First Detection of [13CII] in the Large Magellanic Cloud
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Yoko Okada
University of Cologne
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The [CII] 158 micron line is one of the dominant cooling lines in photon-dominated regions (PDR), and it is often used to trace star formation in distant galaxies. PDR models show that the [CII] emission typically stems from a depth of up to Av ~ 1 on the PDR surface, which corresponds to a [CII] optical depth of around unity. In order to quantify the optical depth of [12CII], we need to compare it with an optically thin line - [13CII] lines. [13CII] observations in several Galactic sources show that the [12CII] emission is indeed often optically thick. In this study, we observed three star-forming regions in the LMC with upGREAT on board SOFIA. We detected two hyperfine components of [13CII] in N159 and N160 in the LMC for the first time. The combined spectra of the hyperfine components 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] 63 micron self-absorption. The derived [12CII] optical depth 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.

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