Staring at the Shadows of Archaic Galaxies: Damped Ly$\alpha$ and Metal
  Absorbers toward a Young $z \sim 6$ Weak-line Quasar
  We characterize the Ly$\alpha$ halo and absorption systems toward PSO
J083+11, a unique $z=6.3401$ weak-line quasar, using Gemini/GNIRS,
Magellan/FIRE, and VLT/MUSE data. Strong absorptions by hydrogen and several
metal lines (e.g., CII, MgII, and OI) are discovered in the spectrum, which
indicates the presence of: (i) a proximate sub-damped Ly$\alpha$ (sub-DLA)
system at $z=6.314$ and (ii) a MgII absorber at $z=2.2305$. To describe the
observed damping wing signal, we model the Ly$\alpha$ absorption with a
combination of a sub-DLA with the neutral hydrogen column density of $\log
N_\mathrm{HI} = 20.03 \pm 0.30$ cm$^{-2}$ and absorption from the intergalactic
medium with a neutral fraction of around 10 percent. The sub-DLA toward PSO
J083+11 has an abundance ratio of [C/O] $=-0.04 \pm 0.33$ and metallicity of
[O/H] $=-2.19 \pm 0.44$, similar to those of low-redshift metal-poor DLAs.
These measurements suggest that the sub-DLA might truncate PSO J083+11's
proximity zone size and complicate the quasar lifetime measurement. However,
this quasar shows no sign of a Ly$\alpha$ halo in the MUSE data cube, where the
estimated $1\sigma$ limit of surface brightness is $2.76 \times 10^{-18}$ erg
s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at aperture size of 1 arcsecond, or equivalent
to a Ly$\alpha$ luminosity of $\leq 43.46$ erg s$^{-1}$. This non-detection,
while being only weak independent evidence on its own, is at least consistent
with a young quasar scenario, as expected for a quasar with a short accretion
timescale.
