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. We study a population of luminous, extremely red quasars at
redshifts $z\sim$~2--4 that has a suite of extreme spectral properties that may
all be related to exceptionally powerful quasar-driven outflows. We present
Keck/KCWI observations of the reddest known ERQ, at $z=$\,2.3184, with an
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
$2242$~km~s$^{-1}$ relative to the halo frame. We propose the inner halo is
dominated by past/moderate-speed outflow and the outer halo dominated by
inflow. The high central concentration of the surface brightness and the
circularly symmetric morphology of the inner halo are consistent with the ERQ
being young. 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,
implying central ionizing photons can escape along the line of sight. While
quasar dominates the ionization, we find no evidence of mechanical quasar
feedback on circumgalactic scales.
