First detection of [13CII] in the Large Magellanic Cloud

TL;DR

This study reports the first detection of the [13CII] line in the Large Magellanic Cloud, revealing that the [12CII] emission is optically thick, which impacts the interpretation of [CII] observations in galaxies.

Contribution

It provides the first observational evidence of [13CII] in the LMC, demonstrating that [12CII] emission is optically thick in this galaxy, similar to Galactic sources.

Findings

[12CII] optical depth is 1-3 in the LMC.

[13CII] line is narrower than [12CII], indicating optical thickness.

[CII] intensity may be underestimated by a factor of ~2 in distant galaxies.

Abstract

[13CII] observations in several Galactic sources show that the fine-structure [12CII] emission is often optically thick (the optical depths around 1 to a few). The aim of our study is to test whether this also affects the [12CII] emission from nearby galaxies like the Large Magellanic Cloud (LMC). We observed three star-forming regions in the LMC with upGREAT on board SOFIA at the frequency of the [CII] line. The 4GHz band width covers all three hyperfine lines of [13CII] simultaneously. For the analysis, we combined the [13CII] F=1-0 and F=1-1 hyperfine components, as they do not overlap with the [12CII] line in velocity. Three positions in N159 and N160 show an enhancement of [13CII] compared to the abundance-ratio-scaled [12CII] profile. This is likely due to the [12CII] line being optically thick, supported by the fact that the [13CII] line profile is narrower than [12CII], the enhancement varies with velocity, and the peak velocity of [13CII] matches the [OI] 63um self-absorption. The [12CII] line profile is broader than expected from a simple optical depth broadening of the [13CII] line, supporting the scenario of several PDR components in one beam having varying [12CII] optical depths. The derived [12CII] optical depth at three positions (beam size of 14arcsec, corresponding to 3.4pc) is 1--3, which is similar to values observed in several Galactic sources shown in previous studies. If this also applies to distant galaxies, the [CII] intensity will be underestimated by a factor of approximately 2.