The Survey of Lines in M31 (SLIM): Investigating the Origins of [CII] Emission

TL;DR

This study uses high-resolution observations of M31 to analyze the origins of [CII] emission, confirming the [CII]-SFR relation at small scales and exploring the diffuse contributions from various stellar populations.

Contribution

It provides the first detailed spatial analysis of [CII] emission in M31 at ~50 pc scales, revealing the significant diffuse component and the effects of stellar populations on [CII] emission.

Findings

The [CII]-SFR correlation persists at ~50 pc scales.

A large fraction of [CII] emission originates outside star-forming regions.

The [CII]/TIR ratio increases with galactocentric radius.

Abstract

The [CII] 158 micron 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 [CII], one might expect this relation to break down at small scales. We investigate the origins of [CII] emission by examining high spatial resolution observations of [CII] in M31, with the Survey of Lines in M31 (SLIM). We present five ~700x700 pc (3"x3") Fields mapping the [CII] emission, Halpha 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 [CII] - SFR correlation holds even at these scales, although the relation typically has a flatter slope than found at larger (~kpc) scales. While the Halpha emission in M31 is concentrated in the SFR regions, we find that a significant amount (~20-90%) of the [CII] 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 [CII]/TIR to dust color in each Field, and find a large scale trend of increasing [CII]/TIR with galactocentric radius. The differences in the relative diffuse fractions of [CII], Halpha and IR tracers are likely caused by a combination of energetic photon leakage from HII 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 [CII] and SFR found in other nearby galaxy studies is retrieved.