# Fluorescent C II* 1335A emission spectroscopically resolved in a galaxy at z = 5.754

## TL;DR

This paper reports the first spectroscopic resolution of the C II*/C II doublet in a high-redshift galaxy, revealing insights into ionization channels and emission mechanisms, with implications for future JWST and ELT studies.

## Contribution

It presents the first spectroscopic resolution of the C II*/C II doublet at high redshift and models the emission components, suggesting ionization escape channels in early galaxies.

## Key findings

- Resolved C II*/C II doublet at z=5.754.
- Multiple emission components required to explain observations.
- Potential use of C II*/C II as a porosity diagnostic in future telescopes.

## Abstract

We report the discovery of the first spectroscopically resolved C II /C II* 1334, 1335A doublet in the Lyman-break galaxy J0215-0555 at z = 5.754. The separation of the resonant and fluorescent emission channels was possible thanks to the large redshift of the source and long integration time, as well as the small velocity width of the feature, 0.6 +- 0.2A. We model this emission and find that at least two components are required to reproduce the combination of morphologies of C II* emission, C II absorption and emission, and Lyman-alpha emission from the object. We suggest that the close alignment between the fluorescence and Lyman-alpha emission could indicate an ionisation escape channel within the object. While the faintness of such a C II /C II* doublet makes it prohibitively difficult to pursue for similar systems with current facilities, we suggest it can become a valuable porosity diagnostic in the era of JWST and the upcoming generations of ELTs.