New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets

TL;DR

This study investigates large radio-optical positional offsets in bright radio-loud active galactic nuclei using VLBI imaging, revealing complex structures and variability as key factors behind the discrepancies.

Contribution

It provides high-resolution VLBI images of AGNs with large offsets, offering insights into the causes of positional misalignments between radio and optical reference frames.

Findings

Complex radio structures contribute to positional offsets.

Time variability affects the apparent source positions.

Proper imaging can clarify the origins of offsets.

Abstract

When comparing modern fundamental reference frames in the radio (International Celestial Reference Frame) and optical (Gaia), a couple of bright radio reference sources appear to have very large radio-optical offsets, from tens up to hundreds of milliarcseconds (mas). The amount of these positional misalignments exceeds the uncertainty of each individual technique by at least an order of magnitude. In most cases, complex and extended radio structure and its time variability, and thus the difficulty in pinpointing the true location of the central engine, is responsible for the large apparent offsets. Sometimes distant parts of the radio structure are not properly detected due to a lack of shorter interferometer baselines. For our 5-GHz very long baseline interferometry (VLBI) experiment using antennas of the European VLBI Network and the enhanced Multi Element Radio Linked Interferometer Network, we selected 10 bright radio-loud active galactic nuclei with extremely large radio-optical offsets. Sensitive imaging involving a wide range of projected baseline lengths, as well as phase-referencing to nearby sources shed light on the possible causes of positional inconsistencies. Here we show results for 3 selected sources from this project.