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

Abstract

Although the {}²{{{P}}}_3/2-{}²{{{P}}}_1/2 transition of [C II] at λ ≃ 158 μ {{m}} 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 μ {{m}} intensity across the entire complex, indicating that both trace ultraviolet radiation fields. The 22 μ {{m}} 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 μ {{m}} 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.