formation rate shows variations between the galaxies as well as between different environments within each galaxy. Conclusions. Our results show that the use of [C <ns0:sc>II</ns0:sc>] as a tracer for star formation is much more tangled than has previously been suggested within the extragalactic literature, which typically focuses on small regions of galaxies and/or uses large-aperture sampling of many different physical environments. As found within resolved observations of the Milky Way, the picture obtained from [C <ns0:sc>II</ns0:sc>] observations is complicated by its local interstellar medium conditions. Future studies will require a larger sample and additional observational tracers, obtained on spatial scales within galaxies, in order to accurately disentangle the origin of [C <ns0:sc>II</ns0:sc>] and calibrate its use as a star formation tracer.