Preprint typeset using LATEX style emulateapj v. 12/16/11 THE SURVEY OF LINES IN M 31 (SLIM): INVESTIGATING THE ORIGINS OF [C ii] EMISSION

The [C ii] 158 µm line is one of the strongest emission lines observed in star-forming galaxies, and has been empirically measured to correlate with the star formation rate (SFR) globally and on ∼ kpc scales. However, due to the multi-phase origins of [C ii], one might expect this relation to break down at small scales. We investigate the origins of [C ii] emission by examining high spatial resolution observations of [C ii] in M 31, with the Survey of Lines in M 31 (SLIM). We present five ∼ 700×700 pc (3′×3′) Fields mapping the [C ii] emission, Hα emission, combined with ancillary infrared (IR) data. We spatially separate star-forming regions from diffuse gas and dust emission on ∼ 50 pc scales. We find that the [C ii] – SFR correlation holds even at these scales, although the relation typically has a flatter slope than found at larger ( ∼ kpc) scales. While the Hα emission in M 31 is concentrated in the SFR regions, we find that a significant amount ( ∼ 20–90%) of the [C ii] emission comes from outside star-forming regions, and that the total IR (TIR) emission has the highest diffuse fraction of all SFR tracers. We find a weak correlation of the [C ii]/TIR to dust color in each Field, and find a large scale trend of increasing [C ii]/TIR with galactocentric radius. The differences in the relative diffuse fractions of [C ii], Hα and IR tracers are likely caused by a combination of energetic photon leakage from H ii regions and heating by the diffuse radiation field arising from older (B-star) stellar populations. However, we find that by averaging our measurements over ∼ kpc scales, these effects are minimized, and the relation between [C ii] and SFR found in other nearby galaxy studies is retrieved