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

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 (${\epsilon }_{\mathrm{PE}}$) 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 ${\epsilon }_{\mathrm{PE}}$. [CII]/TIR fails as a proxy for ${\epsilon }_{\mathrm{PE}}$, 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 ($\mathrm{U}{\mathrm{V}}_{\mathrm{att}}$), and the total attenuated flux ($\mathrm{TO}{\mathrm{T}}_{\mathrm{att}}$). We find [CII]/TIR to be strongly correlated with $\mathrm{U}{\mathrm{V}}_{\mathrm{att}}$/$\mathrm{TO}{\mathrm{T}}_{\mathrm{att}}$, 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 ${\epsilon }_{\mathrm{PE}}^{\mathrm{UV}}$, [CII]/$\mathrm{U}{\mathrm{V}}_{\mathrm{att}}$ 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.

arXiv comment19 pages, 16 figures, accepted for publication in ApJ

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