Resolved ALMA [CII] 158 micron Observations at Cosmic Noon: ISM Structure and Dynamics of Starbursting QSO SDSSJ1000

Abstract: We present spatially resolved Alma Band-9 observations of the [CII] 158 $\mu$m fine structure line from an optically selected quasar, SDSS J100038.01+020822.4 (J1000), at z=1.8275. By utilizing [OI] 63 $\mu$m line observations from Herschel/PACS and constructing a detailed dust SED using Herschel and Spitzer archival imaging data, we show that the [CII] 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 $\sim 5\cdot10^3$ times the local interstellar radiation field (G$0$). We find a factor of 30 variation in spatially resolved [CII]/Far-IR continuum across the source which is explained by the reduced fraction of cooling via [CII] line emission at such high far-UV field strengths. By matching 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 $\sim$3.5 pc perhaps indicating an ISM that is highly fractured due to intense star formation activity. We model the galaxy dynamically and find that the [CII] emission is contained within a compact, dynamically cold disk with v/$\sigma$=6.2, consistent with cosmological simulations. We also report the discovery of a companion galaxy to j1000 confirmed by the detection of [CII] and use recently obtained JWST/NirCAM imaging of the system to argue for J1000 being an interacting system. With total stellar mass $\sim 1.5 \times 10^{10}$ M$\odot$ and main-component dynamical mass $\gtrsim 10^{11}$ M$_\odot$, the J1000 system is a progenitor to the most massive galaxies seen in the local Universe.