Dim cores of radio-bright AGN jets: VLBI and Gaia astrometry pinpoint different parsec-scale features

TL;DR

This study systematically identifies AGNs where the brightest radio feature is a jet component rather than the core, revealing new insights into jet structures and their physical properties using VLBI and Gaia astrometry.

Contribution

The paper introduces a novel method to find AGNs with parsec-scale jet features offset from the core, challenging previous assumptions about jet directions and core brightness.

Findings

Identified 35 AGNs with jet features offset from the core.

Most sources have reversed jet directions compared to previous reports.

Sources show unusually low core brightness temperatures and bright jet components.

Abstract

Astrometry with the very long baseline radio interferometry (VLBI) allows to determine the position of a point close to the source's brightest compact detail at milliarcsecond scales. For most active galactic nuclei (AGNs), this compact detail is the opaque core of the radio jet. Rare cases of sources whose brightest detail is not the core but a prominent jet feature parsecs away from the core have been reported, but such sources remained elusive. In this work, we use a novel method for a systematic search of these sources. We scrutinize the AGNs for which the offset between their coordinates determined with VLBI and Gaia is statistically significant and coincides with the vector between two dominant features in their VLBI images, using publicly available archival multi-frequency data. We find 35 sources whose VLBI coordinates are associated with a bright component of their jet separated by several to tens of mas from the radio core. Their Gaia coordinates, in turn, correspond to the jet origin close to the radio jet core. The previously published jet directions of most of them must be reversed. These sources exhibit atypically low brightness temperatures of the radio cores, down to 10^9 K in the host galaxy frame, and, at the same time, extreme brightness of the dominating jet components. We argue that these bright components are standing shock fronts and discuss possible physical explanations for the low core brightness, such as ineffective particle heating, atypical absorption, or differential Doppler boosting.