Abstract
Abstract The ratio of the [C ii] emission line over the total infrared emission (TIR) is often used as a proxy for the photoelectric (PE) heating efficiency ( ) 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 [C ii]/TIR that we rule out as being due to an intrinsic variation in . [C ii]/TIR fails as a proxy for , because the TIR measures all dust heating, not just the contribution from FUV photons capable of ejecting electrons from dust grains. Using extensive multi-wavelength coverage from the FUV to far-infrared, we infer the attenuated FUV emission ( ), and the total attenuated flux ( ). We find [C ii]/TIR to be strongly correlated with / , indicating that, in M31 at least, one of the dominant drivers for [C ii]/TIR variation is the relative hardness of the absorbed stellar radiation field. We define , [C ii]/ which should be more closely related to the actual PE efficiency, which we find to be essentially constant ( ) in all explored fields in M31. This suggests that part of the observed variation of [C ii]/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, though PE efficiency may also vary across a wider range of environments.
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