Measurements of quasar proximity zones with the Ly$\alpha$ forest of DESI Y1 quasars

TL;DR

This study uses DESI Year 1 data to measure quasar proximity zones via the Lyα forest, revealing that gas overdensities dominate absorption effects and that proximity profiles show no clear dependence on quasar luminosity.

Contribution

First large-scale analysis of quasar proximity zones using DESI data, providing new insights into IGM overdensities and quasar emission properties at high redshift.

Findings

Gas overdensities near quasars are higher than expected, affecting absorption.

Proximity zone profiles show no significant correlation with quasar luminosity.

Enhanced clustering dominates over ionizing radiation in shaping the IGM around quasars.

Abstract

The intergalactic medium (IGM) around quasars is shaped by their dense environments and by their excess ionizing radiation, forming a "quasar proximity zone" whose size and anisotropy depend on the quasar's halo mass, luminosity, age, and radiation geometry. Using over 10,000 quasar pairs from the Dark Energy Spectroscopic Instrument (DESI) Year 1 data, with projected comoving separations $r_{\perp} < 2\,h^{-1}{\rm Mpc}$, we investigate how the proximity zone of foreground quasars at $z\sim2{\rm-}3.5$ affects Lyman-alpha absorption in their background quasars. The large DESI sample enables unprecedented precision in measuring this "transverse proximity" effect, allowing a detailed investigation of the signal's dependence on the projected separation of quasar pairs and the luminosity of the foreground quasar. We find that enhanced gas clustering near quasars dominates over their ionizing effect, leading to stronger absorption on neighboring sightlines. Under the assumption that quasar ionizing luminosity is isotropic and steady, we infer the IGM overdensity profile in the vicinity of quasars, finding overdensities as high as $\Delta \sim 10$ at comoving distance $\sim 1\,h^{-1}{\rm Mpc}$ from the most luminous systems. Surprisingly, however, we find no significant dependence of the proximity profile on the luminosity of the foreground quasar. This lack of luminosity dependence could reflect a cancellation between higher ionizing flux and higher gas overdensity, or it could indicate that quasar emission is highly time variable or anisotropic, so that the observed luminosity does not trace the ionizing flux on nearby sightlines.