Ph.D Thesis

Ph.D StudentBaskin Alexei
SubjectSome Constraints on the Physical Properties of the Broad
Absorption Line and the Broad Emission Line
Regions in Active Galactic Nuclei
DepartmentDepartment of Physics
Supervisor PROF. Ari Laor
Full Thesis textFull thesis text - English Version


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.