2021A&A...647A.189X

Context. We report the relationship between the Gaia-VLBI position differences and the magnitudes of source structure effects in VLBI observations.
Aims. Because the Gaia-VLBI position differences are statistically significant for a considerable number of common sources, we discuss and attempt to explain these position differences based on VLBI observations and available source images at centimeter wavelengths.
Methods. Based on the derived closure amplitude root mean square (CARMS), which quantifies the magnitudes of source structure effects in the VLBI observations used for building the third realization of the International Celestial Reference Frame, the arc lengths and normalized arc lengths of the position differences are examined in detail. The radio-jet directions and the directions of the Gaia-VLBI position differences are investigated for a small sample of sources.
Results. Both the arc lengths and normalized arc lengths of the Gaia and VLBI positions are found to increase with the CARMS values. The majority of the sources with statistically significant position differences are associated with the sources having extended structure. Radio source structure is the one of the major factors of these position differences, and it can be the dominant factor for a number of sources. The vectors of the Gaia and VLBI position differences are parallel to the radio-jet directions, which is confirmed via stronger evidence.