Obscuration of Quasars by Dust and the Reddening Mechanism in   Parkes-Quasars
  A majority of quasar surveys have been based on criteria which assume strong blue continua or a UV-excess. Any amount of dust along the line-of-sight is expected to drastically extinguish the optical/UV flux leading to a selection bias. Radio surveys however should suffer no bias against extinction by dust. Recently, a large complete sample of radio-selected quasars has become available (the `Parkes sample'). A majority of these sources exhibit optical--to--near-infrared continua that are exceedingly `red', very unlike those of quasars selected optically. The purpose of this thesis, broadly speaking, is to explore the problem of incompleteness in optical quasar surveys due to obscuration by dust, and to interpret the relatively `red' continua observed in the Parkes quasar sample. The first part of this thesis explores the observational consequences of an intervening (foreground) cosmological dust component, such as that located in galaxies and clusters. The second part examines the continuum properties of Parkes-quasars in the framework of a number of absorption and emission mechanisms to assess the importance of extinction by dust. It is found that spectroscopic and photometric data for the ``reddest'' quasars (with B-K>5) is best explained by the dust model, rather than ``purely'' by an intrinsic emission property. 
Frank J. Masci