Strong FUV fields drive the [CII]/FIR deficit in z~3 dusty, star-forming
  galaxies
  We present 0.15-arcsec (1 kpc) resolution ALMA observations of the [CII]
157.74 um line and rest-frame 160-um continuum emission in two z~3 dusty,
star-forming galaxies - ALESS 49.1 and ALESS 57.1, combined with resolved
CO(3-2) observations. In both sources, the [CII] surface brightness
distribution is dominated by a compact core $\leq$1 kpc in radius, a factor of
2-3 smaller than the extent of the CO(3-2) emission. In ALESS 49.1, we find an
additional extended (8-kpc radius), low surface-brightness [CII] component.
Based on an analysis of mock ALMA observations, the [CII] and 160-um continuum
surface brightness distributions are inconsistent with a single-Gaussian
surface brightness distribution with the same size as the CO(3-2) emission. The
[CII] rotation curves flatten at $\simeq$2 kpc radius, suggesting the
kinematics of the central regions are dominated by a baryonic disc. Both
galaxies exhibit a strong [CII]/FIR deficit on 1-kpc scales, with
FIR-surface-brightness to [CII]/FIR slope steeper than in local star-forming
galaxies. A comparison of the [CII]/CO(3-2) observations with PDR models
suggests a strong FUV radiation field ($G_0\sim10^4$) and high gas density
($n\mathrm{(H)}\sim10^4-10^5$ cm$^{-3}$) in the central regions of ALESS 49.1
and 57.1. The most direct interpretation of the pronounced [CII]/FIR deficit is
a thermal saturation of the C+ fine-structure levels at temperatures $\geq$500
K, driven by the strong FUV field.
