As the universe evolved from a hot, nearly homogeneous plasma, regions of higher density formed galaxies and quasars, which filled the intervening space with light of all wavelengths, including ultra-violet. At some point, this ultra-violet background reionized the gas between the sources, but not entirely, as absorption by neutral gas has been observed along the line of sight to distant quasars. The interplay of the ultra-violet background with its sources and its absorbers is the topic of this thesis. There are three main problems addressed.
The first question concerns the type of sources, which can loosely be divided between AGN (the class that includes quasars) and normal galaxies. Recent observations have shown a drop in the number of bright AGN, but good constraints on the density of faint AGN do not yet exist. For this reason, the population of faint AGN have been modeled such that these as-yet-unobserved AGN account for the necessary ultra-violet emission. There are two main uncertainties in this calculation. First, a discrepancy exists between the ultra-violet background measurements that come from the proximity effect around quasars and those that simulate the absorbers from the underlying matter distribution. Second, the ionizing emission from normal galaxies could be larger than previously thought. In comparing the models to other data, the contribution of AGN and normal galaxies to the ultra-violet background is found to be roughly comparable.
The second problem in some ways follows from the first in that the proximity effect is used to study properties of AGN themselves. Specifically, the absorption in the vicinity of three AGNs has been observed, with the surprising result that the gas shows no sign of the expected AGN radiation. The possible implication is that AGN shine in narrow beams, or that they shine for only a million years.
Lastly, the ultra-violet background is investigated through its own absorption of gamma rays. The findings suggest that observations at 10GeV may reveal what sources reionized the universe in the first place.
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/09(E), Dissertation Abstracts International|
|Keywords:||Active galactic nuclei, Escape fraction, Gamma ray attenuation, Proximity effect, Qso lifetime, Ultra-violet background|
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