is a thermal saturation of the C$^+$ fine-structure levels at temperatures $\geq500$ K, driven by the strong FUV field.'
author:
- Matus Rybak
- 'G. Calistro Rivera'
- 'J. A. Hodge'
- Ian Smail
- 'F. Walter'
- 'P. van der Werf'
- 'E. da Cunha'
- 'Chian-Chou Chen'
- 'H. Dannerbauer'
- 'R. J. Ivison'
- 'A. Karim'
- 'J. M. Simpson'
- 'A. M. Swinbank'
- 'J. L. Wardlow'
bibliography:
- 'ALESS\_CII.bib'
title: 'Strong FUV fields drive the \[CII\]/FIR deficit in $z\sim3$ dusty, star-forming galaxies'
---

Introduction {#sec:introduction}
============

Dusty, star-forming, submillimeter galaxies (DSFGs, SMGs) are a major contributor to the global star-formation rate between redshifts $z=2-4$, at an epoch when the star-forming activity of the Universe was at its peak (e.g.,@Casey2014). Although few in numbers, thanks to their high star-formation rates (SFR $>$100 M$_\odot$ yr$^{-1}$), up to 20% of all the star formation at $z\sim3$ takes place in SMGs [@Swinbank2014].

The massive dust reservoirs in SMGs absorb the UV/optical radiation from the newborn stars, mostly re-radiating it thermally as a rest-frame FIR/sub-mm continuum[^1]. Therefore, studying the structure and physical properties in these extreme sources requires relying on sub-mm/mm bright tracers – the dust continuum (which directly traces 