2025ApJ...987...61R

We present spatially resolved Atacama Large Millimeter/submillimeter Array Band 9 observations of the [C II] 158 μm fine structure line from an optically selected quasar, SDSS J100038.01+020822.4 (J1000), at z = 1.8275. By utilizing [O I] 63 μm line observations from Herschel/PACS and constructing a detailed dust spectral energy distribution using Herschel and Spitzer archival imaging data, we show that the [C II] line emission is well explained by a photodissociation region (PDR) model, in which the emission arises from the surfaces of molecular clouds exposed to far-UV radiation fields ∼5 × 10³ times the local interstellar radiation field (G₀). We find a factor of 30 variation in spatially resolved [C II]/far-IR continuum across the source, which is explained by the reduced fraction of cooling via [C II] line emission at such high far-UV field strengths. By matching the derived PDR parameters to the observed far-IR line and continuum intensities we derive cloud size scales and find that typical cloud radii in J1000 are ∼3.5 pc, perhaps indicating an interstellar medium that is highly fractured due to intense star formation activity. We model the galaxy dynamically and find that the [C II] emission is contained within a compact, dynamically cold disk with v/σ = 6.2, consistent with the IllustrisTNG50 cosmological simulation. We also report the discovery of a companion galaxy to J1000 confirmed by the detection of [C II] and use recently obtained JWST/NIRCam imaging of the system to argue for J1000 being an interacting system. With total stellar mass ∼ 1.5 × 10¹⁰M_⊙ and main-component dynamical mass ≳ 10¹¹M_⊙, the J1000 system is a progenitor to the most massive galaxies seen in the local Universe.