The Optical Depth of [CII]: the Implications for Galaxies both Near and Far

By Cristian Guevara and Joan Schmelz

Paper: [C II] 158 µm self-absorption and optical depth effects
Guevara, C., et al., 2020/04, A&A, 636A, A16.

Photodissociation Regions (PDRs) are zones of the interstellar medium in which Far-UV photons dominate the thermal balance, chemistry, structure, as well as the distribution of the gas and dust. The incident FUV field photodissociates molecules, photoionizes atoms and molecules, and heats the gas and dust.

The [CII] 158 µm fine structure line in the far IR is one of the brightest emission lines in PDRs and provides an important cooling mechanism for the atomic and molecular gas. The [CII] line is also used as a vital star-formation tracer for both nearby and high redshift galaxies, but these studies assume that the line is optically thin. If this ubiquitous star-formation tracer is to provide physically meaningful astronomical information, it is essential to know whether the line emission is optically thin or thick.

Interest in the optical depth of [CII] has been present since the first observations of [CII] in the early 1980s. Traditionally, [CII] has been assumed to be optically thin, with systematic measurements of a few cases done only in the last few years. This key analysis required the high sensitivity and spectral resolution of the SOFIA upGREAT instrument.

SOFIA observed simultaneously [12CII] and its isotope [13CII]. The [13CII] line is split into three hyperfine satellites due to the extra neutron, located at 11, −63, and 65 km/s from the main [12CII] line. The main objectives of this work were to determine the [12CII] optical depth, study the effects that could result from a non-optically thin line, and determine the physical parameters associated with the ionized carbon.

We observed multiple positions in four different PDRs with a wide range of physical conditions: M43, Horsehead, Monoceros R2, and M17 SW. The results indicate that the [12CII] emission is optically thick, with optical depths ranging between two and seven. In addition, positions in both Monoceros R2 and M17 SW are heavily self-absorbed, with spectral dips that mimic velocity components. Hence, comparisons of velocity profiles without accounting for the optical depth should be treated with caution.

The complexity of the sources and their high optical depth required a more sophisticated approach in order to derive the physical properties of the gas. For these reasons, a double-layered model was developed, with an emitting background traced by the optically thin [13CII], and an absorbing foreground traced by the difference between the [13CII] and [12CII] emission. We found that the background layer has high column density, much higher than expected due to the absorption effects. The ionize foreground is of unknown origin, but must be cold enough to absorb the background radiation and not produce much emission.

The analysis presented here showed that the origin of [CII] emission is more complex than simple models might suggest. The structured line profiles and high optical depth visible in particular in the bright sources of strong [CII] emission in the Milky Way revealed substantially higher [CII] column densities than the ones estimated using an optically thin approximation. Physical parameters derived from the optically thin approximation should also be treated with caution.

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