The Origin of [C ii] 158 μm Emission toward the H ii Region Complex S235

Although the transition of [C ii] at λ ≃ 158 is known to be an excellent tracer of active star formation, we still do not have a complete understanding of where within star formation regions the emission originates. Here, we use SOFIA upGREAT observations of [C ii] emission toward the H ii region complex Sh2-235 (S235) to better understand in detail the origin of [C ii] emission. We complement these data with a fully sampled Green Bank Telescope radio recombination line map tracing the ionized hydrogen gas. About half of the total [C ii] emission associated with S235 is spatially coincident with ionized hydrogen gas, although spectroscopic analysis shows little evidence that this emission is coming from the ionized hydrogen volume. Velocity-integrated [C ii] intensity is strongly correlated with Wide-field Infrared Survey Explorer (WISE) 12 intensity across the entire complex, indicating that both trace ultraviolet radiation fields. The 22 and radio continuum intensities are only correlated with [C ii] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [C ii] and molecular gas tracers are poor across the region. We find similar results for emission averaged over a sample of external galaxies, although the strength of the correlations is weaker. Therefore, although many tracers are correlated with the strength of [C ii] emission, only WISE 12 emission is correlated on small scales of the individual H ii region S235 and also has a decent correlation at the scale of entire range of galaxies. Future studies of a larger sample of Galactic H ii regions would help to determine whether these results are truly representative. © 2019. The American Astronomical Society. All rights reserved