The [CII] 158μm line emission in nearby Galaxies

The 158.7um emission line from the singly ionized carbon is the brightest far-IR line emitted by galaxies, usually amounting to 1% or more of the total far-IR continuum. Since the line acts as the main cooling channel for photodissociation regions in the interstellar medium (ISM), it is essential to study the properties of the ISM in high redshift galaxies, as recently shown by ALMA observations. However, because of the low ionization potential of carbon, C+ exists in a variety of environments, from HII regions to warm molecular clouds. Moreover, mechanisms different than stellar emission can significantly contribute to the emission. It is therefore essential to study in detail the contribution of the different ISM regions and mechanism of emission of C+ in nearby galaxies to better infer the properties of the ISM in distant galaxies. We will show how Herschel and SOFIA observations of nearby extended galaxies have shed new light on the C+ emission origin and its relationship with star formation rates in galaxies.