Probing the Inner Circumgalactic Medium and Quasar Illumination around the Reddest `Extremely Red Quasar' (ERQ)

Dusty quasars might be in a young stage of galaxy evolution with prominent quasar feedback. A recently discovered population of luminous, extremely red quasars at $z\sim$~2--4 has extreme spectral properties related to exceptionally powerful quasar-driven outflows. We present Keck/KCWI observations of the reddest known ERQ, at $z=$\,2.3184, with extremely fast [\ion{O}{III}]~$\lambda$5007 outflow at $\sim$6000~km~s$^{-1}$. The Ly$\alpha$ halo spans $\sim$100~kpc. The halo is kinematically quiet, with velocity dispersion $\sim$300~km~s$^{-1}$ and no broadening above the dark matter circular velocity down to the spatial resolution $\sim$6~kpc from the quasar. We detect spatially-resolved \ion{He}{II}~$\lambda$1640 and \ion{C}{IV}~$\lambda$1549 emissions with kinematics similar to the Ly$\alpha$ halo and a narrow component in the [\ion{O}{III}]~$\lambda$5007. Quasar reddening acts as a coronagraph allowing views of the innermost halo. A narrow Ly$\alpha$ spike in the quasar spectrum is inner halo emission, confirming the broad \ion{C}{IV}~$\lambda$1549 in the unresolved quasar is blueshifted by $2240$~km~s$^{-1}$ relative to the halo frame. We propose the inner halo is dominated by moderate-speed outflow driven in the past and the outer halo dominated by inflow. The high central concentration of the halo and the symmetric morphology of the inner region are consistent with the ERQ being in earlier evolutionary stage than blue quasars. The \ion{He}{II}~$\lambda$1640/Ly$\alpha$ ratio of the inner halo and the asymmetry level of the overall halo are dissimilar to Type~II quasars, suggesting unique physical conditions for this ERQ that are beyond orientation differences from other quasar populations. We find no evidence of mechanical quasar feedback in the Ly$\alpha$-emitting halo.