VLBI-Gaia offsets favor parsec-scale jet direction in active galactic nuclei

Context. The data release 1 (DR1) of milliarcsecond-scale accurate optical positions of stars and galaxies was recently published by the space mission Gaia. Aims. We study the off sets 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. Methods. We analyzed VLBI and Gaia positions and determined the direction of jets in 2957 AGNs from their VLBI images. Results. We find that there is a statistically significant excess of sources with VLBI-to-Gaia position off set in directions along and opposite to the jet. Off sets along the jet vary from 0 to tens of mas. Off sets in the opposite direction do not exceed 3 mas. Conclusions. The presense of strong, extended parsec-scale optical jet structures in many AGNs is required to explain all observed VLBI-Gaia off sets along the jet direction. The off sets 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.