Tracing Milky Way scattering by compact extragalactic radio sources
  We used archival very long baseline interferometry (VLBI) data of active
galactic nuclei (AGN) observed from 1.4 GHz to 86 GHz to measure the angular
size of VLBI radio cores in 9525 AGNs. We analysed their sky distributions,
frequency dependencies and created the distribution map of large-scale
scattering properties of the interstellar medium in our Galaxy for the first
time ever. Significant angular broadening of the measured AGN core sizes is
detected for the sources seen through the Galactic plane, and this effect is
especially strong at low frequencies (e.g. at 2 GHz). The scattering screens
containing electron density fluctuations of hot plasma are mainly concentrated
in the Galactic plane and manifest clumpy distribution. The region of the
strongest scattering is the Galactic centre, where the Galactic bar and the
compact radio source Sagittarius A* are located. We have also found the
enhancement of scattering strength in regions of the Cygnus constellation,
supernova remnants Taurus A, Vela, W78 and Cassiopeia A, and the Orion Nebula.
Using multi-frequency observational data of AGN core sizes, we separated the
contribution of the intrinsic and scattered sizes to the measured angular
diameter for 1546 sources. For the sources observed through the Galactic plane,
the contribution of the scattered size component is systematically larger than
for those seen outside the Galactic plane. The derived power-law scattering
indices are found to be in a good agreement with theoretical predictions for
the diffractive dominated scattering of radio emission in a hot turbulent
plasma.
