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

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 ( mas) dual and off-nucleus AGN to poorly explored regime between ∼ 10 pc and ∼ 1 kpc, with Gaia's full sky coverage and depth to G ∼ 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.

AGNs on the Move: A Search for Off-nuclear AGNs from Recoiling Supermassive Black Holes and Ongoing Galaxy Mergers with the Zwicky Transient Facility

“…A Gaia analysis of a sample of lowredshift, unobscured broad-line AGNs from the Sloan Digital Sky Survey (SDSS) showed that at least 99% were within 1 kpc of the host, 90% were within 500 pc, and 40% were within 100 pc (Shen et al 2019). That study used a technique called varstrometry (Hwang et al 2020) to measure AGN-host spatial offsets via the astrometric jitter of the photocenter induced by the AGN flux variability, allowing them to rule out the existence of a substantial offset AGN population on 10 pc-1 kpc scales at redshifts of 0.3 < z < 0.8.…”

Dual AGN in the Horizon-AGN simulation and their link to galaxy and massive black hole mergers, with an excursus on multiple AGN

“…Searches are generally of two types, either blind searches that search surveys for two AGN at small separation or in the same galaxy, for instance through spectroscopic signatures (Comerford et al 2013;Kim et al 2020;Hwang et al 2020), or assisted searches that look for companions near detected AGN (e.g., Koss et al 2012;Silverman et al 2020). After selection of candidates, additional tests are often needed to confirm the dual nature of the selected AGN (e.g., Rosario et al 2011;Rubinur et al 2019;Gabányi et al 2016;Foord et al 2020;Comerford et al 2012).…”

Gaia: The Galaxy in six (and more) dimensions

“…This opens up a range of time-domain astrophysical studies based on photometry (light-curves), spectroscopy (RV curves), and astrometry (centroid motions), obtained quasi-simultaneously in case of bright sources (Gaia Collaboration et al, 2019). The repeated observations allow to study periodically varying objects such as variable stars or quasars (Gaia Collaboration et al, 2017;Hwang et al, 2019) or transient objects such as novae, supernovae, and microlensing events (Wyrzykowski, 2016). In addition repeated observations allow to detect planets through astrometric variability, opening a discovery window that is complementary to other techniques (Perryman et al, 2014).…”

“…According to the NASA ADS, the majority of the new publications are related to the fields of Galactic halo and disk studies (see below), but also to solar system studies (Cellino et al, 2007;Gaia Collaboration et al, 2018d), exoplanet and host star characterization (Kervella et al, 2019), characterization of variable or binary objects (Ziegler et al, 2018) and of objects with astroseismological data (Berger et al, 2018). Work was done also in the field of extragalactic studies, for example on Quasars variability (Hwang et al, 2019) or gravitational lensing (Wertz et al, 2019). Gaia DR2 data have even been used to search for a plausible home star for the interstellar object 'Oumuamua (Bailer-Jones et al, 2018), which recently was discovered transiting in the solar system 12 .…”