Varstrometry for Off-nucleus and Dual Sub-kpc AGN (VODKA): Very Long Baseline Array Searches for Dual or Off-nucleus Quasars and Small-scale Jets

TL;DR

This study uses VLBA observations and a novel Gaia-based astrometric technique to identify and analyze candidate dual and off-nucleus quasars at sub-kiloparsec scales, revealing potential dual systems and small-scale jets.

Contribution

Introduces varstrometry as a new method for detecting dual and off-nucleus quasars at parsec scales using Gaia and VLBA data, with initial candidate identification.

Findings

8 out of 23 candidates show multiple radio components or significant positional offsets.

3 candidates likely host small-scale jets based on radio morphology.

3 candidates are potential dual quasars at parsec scales.

Abstract

Dual and off-nucleus active supermassive black holes are expected to be common in the hierarchical structure formation paradigm, but their identification at parsec scales remains a challenge due to strict angular resolution requirements. We conduct a systematic study using the Very Long Baseline Array (VLBA) to examine 23 radio-bright candidate dual and off-nucleus quasars. The targets are selected by a novel astrometric technique ("varstrometry") from Gaia, aiming to identify dual or off-nucleus quasars at (sub)kilo-parsec scales. Among these quasars, 8 exhibit either multiple radio components or significant (>3$\sigma$) positional offsets between the VLBA and Gaia positions. The radio emission from the three candidates which exhibit multiple radio components is likely to originate from small-scale jets based on their morphology. Among the remaining five candidates with significant VLBA-Gaia offsets, three are identified as potential dual quasars at parsec scales, one is likely attributed to small-scale jets, and the origin of the last candidate remains unclear. We explore alternative explanations for the observed VLBA-Gaia offsets. We find no evidence for optical jets at kilo-parsec scales, nor any contamination to Gaia astrometric noise from the host galaxy; misaligned coordinate systems are unlikely to account for our offsets. Our study highlights the promise of the varstrometry technique in discovering candidate dual or off-nucleus quasars and emphasizes the need for further confirmation and investigation to validate and understand these intriguing candidates.