The Survey of Lines in M31 (SLIM): The Drivers of the [C II]/TIR Variation

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 (ϵPE) 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 ϵPE. [C ii]/TIR fails as a proxy for ϵPE, 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 (UVatt), and the total attenuated flux (TOTatt). 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 ϵPEUV, [C ii]/UVatt 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.