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We have measured the angular sizes of radio cores of active galactic nuclei(AGN) and analyzed their sky distributions and frequency dependencies to studysynchrotron opacity in AGN jets and the strength of angular broadening in theinterstellar medium. We have used archival very long baseline interferometry(VLBI) data of more than 3000 compact extragalactic radio sources observed atfrequencies, $\nu$, from 2 to 43 GHz to measure the observed angular size ofVLBI cores. We have found a significant increase in the angular sizes of theextragalactic sources seen through the Galactic plane ($|b|\lesssim10^\circ$)at 2, 5 and 8 GHz, about 1/3 of which show significant scattering. Thesesources are mainly detected in directions to the Galactic bar, the Cygnusregion, and a region with galactic longitudes $220^\circ\lesssiml\lesssim260^\circ$ (the Fitzgerald window). The strength of interstellarscattering of the AGNs is found to correlate with the Galactic H$\alpha$intensity, free-electron density, and Galactic rotation measure. The dependenceof scattering strengths on source redshift is insignificant, suggesting thatthe dominant scattering screens are located in our Galaxy. The observed angularsize of Sgr A$^\ast$ is found to be the largest among thousands of AGN observedover the sky; we discuss possible reasons of this strange result. Excludingextragalactic radio sources with significant scattering, we find that angularsize of opaque cores in AGN scales typically as $\nu^{-1}$ confirmingpredictions of a conical synchrotron jet model with equipartition.

monthly notices of the royal astronomical society

Milky Way scattering properties and intrinsic sizes of active galactic  nuclei cores probed by very long baseline interferometry surveys of compact  extragalactic radio sources