Resolved ALMA [CII] 158 μm Observations at Cosmic Noon: Interstellar Medium Structure and Dynamics of the Starbursting QSO SDSS J1000

We present spatially resolved Atacama Large Millimeter/submillimeter Array Band 9 observations of the [C ii] 158 mu m fine structure line from an optically selected quasar, SDSS J100038.01+020822.4 (J1000), at z = 1.8275. By utilizing [O i] 63 mu m 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 similar to 5 x 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 similar to 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/sigma = 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 similar to 1.5 x 1010 M circle dot and main-component dynamical mass greater than or similar to 1011 M circle dot, the J1000 system is a progenitor to the most massive galaxies seen in the local Universe.