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

## TL;DR

This study investigates the variation of the [CII]/TIR ratio in M31, revealing it is driven by the hardness of the absorbed stellar radiation field rather than intrinsic photoelectric efficiency changes.

## Contribution

The paper demonstrates that [CII]/TIR variation in M31 is primarily due to radiation field hardness, not changes in PE efficiency, challenging its use as a direct proxy for PE efficiency.

## Key findings

- [CII]/TIR correlates with the ratio of attenuated UV to total flux.
- The PE efficiency, defined as [CII]/UV_att, remains constant at about 1.85%.
- Variation in [CII]/TIR in other galaxies may be due to radiation field hardness.

## Abstract

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.