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

We have measured the angular sizes of radio cores of active galactic nuclei (AGNs) and analysed their sky distributions and frequency dependences to study synchrotron opacity in AGN jets and the strength of angular broadening in the interstellar medium. We have used archival very long baseline interferometry (VLBI) data of more than 3000 compact extragalactic radio sources observed at frequencies, nu, from 2 to 43 GHz to measure the observed angular size of VLBI cores. We have found a significant increase in the angular sizes of the extragalactic sources seen through the Galactic plane (vertical bar b vertical bar less than or similar to 10 degrees) at 2, 5 and 8 GHz, about one-third of which show significant scattering. These sources are mainly detected in directions to the Galactic bar, the Cygnus region and a region with galactic longitudes 220 degrees less than or similar to l less than or similar to 260 degrees (the Fitzgerald window). The strength of interstellar scattering of the AGNs is found to correlate with the Galactic Ha intensity, free-electron density and Galactic rotation measure. The dependence of scattering strengths on source redshift is insignificant, suggesting that the dominant scattering screens are located in our Galaxy. The observed angular size of Sgr A* is found to be the largest among thousands of AGNs observed over the sky; we discuss possible reasons for this strange result. Excluding extragalactic radio sources with significant scattering, we find that the angular size of opaque cores in AGNs scales typically as nu(-1), confirming predictions of a conical synchrotron jet model with equipartition.