Strong Far-ultraviolet Fields Drive the [C ii]/Far-infrared Deficit in z ∼ 3 Dusty, Star-forming Galaxies

We present 0.″15 (1 kpc) resolution ALMA observations of the [C ii] 157.74 μm line and rest-frame 160 μm 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 [C ii] surface brightness distribution is dominated by a compact core ≤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 [C ii] component. Based on an analysis of mock ALMA observations, the [C ii] and 160 μm continuum surface brightness distributions are inconsistent with a single-Gaussian surface brightness distribution with the same size as the CO (3-2) emission. The [C ii] rotation curves flatten at ≃2 kpc radius, suggesting that the kinematics of the central regions are dominated by a baryonic disk. Both galaxies exhibit a strong [C ii]/far-IR (FIR) deficit on 1 kpc scales, with FIR surface brightness to [C ii]/FIR slope steeper than in local star-forming galaxies. A comparison of the [C ii]/CO (3-2) observations with photodissociation region models suggests a strong far-UV (FUV) radiation field (G ₀ ∼ 10⁴) and high gas density (n(H) ∼ 10⁴-10⁵ cm^-3) in the central regions of ALESS 49.1 and ALESS 57.1. The most direct interpretation of the pronounced [C ii]/FIR deficit is a thermal saturation of the C⁺ fine-structure levels at temperatures ≥500 K, driven by the strong FUV field.