SOFIA Reveals the Complex Nature of a 'Simple' Star Formation Tracer

By Yoko Okada, University of Cologne

Paper: First Detection of [13C II] in the Large Magellanic Cloud
Y. Okada, et al., 2019, A&A, 631L, 12O.

The cosmic cycle of star formation is an essential part of the evolution of the universe. The far-infrared fine structure line from singly ionized carbon (C+) at 158 μm (transition 3P3/2-3P2/1; hereafter [CII]) is one of the important tracers of star-forming activity in near and far galaxies. The ultraviolet radiation from massive stars creates warm layers on the surface of the surrounding molecular clouds, where the energy balance is maintained by cooling the gas through emission lines like [CII] 158 μm.

Observations of [CII] have become more important recently because the cosmological redshift moves the line from distant galaxies into the band detectable by the Atacama Large Millimeter/submillimeter Array (ALMA). Many scientists use the strength of this spectral line as a direct indicator of the star formation rate in those galaxies that appear only as a point source. Observations in the nearby universe are helping to determine if this simple assumption is valid.

Recent SOFIA observations have revealed more and more cases of optically thick [CII] emission from Galactic star-forming regions. Now we have the first results from the Large Magellanic Cloud, one of the best-studied star formation laboratories outside our Galaxy. These results indicate that the intensity of this line may be underestimated by a factor of approximately two.

The only observational way to estimate the optical depth of the [CII] line is to compare it with the hyperfine emission from its isotope − 13C+. Because the wavelength of the [13CII] emission is less than 0.03 μm away from the [12CII] line, very high spectral resolution is required to separate these features. The heterodyne instrument upGREAT (German Receiver for Astronomy at Terahertz Frequencies) onboard SOFIA provides this opportunity.

SOFIA detected [13CII] in three active star forming regions in the Large Magellanic Cloud for the first time. Results indicate that the intensity of the [12CII] emission is lower by a factor of about two compared to that expected from the [13CII] emission. The most likely explanation for this disagreement is that the [12CII] emission is optically thick.

The alternative explanation — that the isotopic ratio 12C/13C is lower than reported in the literature — can be excluded for two reasons. The first is that the intensity ratio [12CII]/[13CII] varies over different velocity bins and is lowest at the peak of the line. Since there is little line-of-sight contamination towards the Large Magellanic Cloud, different velocities most likely correspond to different cloud components within the same region. Thus, they should have the same isotopic ratio. The second reason is that the fine-structure line from the neutral oxygen profile at 63 μm also indicates self-absorption at the velocity where the intensity ratio of [12CII]/[13CII] is lowest.

Optically thick [CII] emission is not limited to small-scale, extreme regions but turns out to be significant over an area of 4-by-4 sq pc in the Large Magellanic Cloud. This is consistent with the large-scale map obtained by upGREAT for the Orion Nebula. These results provide a warning to astronomers that the optical depth effect should not be ignored when using [CII] as a star-formation tracer.

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