Optical spectral properties of active galactic nuclei and quasars

by Yee, Howard Kwong

Abstract (Summary)
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Four separate investigations dealing with the properties of optical continuum and emission-lines of active galactic nuclei (AGN) and quasars are presented. Multichannel scans of 3CR radio galaxies are decomposed by using a two-component model -- an elliptical galaxy and a power-law nonthermal component. It is found that there is a strong correlation between the luminosity of the power-law component and the strength of the Balmer emission-lines. In most cases, by extrapolating to the Lyman continuum, the power-law models derived provide enough ionizing radiation to account for the Balmer line strengths. Extending the study of radio galaxies to include Seyfert galaxies and quasars, it is found that there is a strong continuity between broad-line AGN's and quasars in terms of similarities in the correlations between line luminosities and nonthermal continuum luminosity. There is a tight correlation between Hβ and nonthermal luminosity for all objects, suggesting that the Balmer lines arise from recombination after photoionization by the central continuum, and that parameters such as the overall spectral index and the covering factor are similar. Relative to the nonthermal continuum, the forbidden lines of the broad-line objects are approximately one order of magnitude weaker than those of the narrow-line objects. This is consistent with the view that forbidden lines are suppressed in the dense broad-line region of the broad-line objects. Next, a study of the variability of absolute optical energy distribution and emission-lines of the N-galaxies 3C 382 and 3C 390.3 is made. By assuming that the nuclear component retains its spectral shape despite the variation, consistent decomposition of the spectrum into its nuclear and ellitipcial galaxy component can be made. Within each object, the narrow forbidden lines have remained constant, but the broad Balmer lines vary with the luminosity of the central component (L[subscript NT]) as a power-law: L[...] with [Gamma]0.5. This relation can be explained by having two components of broad-line gas clouds â one having a time-scale for variability of a few months, the other, years. Variability of Balmer line profiles in these two objects are also discussed. Lastly, a preliminary study of surface photometry of Markarian Seyfert galaxies are presented. It is found that the properties of the underlying galaxies such as scale-length and surface brightness of the disk, color, and total brightness, do not depart systematically from those of luminous normal spiral galaxies. In addition, along with the results from the study of radio galaxies, it is found that there is no correlation between the luminosity of nuclear component and the brightness of the host-galaxy.
Bibliographical Information:

Advisor:J. Beverley Oke

School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis



Date of Publication:10/30/1980

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