QSO Absorption Lines and the Ionizing Field at High Redshifts

Citation

Zuo, Lin (1992) QSO Absorption Lines and the Ionizing Field at High Redshifts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vsk7-nv30. https://resolver.caltech.edu/CaltechETD:etd-09232008-081252

Abstract

In this thesis we explore the relationship between the QSO absorption line systems and the metagalactic ionizing field at high redshifts.

In the first introductory chapter we describe the recent developments in the field of QSO absorption lines and the ionizing radiation background. We concentrate on compiling the new observational results and address why studies of QSO absorption line systems and ionizing field are important to our understandings of the formation and evolution of the large scale structure of the universe.

In the second chapter Markoff's method has been used to derive a general formalism to deal with the absorptions produced by randomly distributed discrete clouds, such as the QSO absorption line systems. Some analytical forms are obtained for the effective optical depth τ_eff, the count reduction factor f_c, and the optical depth probability distribution function P(τ). We demonstrate that the spectrum of ionizing background is very different from the intrinsic source spectrum. We calculate the QSO contributions to J_νL(Z_obs) by using simple analytical expressions. We show that because of the Lyman continuum absorption produced by QSO absorption line systems, it is very difficult to find a "clear" line of sight to conduct the Hell Gunn-Peterson test. We have also found that dust grains in the QSO damped Lyα systems produce a marginally significant obscuration for z = 3 quasars: the count reduction factor is 1⁄1.7 at z = 3. The reddening is shown to be small for a flux limited QSO sample.

In the following two chapters we discuss fluctuations and intensity correlation in the ionizing field. We derive the intensity probability distribution function P(J) for randomly distributed point sources. We show that absorptions by QSO absorption line systems reduce the total number of sources involved in producing the ionizing background and therefore enhance the fluctuation significantly, if QSOs are the main ionizing sources. We have calculated the intensity correlation function ξ_j for randomly distributed QSOs. The QSO Lyα clouds can be used as intensity indicators to reveal the intensity correlation at high redshifts. We have measured the equivalent width correlation function ξ_1⁄w for several selected QSOs and have found, in some cases, strong correlation signals at small separations. Careful examination shows that such signals are mainly generated by the lines near the QSO emission redshifts. One explanation is that the high S⁄N near QSO emission red-shifts enable us to detect very weak lines which result in the correlation signal. The other explanation is that the correlated intensities of ionizing field near QSOs have caused the observed equivalent width correlation. If this latter explanation is correct, from the affected range by QSOs we conclude that J_νL is less than 10^-21 ergs s^-1cm^-2Hz^-1sr^-1 at z ~ 3.5.

The last two chapters deal with the ionization structure of the QSO Lyα clouds. We solve the coupled ionization balance and radiation transfer problem (the "inverse HII region" problem) for the non-uniform spherical absorbing clouds and calculate the HI column density distribution f(N). We show that with an appropriate density gradient in the clouds we can reproduce the observed overall power law distribution and the apparent excess of absorption systems with N ≥ 2 x 10²⁰cm^-2. The calculated f(N) is not sensitive to the input ionizing spectra and is a generic feature of the density profile. In our model all QSO absorption lines have the same origin: the ionized outer envelopes produce the Lyα forest lines while the neutral cores result in damped Lyα systems. We discuss the consequences of such models and propose star-forming dwarf galaxies as primary candidates for QSO absorption line systems. Our calculations also suggest that uniform cloud models are highly unlikely. To calculate the absorptions produced by HeI and HeII in QSO Lyα clouds we also solve the ionization structure for the clouds containing both H and He. We explore the variation of HeI and HeII distributions under different input ionizing spectra and discuss how to get a self-consistent spectral shape for the ionizing background.

Thesis Files