Investigation of Milliarcsecond‐Scale Radio Structure in Active Galactic Nuclei

Abstract

As the most powerful engines in the universe, active galactic nuclei (AGNs) have been studied extensively with very long baseline interferometry (VLBI). Brief background information on AGNs, their observed properties and classification, and the VLBI technique used for observations and data reduction is presented in chapter 1. In chapter 2, Very Long Baseline Array (VLBA) observations of a sample of 90 compact extragalactic radio sources at 15 GHz are presented. Most of the sources in the sample are selected from the AGNs that have been imaged with the VLBA alone at 6 cm as part of the VSOP prelaunch survey (E. B. Fomalont et al. 2000, ApJS, 131, 95). These observations can extend the study for the kinematics of AGNs; provide the useful information for the jet properties on parsec scale such as jet curvature, parsec/kiloparsec alignment, and morphology; and test various unification models of AGNs. The obtained total intensity images of the 90 sources are displayed in this chapter. The model fit results and brief comments on individual sources are also presented. From the images, most of the sources in the sample show asymmetric core-jet structures on milliarcsecond scales. The range of core and jet strengths, jet lengths, and bend is wide. Structural changes and superluminal motion are also detected in some sources. Statistical study finds that the jet and core spectral indices of the compact radio sources do not change apparently over the large redshift range. Furthermore, the two-epoch observations of 24 sources in the sample provide a rough test for the relation of angular velocity versus redshift (m-z). For sources whose redshifts are lower than 3, the angular velocities of secondary components of the sources decrease with increasing redshifts, which is in agreement with the standard cosmological model. With their high resolution, VLBI polarimetry studies on AGNs have given numerous insights into magnetic-field morphology, physical conditions in the inner regions of AGNs, and the conditions in the narrow-line regions of such sources through the investigation of Faraday rotation. Polarimetry observations for quasar 3C 147 with VLBA are the focus of chapter 3. The derived VLBI total intensity structure of 3C 147 can been divided into two parts: the complex core region (head of the jet) and the more or less collimated jet. The jet extending to the southwest in structural position angle∼ 130 is followed out to a distance of about 200 mas from the core. The jet shows several gentle wiggles and turns sharply toward the north near the end of its detected length. Polarization of this source was detected in one bright feature in the inner jet. The rotation measure (RM) of this feature (∼ 1300 rad m ) agrees with 2