Searching for the shadows of giants II: the effect of local ionisation on the Lyman-$\alpha$ absorption signatures of protoclusters at redshift $z\sim 2.4$

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 $M_{z=0}\simeq 10^{14}{M}_{\odot }$ protoclusters at $z\simeq 2.4$. 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 $T>10^6\mathrm{K}$ 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 $\lesssim 5h^{-1}\text{cMpc}$ 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 $\sim 4h^{-1}\text{cMpc}$, 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.

14 pages (including 1 appendix), 5 figures (plus 2 in the appendix), 1 table. Accepted for publication in MNRAS

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• Characterizing Protoclusters and Protogroups at z $\sim$ 2.5 Using Ly$\alpha$ Tomography
• Observational Evidence for Large-Scale Gas Heating in a Galaxy Protocluster at z=2.30
• Detecting preheating in proto-clusters with Lyman-$\alpha$ Forest Tomography
• LATIS: A Sample of IGM-selected Protoclusters and Protogroups at $z\sim 2.5$
• LATIS: Comparing Galaxy and IGM Tomography Maps as Tracers of Large-scale Structure and Protoclusters at $z\sim 2.5$
• Lyα Intensity Mapping in HETDEX: Galaxy-Lyα Intensity Cross-Power Spectrum