Systematic Bias in Ionizing Radiation Escape Fraction Measurements from Foreground Large-Scale Structures

TL;DR

This study examines how large-scale structures in the universe bias measurements of ionizing radiation escape fractions by analyzing the relationship between Lyman-alpha forest transmission and galaxy density at high redshift.

Contribution

It demonstrates a statistically significant correlation between galaxy density and Lyman-alpha optical depth, highlighting the importance of accounting for IGM structure in escape fraction measurements.

Findings

Overdense regions have higher neutral gas content.

Photometric methods enable larger samples of lines of sight.

Large-scale structure impacts ionizing radiation escape estimates.

Abstract

We investigate the relationship between the Lyman-alpha (Lya) forest transmission in the intergalactic medium (IGM) and the environmental density of galaxies, focusing on its implications for the measurement of ionizing radiation escape fractions. Using a sample of 268 spectroscopically confirmed background galaxies at 2.7<z<3.0 and a galaxy density map at z~2.5 within the COSMOS field, we measure the Lya transmission photometrically, leveraging the multiwavelength data available from the COSMOS2020 catalog. Our results reveal a weak but statistically significant positive correlation between Lya optical depth and galaxy density contrast, suggesting that overdense regions are enriched in neutral gas, which could bias escape fraction measurements. This emphasizes the need to account for the large-scale structure of the IGM in analyses of ionizing radiation escape fractions, and highlights the advantages of a photometric approach for increasing the number of sampled lines of sight across large fields. The photometric redshifts provided by upcoming all-sky surveys, such as Euclid, will make it possible to account for this effect across widely separated fields.