VLBI-Gaia offsets favor parsec-scale jet direction in Active Galactic
  Nuclei
  The data release 1 (DR1) of milliarcsecond-scale accurate optical positions
of stars and galaxies was recently published by the space mission Gaia. We
study the offsets of highly accurate absolute radio (very long baseline
interferometry, VLBI) and optical positions of active galactic nuclei (AGN) to
see whether or not a signature of wavelength-dependent parsec-scale structure
can be seen. We analyzed VLBI and Gaia positions and determined the direction
of jets in 2957 AGNs from their VLBI images. We find that there is a
statistically significant excess of sources with VLBI-to-Gaia position offset
in directions along and opposite to the jet. Offsets along the jet vary from
zero to tens of mas. Offsets in the opposite direction do not exceed 3 mas. The
presense of strong, extended parsec-scale optical jet structures in many AGNs
is required to explain all observed VLBI-Gaia offsets along the jet direction.
The offsets in the opposite direction shorter than 1 mas can be explained
either by a non-point-like VLBI jet structure or a "core-shift" effect due to
synchrotron opacity.
