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A High-Resolution Study of the High-Redshift Intergalactic Medium


Becker, George Dana (2007) A High-Resolution Study of the High-Redshift Intergalactic Medium. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TND7-F429.


We present a study of the structure and evolution of the high-redshift intergalactic medium (IGM) using a variety of approaches based on high-resolution quasar spectra. We first use the cross-correlation of transmitted flux towards two pairs of quasars with moderate angular separation to demonstrate that filamentary structures in the IGM at z = 4 are partially coherent on roughly 1 Mpc scales. We then measure the ionization state of the high-redshift IGM using three techniques: a search for low-ionization metal lines at z > 5, an analysis of the evolution of Lyman-alpha optical depths over 2 < z < 6, and a measurement of the UV background at 4 < z < 5 using the quasar proximity effect. We identify an excess of O I systems towards the z = 6.4 quasar SDSS J1148+5251, but do not detect a "forest" of O I lines elsewhere that would strongly indicate the end of reionization at z = 6. Using a large sample of quasars spanning 2.0 < z < 6.4, we show that a standard Lyman-alpha optical depth distribution that assumes an isothermal IGM and uniform ionizing background produces poor fits to the observed flux distributions unless large corrections are made to the quasar continua. In contrast, an empirical lognormal optical depth distribution fits the data at all redshifts. A simple evolution of the lognormal distribution accounts for the observed mean Lyman-alpha and Lyman-beta transmitted fluxes over 1.6 < z < 6.2, which further suggests that no sudden change in the intergalactic medium such as reionization is occurring at z = 6. We measure the metagalactic UV background at 4 < z < 5 using the quasar proximity effect in a sample of 16 quasars. We combine accurate redshifts from Mg II emission and other indicators with a new method for estimating the size of a quasar's proximity region to demonstrate that the relative sizes of the proximity regions are generally consistent with the expected trend in quasar luminosity. Our estimate of the background H I ionization rate at z = 4-5 is consistent with little or no evolution from z > 2.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:intergalactic medium; quasar absorption lines; quasars; reionization
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Sargent, Wallace L. W.
Group:Astronomy Department
Thesis Committee:
  • Steidel, Charles C. (chair)
  • Hillenbrand, Lynne A.
  • Rauch, Michael
  • Ellis, Richard S.
  • Phinney, E. Sterl
  • Sargent, Wallace L. W.
Defense Date:24 July 2006
Non-Caltech Author Email:george.becker (AT)
Record Number:CaltechETD:etd-05312007-141523
Persistent URL:
Becker, George Dana0000-0003-2344-263X
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2340
Deposited By: Imported from ETD-db
Deposited On:01 Jun 2007
Last Modified:30 May 2023 22:32

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PDF (gdb_thesis_double.pdf) - Final Version
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