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Title: Exploring the ionization state of the intergalactic medium with Lyman-α absorption
Author: Bolton, J. S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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Within the redshift range 2 ≤ z ≤ 4, a substantial contribution to the UV background at 1 Ryd from galaxies is required. Quasars dominate the ionizing photon emission at 4 Ryd. A significant contribution to the UV background at 4 Ryd by thermal emission from hot gas in galaxies and galaxy groups appears unlikely. The small spatial variance expected for a UV background dominated by ionizing emission from hot gas cannot be reconciled with the large fluctuations observed in the ratio of He II to H I column densities in the Lyα forest. it is shown that these large column density ratio fluctuations are expected if, at each point in space, only a small number of quasars contribute to the UV background at 4 Ryd. This is expected at the tail-end of the He II ionizing photons is similar to the average separation between quasars. At higher redshifts, z ≃ 6, the almost complete attenuation of quasar spectra blueward of the Lyα emission line indicates that the neural hydrogen fraction in the IGM is increasing with lookback time. Radiative transfer simulations are used to perform an in depth examination of the small regions of high transmissivity immediately blueward of the Lyα emission lines, produced by quasar emission. The sizes of these highly ionized near-zones cannot be used to reliably constrain the ionization state of the hydrogen in the surrounding IGM; the near-zone sizes provide no clear evidence for the tail-end of H I reionization occurring just above z = 6. The relative sizes of the Lyα and Lyβ near-zones, as well as the widths of the absorption features within the Lyα near-zones, should provide more sensitive measurements of the IGM neutral hydrogen fraction at these redshifts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available