Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

Abstract

Local variations in the intergalactic medium (IGM) neutral hydrogen fraction will affect the Ly-α 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-α absorption associated with Mz = 0 ≃ 1014 M⊙ protoclusters at z ≃ 2.4. We find that protocluster galaxy overdensities exhibit a weak anti-correlation with Ly-α transmission in IGM transmission maps, but local H$\, \rm \scriptstyle I$ 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 ≲ 5h−1 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-α transmission maps over scales of ∼4h−1 cMpc, as is typically done in observations. However, if calibrating the relationship between the matter density and Ly-α transmission in tomographic maps using simple analytical models for the Ly-α forest opacity, the presence of hot gas around haloes can still result in systematically lower estimates of Mz = 0 for the most massive protoclusters.

Detecting preheating in proto-clusters with Lyman-$α$ Forest Tomography

“…Recently, Miller et al (2021) analyzed proto-clusters in the Illustris TNG100 hydrodynamical simulation and studied the effect of AGN on the thermal and ionisation properties. They found that the AGN affects the Lyα transmission within ∼ 1 h −1 Mpc of the central halo at z ∼ 2.4, but the effects are minor when smoothed over the ≥ 4 h −1 Mpc scales typically used to search for proto-clusters in the Lyα forest.…”

A population of ultraviolet-dim protoclusters detected in absorption

“…This is expected to have a mild effect on the protocluster signal in most cases. 48 More importantly, it can only reduce the observed absorption, and thus cannot account for our main result, the low galaxy density observed around the strongest absorption peaks. The same comment applies to any "pre-heating" of the intracluster medium 49 beyond the feedback implemented in TNG.…”

Characterizing Protoclusters and Protogroups at z ∼ 2.5 Using Lyα Tomography

“…This requires radiative transfer simulations, which are computationally demanding. However, Miller et al (2021) placed an approximate bound on this effect by sampling the AGN luminosity function and assigning AGNs to massive halos in a one-to-one rank order fashion (implying a duty cycle of one). They conclude local ionization usually has a small effect on the Lyα absorption observed in tomographic maps, even within massive protoclusters.…”