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

TL;DR

This study uses VLBI data of over 3000 extragalactic radio sources to analyze AGN core sizes and interstellar scattering, revealing Galactic plane scattering effects and confirming synchrotron jet models.

Contribution

It provides a comprehensive analysis of AGN core sizes, scattering effects, and their Galactic dependencies using extensive archival VLBI data, with new insights into scattering regions and core size scaling.

Findings

Significant scattering observed near Galactic plane regions.

Scattering strength correlates with Hα intensity, electron density, and rotation measure.

AGN core size scales as ν^{-1}, consistent with synchrotron jet models.

Abstract

We have measured the angular sizes of radio cores of active galactic nuclei (AGN) and analyzed their sky distributions and frequency dependencies 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 ($|b|\lesssim10^\circ$) at 2, 5 and 8 GHz, about 1/3 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 $220^\circ\lesssim l\lesssim260^\circ$ (the Fitzgerald window). The strength of interstellar scattering of the AGNs is found to correlate with the Galactic H$\alpha$ 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$^\ast$ is found to be the largest among thousands of AGN observed over the sky; we discuss possible reasons of this strange result. Excluding extragalactic radio sources with significant scattering, we find that angular size of opaque cores in AGN scales typically as $\nu^{-1}$ confirming predictions of a conical synchrotron jet model with equipartition.