2006A&A...453...77R

The [CII] 158µm 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. 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. 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 (Hα, Spitzer 24µm). The similarity of the [CII] emission with the Hα and 24µm images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The [CII] cooling rate per H atom increases dramatically from ∼2.7x10^–26erg/s/atom in the border of the map to ∼1.4x10^–25erg/s/atom in the regions of star formation. The [CII]/FIR_42–122 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 G₀∼100 and n∼3000 for the PDR emission in the regions of star formation in the arm of M31.