Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei

Abstract

We have used the Very Long Baseline Array at 15 GHz to image the structure of 132 strong compact active galactic nuclei (AGNs) and quasars with a resolution better than 1 mas and a dynamic range typically exceeding 1000:1. These observations were made as part of a program to investigate the subparsec structure of quasars and AGNs and to study the changes in their structure with time. Many of the sources included in our study, particularly those located south of declination +35?, have not been previously imaged with milliarcsecond resolution. Each of the sources has been observed at multiple epochs. In this paper, we show images of each of the 132 sources that we have observed. For each source we present data at the epoch that had the best quality data. In a future paper we will discuss the kinematics derived from the observations at all epochs. Most of the sources we have observed show the canonical core-jet morphology with structure somewhat characteristic of the jets seen on arcsecond scales with the Very Large Array and Westerbork telescopes. The milliarcsecond jets generally appear one-sided, but two-sided structure is often found in lower luminosity radio galaxies and in high-luminosity quasars with gigahertz-peaked spectra. In many cases there is significant curvature, sometimes up to 90? or more, particularly close to the core. In other cases the jets have a more gradual curvature. In some sources there are multiple bends or twists along the jet, suggestive of a three-dimensional curved structure. Many of the jets may be described by a small number of apparently discrete components, but in other cases there appears to be a monotonically decreasing distribution of radio emission. Usually the structure is unresolved along the direction perpendicular to the jet, but a few sources have broad plumes. Much of the visible parsec-scale structure in compact radio sources can probably be explained as the projection of a relativistic beamed, twisted jet, which appears bright at those positions where it approaches the viewer. In some low-luminosity radio galaxies, the structure appears more symmetric at 2 cm than at longer wavelengths. The apparent long-wavelength asymmetry in these sources is probably due to absorption by intervening ionized material. A few sources contain only a single component, with any secondary feature at least 1000 times weaker. Peak rest-frame brightness temperatures are typically of the order of 1011?1012 K, with no evidence for any excess over the limit of 1012 K expected from inverse Compton cooling. We find no obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.