# The Origin of [CII] 158um Emission toward the HII Region Complex S235

## TL;DR

This study uses SOFIA and GBT observations to investigate the origin of [CII] 158um emission in the S235 HII region, revealing its association with UV radiation fields and complex spatial relationships with ionized and molecular gas.

## Contribution

It provides detailed spatial and spectroscopic analysis of [CII] emission origins in a Galactic HII region, highlighting the correlation with UV radiation indicators and the limited association with ionized hydrogen.

## Key findings

- Approximately half of [CII] emission coincides with ionized hydrogen gas.
- [CII] intensity correlates strongly with WISE 12um emission across the complex.
- Correlations between [CII] and molecular gas tracers are weak.

## Abstract

Although the 2P3/2-2P1/2 transition of [CII] at 158um 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 [CII] emission toward the HII region complex Sh2-235 (S235) to better understand in detail the origin of [CII] 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 [CII] 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 [CII] intensity is strongly correlated with WISE 12um intensity across the entire complex, indicating that both trace ultra-violet radiation fields. The 22um and radio continuum intensities are only correlated with [CII] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [CII] 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 [CII] emission, only WISE 12um emission is correlated on small-scales of the individual HII region S235 and also has a decent correlation at the scale of entire galaxies. Future studies of a larger sample of Galactic HII regions would help to determine whether these results are truly representative.