Searching for the shadows of giants II: the effect of local ionisation on the Lyman-$\alpha$ absorption signatures of protoclusters at redshift $z\sim2.4$

TL;DR

This study uses simulations to examine how local ionisation effects from AGN and hot gas influence Lyman-alpha absorption signatures of protoclusters at redshift 2.4, impacting their detection and mass estimation in tomographic surveys.

Contribution

It investigates the impact of local ionisation variations on Lyman-alpha signatures of protoclusters, highlighting the minimal effect on detection but potential bias in mass estimates.

Findings

Strong local ionisation effects are confined within ~5h^{-1} cMpc of dark matter haloes.

Smoothing over ~4 h^{-1} cMpc minimizes the impact of local ionisation on protocluster detection.

Hot gas can cause systematic underestimation of protocluster masses when calibrating Ly-alpha transmission models.

Abstract

Local variations in the intergalactic medium (IGM) neutral hydrogen fraction will affect the Ly-$\alpha$ absorption signature of protoclusters identified in tomographic surveys. Using the IllustrisTNG simulations, we investigate how the AGN proximity effect and hot, collisionally ionised gas arising from gravitational infall and black hole feedback changes the Ly-$\alpha$ absorption associated with $M_{z=0}\simeq10^{14}\,M_\odot$ protoclusters at $z\simeq2.4$. We find that protocluster galaxy overdensities exhibit a weak anti-correlation with Ly-$\alpha$ transmission in IGM transmission maps, but local HI ionisation enhancements due to hot $T>10^{6}\rm\,K$ gas or nearby AGN can disrupt this relationship within individual protoclusters. On average, however, we find that strong reductions in the IGM neutral fraction are limited to within $\lesssim 5h^{-1}\,\textrm{cMpc}$ of the dark matter haloes. Local ionisation enhancements will therefore have a minimal impact on the completeness of protocluster identification in tomographic surveys if smoothing Ly-$\alpha$ transmission maps over scales of $\sim4 h^{-1}\,\textrm{cMpc}$, as is typically done in observations. However, if calibrating the relationship between the matter density and Ly-$\alpha$ transmission in tomographic maps using simple analytical models for the Ly-$\alpha$ forest opacity, the presence of hot gas around haloes can still result in systematically lower estimates of $M_{z=0}$ for the most massive protoclusters.