  The ratio of the [CII] 158$\,\mu$m emission line over the total infrared
emission (TIR) is often used as a proxy for the photoelectric (PE) heating
efficiency ($\epsilon_{\rm PE}$) of the far-ultraviolet (FUV) photons absorbed
by dust in the interstellar medium. In the nearby galaxy M31, we measure a
strong radial variation of [CII]/TIR that we rule out as being due to an
intrinsic variation in $\epsilon_{\rm PE}$. [CII]/TIR fails as a proxy for
$\epsilon_{\rm PE}$, because the TIR measures all dust heating, not just the
contribution from FUV photons capable of ejecting electrons from dust grains.
Using extensive multiwavelength coverage from the FUV to far-infrared (FIR), we
infer the attenuated FUV emission ($\rm UV_{att}$), and the total attenuated
flux ($\rm TOT_{att}$). We find [CII]/TIR to be strongly correlated with $\rm
UV_{att}$/$\rm TOT_{att}$, indicating that, in M31 at least, one of the
dominant drivers for [CII]/TIR variation is the relative hardness of the
absorbed stellar radiation field. We define $\rm{ \epsilon_{PE}^{UV}}$,
[CII]/$\rm{ UV_{att}}$ which should be more closely related to the actual PE
efficiency, which we find to be essentially constant ($1.85 \pm 0.8 \%$) in all
explored fields in M31. This suggests that part of the observed variation of
[CII]/TIR in other galaxies is likely due to a change in the relative hardness
of the absorbed stellar radiation field, caused by a combination of variations
in the stellar population, dust opacity and galaxy metallicity, although PE
efficiency may also vary across a wider range of environments.

Astrophysics of Galaxies