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

## Author(s): Kapala, Maria J; Groves, Brent; Sandstrom, Karin M.; Jarrett, Thomas; da Cunha, Elisabete; et al

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 Abstract: The ratio of the [C II] 158 mm emission line over the total infrared emission (TIR) is often used as a proxy for the photoelectric (PE) heating efficiency (epsilon(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 [C II]/TIR that we rule out as being due to an intrinsic variation in. PE. [C II]/TIR fails as a proxy for. PE, 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 UVatt/TOTatt, 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. PE UV, [C II]/UVatt which should be more closely related to the actual PE efficiency, which we find to be essentially constant (1.85 +/- 0.8%) 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. Publication Date: 20-Jun-2017 Electronic Publication Date: 23-Jun-2017 Citation: Kapala, Maria J, Groves, Brent, Sandstrom, Karin, Jarrett, Thomas, da Cunha, Elisabete, Croxall, Kevin, Dalcanton, Julianne, Draine, Bruce, Glover, Simon, Schinnerer, Eva. (2017). The Survey of Lines in M31 (SLIM): The Drivers of the [CII]/TIR Variation. ASTROPHYSICAL JOURNAL, 842 (10.3847/1538-4357/aa7265 DOI: doi:10.3847/1538-4357/aa7265 ISSN: 0004-637X EISSN: 1538-4357 Type of Material: Journal Article Journal/Proceeding Title: ASTROPHYSICAL JOURNAL Version: Final published version. Article is made available in OAR by the publisher's permission or policy. Notes: Supplementary data may be available at http://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2017ApJ...842..128K , http://ned.ipac.caltech.edu/cgi-bin/nph-objsearch?search_type=Search&refcode=2017ApJ...842..128K and http://archives.esac.esa.int/hsa/whsa/?ACTION=PUBLICATION&ID=2017ApJ...842..128K

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