# Implications of $z \sim 6$ Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes

## TL;DR

This study analyzes quasar proximity zones at high redshift, finding a shallower evolution than previously thought and good agreement with simulations, while identifying outliers possibly due to short quasar lifetimes or local absorption.

## Contribution

It provides a homogeneous analysis of 34 high-redshift quasars, updates systemic redshift measurements, and compares observations with simulations revealing new insights into reionization and quasar lifetimes.

## Key findings

- Proximity zone size evolution is shallower than previously reported.
- Good agreement between observed and simulated proximity zones regardless of IGM ionization.
- Identification of outliers with very small proximity zones potentially indicating short quasar lifetimes.

## Abstract

We study quasar proximity zones in the redshift range $5.77 \leq z \leq 6.54$ by homogeneously analyzing $34$ medium resolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude measurements. Whereas previous studies found strong evolution of proximity zone sizes with redshift, and argued that this provides evidence for a rapidly evolving intergalactic medium (IGM) neutral fraction during reionization, we measure a much shallower trend $\propto(1+z)^{-1.44}$. We compare our measured proximity zone sizes to predictions from hydrodynamical simulations post-processed with one-dimensional radiative transfer, and find good agreement between observations and theory irrespective of the ionization state of the ambient IGM. This insensitivity to IGM ionization state has been previously noted, and results from the fact that the definition of proximity zone size as the first drop of the smoothed quasar spectrum below the $10\%$ flux transmission level probes locations where the ionizing radiation from the quasar is an order of magnitude larger than the expected ultraviolet ionizing background that sets the neutral fraction of the IGM. Our analysis also uncovered three objects with exceptionally small proximity zones (two have $R_p < 1$proper Mpc), which constitute outliers from the observed distribution and are challenging to explain with our radiative transfer simulations. We consider various explanations for their origin, such as strong absorption line systems associated with the quasar or patchy reionization, but find that the most compelling scenario is that these quasars have been shining for $\lesssim 10^5$yr.