Searching for the shadows of giants: the absorption signatures of protoclusters in the Lyman-alpha forest

Miller, Joel Sebastian Arthur (2022) Searching for the shadows of giants: the absorption signatures of protoclusters in the Lyman-alpha forest. PhD thesis, University of Nottingham.

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In this thesis, I use large scale state-of-the-art hydrodynamical cosmological simulations to investigate the Ly-α absorption properties of protocluster regions at redshift z∼2.4. Using the IllustrisTNG simulations, I investigate how the AGNproximity effect and hot, collisionally ionised gas arising from gravitational infall and black hole feedback changes the Ly-α absorption associated with the z≃2.4 progenitors of M≥10^14 M_☉ clusters. Within these protoclusters, I find that galaxy overdensities exhibit a weak anti-correlation with Ly-α transmission in tomographic maps of the intergalactic medium (IGM), but local HI ionisation enhancements due to hot T>10^6 K gas or nearby AGN can disrupt this relationship within individual protoclusters. On average, however, I find that strong reductions in the IGM neutral fraction are limited to within ≲5 h^−1 cMpc of the dark matter haloes. Local ionisation enhancements will therefore have a limited impact on the completeness of protocluster identification in tomographic surveys if smoothing Ly-α transmission maps over scales of ∼4 h^−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 may result in systematically lower estimates of the z=0 descendant mass of the most massive protoclusters.

I then use simulations from the Sherwood, EAGLE and Illustris projects to examine the Ly-α absorption signatures of z∼2.4 protoclusters with a descendant mass of M≥10^14 M_☉. I find there is a weak correlation between the mass overdensity, δ_m, and the Ly-α effective optical depth relative to the mean, δ_τeff, averaged over 15 h^−1 cMpc scales; however, the scatter in the δ_m–δ_τeff plane means it is not possible to uniquely identify large scale overdensities with strong Ly-α absorption. Assuming perfect removal of contamination by Ly-α absorbers with damping wings, more than half of the remaining sightlines with δ_τeff>3.5 (CoSLAs) trace protoclusters. It is furthermore possible to identify a model dependent δ_τeff threshold that eliminates the contamination from filamentary structure and Lyman-limit systems that are preferentially aligned along the line of sight. However, such regions are rare: excluding absorption caused by damped systems, less than 0.1% of sightlines that pass through a protocluster have δ_τeff>3.5, meaning that any protocluster sample selected in this manner will also be highly incomplete.

Finally, I use the TNG300-1 simulation from the IllustrisTNG project to study the extent to which simulation box size—and the resulting presence of the most massive protoclusters—is responsible for the differences between the CoSLA populations observed in large N-body simulations and those in comparatively small hydrodynamical simulations. Using the better statistics afforded by the larger simulation volume, I confirm that CoSLAs are rare objects, finding a number of CoSLAs per unit redshift dn/dz∼1×10^−2 at redshift z=2.444. Furthermore, I find the δτeff–δm relation in a 205^3 h^−3 Mpc^3 volume is consistent with that for a smaller 75^3 h^−3 Mpc^3 box size, with systems with δ_m>1 existing over a wide range of δ_τeff values. I find the CoSLAs in TNG300-1 to have a median δ_m=0.45±0.05, which remains somewhat lower than the δ_m=0.64±0.38 previously found in larger N-body simulations. These findings suggest that the differences between the CoSLA populations in hydrodynamical and N-body simulations are not solely the result of the differing simulation box sizes. Instead I suggest that these discrepancies are caused by the modelling of the hot, collisionally ionised, gas that surrounds massive haloes in protocluster regions.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Bolton, James S.
Hatch, Nina A.
Keywords: Protoclusters, Lyman-alpha forest, IGM, Cosmological simulations
Subjects: Q Science > QB Astronomy
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 67235
Depositing User: Miller, Joel
Date Deposited: 31 Jul 2022 04:40
Last Modified: 31 Jul 2022 04:40

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