|Ph.D Student||Baskin Alexei|
|Subject||Some Constraints on the Physical Properties of the Broad|
Absorption Line and the Broad Emission Line
Regions in Active Galactic Nuclei
|Department||Department of Physics||Supervisor||Professor Ari Laor|
|Full Thesis text|
The most prominent emission features in the optical-UV spectrum of active galactic nuclei (AGN) are the broad emission lines. Despite the large range of AGN luminosity (1039-1048 erg/s), the broad emission lines have similar properties in all AGN. What produces this similarity? In this thesis, I attempt to provide a possible solution. Photoionization inevitably transfers momentum to the photoionized gas. Yet, most of the photoionized gas in the broad-line region (BLR) follows Keplerian orbits, which suggests the BLR is roughly radially static. Thus, the photoionized layer of the gas must develop a pressure gradient due to the incident ionizing radiation. I present solutions for the structure of such a hydrostatic photoionized gas layer in the BLR. The radiation pressure confinement/compression (RPC) of the photoionized layer by the incident radiation leads to a universal ionization parameter U~0.1 in the inner region of the layer, independent of luminosity and distance from the ionizing source. Thus, RPC naturally explains the universality of the BLR properties in AGN.
Approximately 10-20% of AGN present broad absorption lines (BALs) in their UV spectrum, which show a large diversity of absorption profile properties. I study BAL outflows both observationally and theoretically. Observationally, I explore what parameters underlie the diversity of C IV BAL properties, using the Sloan Digital Sky Survey Data Release 7 quasar catalogue. I find that the He II emission equivalent-width (EW) and the continuum slope in the 1700-3000 A range set the BAL properties and the observed fraction of AGN that have BALs. I suggest that a lower He II EW may indicate a softer ionizing continuum, which allows the outflow to reach higher velocities before being over-ionized, and to produce a larger covering factor. A bluer continuum may indicate less inclined systems, where the outflows reach larger velocities due to the higher observed disc luminosity for a disc closer to face-on.
I also explore the theoretical idea that radiation pressure can lead to a large increase in the gas metallicity. I calculate the photon and gas densities which allow decoupling of metal ions from the mostly-H gas, and produce an outflow composed mostly of metals. In an additional study, I apply the RPC model to the BAL absorbing gas. This model naturally explains the small filling factor (f<10-3) of BAL gas in the radial direction, which is the only remaining solution to prevent over-ionization of the BAL outflow.