[CII] emission and star formation in the spiral arms of M 31

Context. The [CII] 158 mu m line is the most important coolant of the interstellar medium in galaxies but substantial variations are seen from object to object. The main source of the emission at a galactic scale is still poorly understood and candidates range from photodissociation regions (PDRs) to the cold neutral or diffuse warm ionized medium. Previous studies of the [CII] emission in galaxies have a resolution of several kpc or more so the observed emission is an average of different ISM components. Aims. The aim of this work is to study, for the first time, the [ CII] emission at the scale of a spiral arm. We want to investigate the origin of this line and its use as a tracer of star formation. Methods. We present [CII] and [OI] observations of a segment of a spiral arm of M 31 using the Infrared Space Observatory. The [CII] emission is compared with tracers of neutral gas (CO, HI) and star formation (H alpha, Spitzer 24 mu m). Results. The similarity of the [CII] emission with the H alpha and 24 mu m images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The [C-II] cooling rate per H atom increases dramatically from similar to 2.7 x 10(-26) erg s(-1) atom(-1) in the border of the map to similar to 1.4 x 10(-25) erg s(-1) atom(-1) in the regions of star formation. The [CII]/FIR42-122 ratio is almost constant at 2%, a factor 3 higher than typically quoted. However, we do not believe that M 31 is unusual. Rather, the whole-galaxy fluxes used for the comparisons include the central regions where the [CII]/FIR ratio is known to be lower and the resolved observations neither isolate a spiral arm nor include data as far out in the galactic disk as the observations presented here. A fit to published PDR models yields a plausible average solution of G(0) similar to 100 and n similar to 3000 for the PDR emission in the regions of star formation in the arm of M31.