Measuring the Density Fields around Bright Quasars at $z\sim 6$ with XQR-30 Spectra

TL;DR

This study uses high-resolution spectra of $z\sim 6$ quasars to measure the intergalactic density field around quasars, compare it with simulations, and constrain cosmological parameters like $\sigma_8$, revealing environmental variations and host halo masses.

Contribution

First measurement of the density field around $z\sim 6$ quasars using high-resolution spectra, providing new constraints on cosmology and quasar host halo masses.

Findings

Good agreement between observed and simulated density CDFs at 1.5-3 pMpc.

Constraint on $\sigma_8=0.6\pm 0.3$ from proximity zone densities.

Higher than predicted density within 1.5 pMpc suggests massive quasar host halos.

Abstract

Measuring the density of the intergalactic medium using quasar sightlines in the epoch of reionization is challenging due to the saturation of Lyman-$\alpha$ absorption. Near a luminous quasar, however, the enhanced radiation creates a proximity zone observable in the quasar spectra where the Lyman-$\alpha$ absorption is not saturated. In this study, we use $10$ high-resolution ($R\gtrsim 10,000$) $z\sim 6$ quasar spectra from the extended XQR-30 sample to measure the density field in the quasar proximity zones. We find a variety of environments within $3$ pMpc distance from the quasars. We compare the observed density cumulative distribution function (CDF) with models from the $\textit{Cosmic Reionization on Computers}$ simulation, and find a good agreement between $1.5$ to $3$pMpc from the quasar. This region is far away from the quasar hosts and hence approaching the mean density of the universe, which allows us to use the CDF to set constraints on the cosmological parameter $\sigma_8=0.6\pm0.3$. The uncertainty is mainly due to the limited number of high-quality quasar sightlines currently available. Utilizing the more than $>200$ known quasars at $z\gtrsim 6$, this method will allow us in the future to tighten the constraint on $\sigma_8$ to the percent level. In the region closer to the quasar within $1.5$ pMpc, we find the density is higher than predicted in the simulation by $1.23 \pm 0.17$, suggesting the typical host dark matter halo mass of a bright quasar ($M_{\rm 1450}<-26.5$) at $z\sim 6$ is $\log_{\rm 10} (M/M_\odot)=12.5^{+0.4}_{-0.7}$.