Evidence of the $Gaia$--VLBI position differences being related to radio source structure

TL;DR

This study investigates the relationship between Gaia and VLBI position differences and radio source structure effects, finding that source structure significantly influences positional discrepancies and aligns with radio jet directions.

Contribution

It provides evidence linking Gaia-VLBI position differences to radio source structure effects, emphasizing the impact of source extension and jet orientation on positional accuracy.

Findings

Position differences increase with source structure effects.

Most significant differences are associated with extended sources.

Position differences align with radio jet directions.

Abstract

We report the relationship between the $Gaia$--VLBI position differences and the magnitudes of source structure effects in VLBI observations. Because the $Gaia$--VLBI position differences are statistically significant for a considerable number of common sources, we attempt to discuss and explain these position differences based on VLBI observations and available source images at cm-wavelengths. 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. 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 dominate 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 with stronger evidence.