Details

Autor(en) / Beteiligte

• Robinson, Andrew

Titel

A Study of Emission Line Variability in Active Galactic Nuclei

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1985

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• This work is a detailed study of the time variations which have been observed in the emission lines of Type 1 Seyfert Nuclei and the related Broad Line Radio Galaxies. The existing observations of these phenomena are extensively reviewed and it is concluded that such variations are common in these objects, affect mainly the lines emitted in the high density, high velocity dispersion broad line region (BLR) and are closely related to the corresponding variations in the power law continuum emitted by the central energy source. In the light of the observational evidence, the most probable cause of the emission line variations is time dependent photoionization of the BLR gas by a variable UV continuum. In this case, the line variations potentially convey much important information on the dimensions, structure and velocity field of the BLR. Therefore, in order to confirm the causal link between the continuum and emission line variations and also to provide a theoretical basis for the interpretation of the observations, a computer model of the BLR has been developed. This model involves a simplified time dependent photoionization calculation which includes the effect of light travel times and velocity fields in a three dimensional model BLR. Calculations for the general case, using parameters typical of type 1 Seyfert galaxies, show that the observed characteristics of the emission line variations can be explained by time dependent photoionization by a central source, combined with light travel time effects in the BLR. However, the line response is sensitive to many properties of the active nucleus and so model fitting to observations of individual objects is a more promising method of investigating the BLR. Hence, various multi-component models of the BLR of NGC 4151, the most intensively studied Sy 1, have also been constructed. It is found that a small, inner component, within 20 light days of the central energy source, is required in order to explain the observed variations of the CIV A1549 line. The changes in the profile of this line suggest that the velocity field of this component is either rotational or completely random. A general consequence of this phenomenon, which has implications for other studies of Seyfert galaxies, is that the emission line spectra of these objects are time dependent, perhaps even to the extent of affecting their classification as type 1 or type 2 nuclei.