# Varstrometry for Off-nucleus and Dual sub-Kpc AGN (VODKA): Methodology and Initial Results with Gaia DR2

## TL;DR

This paper introduces varstrometry, a Gaia-based method for detecting small-scale dual and off-nucleus AGN through variability-induced astrometric jitter, demonstrating its feasibility with initial data and discussing its potential for systematic discovery.

## Contribution

The paper presents a novel methodology using Gaia astrometry to identify sub-kpc dual and off-nucleus AGN, extending searches to smaller scales and demonstrating initial results with Gaia DR2 data.

## Key findings

- Feasibility of varstrometry demonstrated with Gaia DR2 data.
- Several candidate dual or off-nucleus AGN identified.
- Impact of host galaxies on astrometric accuracy discussed.

## Abstract

Gaia's milli-arcsec (mas) astrometric precision allows systematic identification of optically-selected sub-kpc dual active galactic nuclei (AGN), off-nucleus AGN, and small-scale lensed quasars by `varstrometry' -- where variability-induced astrometric jitter, i.e., temporal displacements of photocenter in unresolved sources, can be reasonably well detected or constrained. This approach extends systematic searches for small-scale ($\gtrsim$ mas) dual and off-nucleus AGN to poorly explored regime between $\sim 10$ pc and $\sim 1$ kpc, with Gaia's full sky coverage and depth to $G\sim 21$. We outline the general principles of this method and calculate the expected astrometric signals from the full time series of photocenter measurements and light curves. We demonstrate the feasibility of varstrometry by using Gaia DR2 data on a sample of variable pre-main sequence stars with known close companions. We find that extended host galaxies have a significant impact on the accuracy of astrometric and photometric variability in Gaia DR2, a situation to be improved in future Gaia releases. Using spectroscopically confirmed SDSS quasars, we present several examples of candidate sub-kpc off-nucleus or dual AGN selected from Gaia DR2. We discuss the merits and limitations of this method and follow-up strategy for promising candidates. We highlight Gaia's potential of systematically discovering and characterizing the sub-kpc off-nucleus and dual AGN population in the entire optical sky.