Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure

We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources. With projected baselines out to 440 million wavelengths we are able to investigate source structure on typical angular scales as small as 0. 05 mas. For 171 sources in our sample, more than half of the flux density seen by the VLBA remains unresolved on the longest baselines. The 163 sources in our list with median 15 GHz correlated flux density in excess of 0. 5 Jy on these long baselines will be useful as fringe-finders for short wavelength VLBA observations. We have modeled the core of each source as an elliptical Gaussian component. For about 60% of the sources, at least at one epoch, the core component appears unresolved generally along a direction transverse to the jet direction. These core components are usually less than 0. 05 mas in size. IDV sources show, in general, more compact and core dominant structure on sub-milliarcsecond scales that non IDV sources; IDVs with higher amplitude of intraday variations tend to have higher unresolved VLBA flux density. The most variable sources tend to have the most compact VLBA structure. The gamma-ray loud AGN appear to have more compact VLBA structure than the gamma-ray quiet AGN. This suggests that the mechanism of radio emission of compact components and the mechanism of gamma-ray emission are related. Typical estimates or lower limits to the brightness temperature of the jet cores are in the range of 10¹1 to 10¹3 K but extend up to 5x10¹3 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed only in radio sources with high observed brightness temperatures as would be expected from relativistic beaming (abridged).