1997PASP..109.1191S

begin{tex}This dissertation presents the results wherein a total of 48 southern and equatorial extragalactic radio sources were investigated with two sessions of a snap-shot VLBI imaging survey at 5∼GHz. The survey was the first of its kind in the southern hemisphere and filled a gap in VLBI observations of southern radio sources. The fringes were detected for 45 sources, and images were obtained for 43. Of these 43 sources imaged, 21 had not previously been mapped with VLBI at milliarcsecond scale. Based on these observations and upon other published results, a thorough description of structures was presented for each source. Superluminal motion was confirmed and/or inferred in 6 sources by comparing with previous maps and the temporal variability of the radio flux density. No significant differences in the source morphologies were found between the survey and northern hemisphere VLBI surveys (e.g., Pearson & Readhead survey). The bending of the jet from milliarcsecond to arcsecond scales seems to be a common feature in blazars. A statistical comparison with γ–ray detection suggests the dominance of blazars among the detected γ–ray sources; this might imply the importance of the beaming effect in the γ–ray emission. This dissertation also describes the study of a southern millimeter bright blazar, OV–236, through multi–frequency, multi–epoch VLBI observations. An analysis of the observational data reveals, for the first time, that a bent, superluminal (β_app~≃2.08) subparsec–scale jet exists in OV–236, which substantiates other evidence of an enhanced jet–curvature in the vicinity of the core. There is a large difference (75^○) in the position angle between the parsec and subparsec scale jet. The limb brightening, seen at the east end of parsec–scale jet and also at the west end of the core, may signify an interaction of the source with ambient media, further suggesting that the bending of the subparsec–scale jet might be caused by such an effect and that a similar bending might occur within the parsec–scale jet. There is probably a correlation between the flux density outbursts and the genesis of new components. Finally, some suggestions are made for the future research. In brief, these are: 1) Survey the remaining sources to form a complete sample for statistical analysis; and 2) Conduct follow–up observations of some particularly interesting sources, such as superluminal candidates, compact steep spectrum sources and γ–ray–loud blazars, to obtain detailed information on structural evolution. A proposal entitled ``Multi–Frequency High–Resolution Imaging Observations of OV–236'' was submitted to the VLBI Space Observatory Programme (VSOP) and has been selected to be part of the VSOP Key Science Program. The aim is to obtain the spectrum and to derive the distributions of relativistic electrons and the magnetic field as well as to study the fine structure within the curved superluminal jet.