Abstract
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 ionized gas arising from gravitational infall and black hole feedback changes the Ly α absorption associated with $M_{z=0}\simeq 10^{14}\, {\rm M}_\odot$ protoclusters at z ≃ 2.4. We find that protocluster galaxy overdensities exhibit a weak anticorrelation with Ly α transmission in IGM transmission maps, but local H$\, \rm \scriptstyle I$ ionization enhancements due to hot $T\gt 10^{6}\rm \, K$ 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 ionization 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 $\sim 4 h^{-1}\, \textrm {cMpc}$, 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.
Highlights
A fundamental prediction of Lambda cold dark matter ( CDM) cosmogonies is that galaxy clusters are built from the assembly of lower mass haloes such as galaxy groups and isolated galaxies (e.g. White & Frenk 1991)
We have investigated the effect that local ionization variations in the intergalactic medium (IGM), due the proximity effect from AGN and hot, T > 106 K gas from shocks and AGN feedback, have on the Ly α absorption signature of protoclusters in the IllustrisTNG simulations at redshift z 2.4
We consider three different local ionization models in our analysis: a fiducial model with a spatially uniform UV background model, a second model that ignores the effect of collisional ionization and self-shielding on the H I fraction in the IGM, and final model where we incorporate spatial variations in the UV background due to the proxmity effect from AGN
Summary
A fundamental prediction of Lambda cold dark matter ( CDM) cosmogonies is that galaxy clusters are built from the assembly of lower mass haloes such as galaxy groups and isolated galaxies (e.g. White & Frenk 1991). A large sample of QSO sightlines can be used for this purpose (see e.g. Ravoux et al 2020), but recent observational advances have allowed spectra from background star-forming galaxies to be used, providing the high density of sight-lines needed to reconstruct the Ly α transmission on scales of a few comoving Mpc (Lee et al 2016, 2018; Newman et al 2020; Mukae et al 2020a). This approach has been successfully used to locate dense structures in the early Universe – some of which are expected to be protoclusters. Throughout this paper, we refer to comoving distance units using the prefix ‘c’ and to proper distance units using the prefix ‘p’
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