2020MNRAS.494.5542R

We present a sub-kpc resolved study of the interstellar medium properties in SDP.81, a z = 3.042 strongly gravitationally lensed, dusty star-forming galaxy, based on high-resolution, multiband ALMA observations of the far-infrared (FIR) continuum, CO ladder, and the [C II] line. Using a visibility-plane lens modelling code, we achieve a median source-plane resolution of ∼200 pc. We use photon-dominated region (PDR) models to infer the physical conditions - far-ultraviolet (FUV) field strength, density, and PDR surface temperature - of the star-forming gas on 200-pc scales, finding a FUV field strength of ∼10³-10⁴G₀, gas density of ∼10⁵ cm^–3, and cloud surface temperatures up to 1500 K, similar to those in the Orion Trapezium region. The [C II] emission is significantly more extended than that FIR continuum: ∼50 per cent of [C II] emission arises outside the FIR-bright region. The resolved [C II]/FIR ratio varies by almost 2 dex across the source, down to ∼2 x 10^–4 in the star-forming clumps. The observed [C II]/FIR deficit trend is consistent with thermal saturation of the C⁺ fine-structure-level occupancy at high gas temperatures. We make the source-plane reconstructions of all emission lines and continuum data publicly available.