Staring at the Shadows of Archaic Galaxies: Damped Ly$\alpha$ and Metal Absorbers toward a Young $z\sim 6$ Weak-line Quasar

Abstract

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 $\le 43.46$ 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.

22 pages, 12 figures, and 1 table. Replaced to match version accepted for publication in The Astronomical Journal. We welcome the comments from the reader. Related paper: arXiv:2009.07784