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UBC Theses and Dissertations

A study of unified models of radio-loud active galactic nuclei through radio source evolution Xu, Lienong


The study of Active Galactic Nuclei (AGN) unified models attempts to unite quasi-stellar objects (QSOs) and radio galaxies (RGs) through two orienta- tion effects: (1) from twin-jet relativistic ejection of synchrotron radiating plasma along an axis; and from (2) a central dusty torus aligned at 90⁰ to the axis and hiding the black-hole (BH) accretion disk cone from side-on viewing. Random orientation of the axis to our line of sight provides such orientation-dependent appearances that QSOs and RGs were originally clas- sified as entirely different. However, if unified models are correct, we gain deeper theoretical understanding of the interplay among AGN, black-holes and host galaxy properties. This should provide an answer to what causes the origin and diversity of AGN populations. The subset of AGN populations with powerful radio frequency emission is called radio-loud (RL) AGN. The RL AGN unification scheme claims that powerful radio galaxies are the parent population of RL QSOs or BL Lac objects. RL AGN unified models cannot ignore the impact of strong cosmic evolution of powerful radio sources. Moreover, radio-source counts of the populations differ at varying radio frequencies. Detailed modelling of the radio source evolution and radio luminosity function must be ac- counted for by cosmic evolution in order to properly interpret any unified model. Preliminary investigations of new techniques to incorporate updated data are described here. These include both evolution-model tests and tests on directly constructing the local radio luminosity function from new low- frequency data. Despite some successes, the subject of radio-loud AGN unified mod- elling remains active, since there are still unresolved issues, for example, contradictory results are revealed in existing samples that compare the pro- jected linear sizes of radio galaxies with radio-loud QSOs. Whether this is merely the result of selection effects or there are complications in radio-loud unified models remains unclear. The surface-brightness sensitivity of new- generation low-frequency radio telescopes (e.g. the Murchison Wide Field Array, MWA) may shed light on resolving these discrepancies.

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