  The CII 158 microns 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.
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
(H\alpha, Spitzer 24 mu.) The similarity of the CII emission with the Ha and 24
mu 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 ~2.7e-26 ergs/s/atom in the border of the map
to ~ 1.4e-25 ergs/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 G_0~100 and n~3000 for the PDR emission in the regions of star
formation in the arm of M31.
