A High Resolution Search for Dual AGN Candidates in Mergers: A Pre-Selection Strategy using Keck AO

TL;DR

This study develops a method combining WISE and SDSS data to pre-select galaxy mergers likely hosting dual active galactic nuclei, and confirms substructure with Keck AO imaging, improving detection efficiency.

Contribution

The paper introduces a novel pre-selection strategy using infrared-optical offsets to identify advanced galaxy mergers with unresolved dual AGN candidates, validated by high-resolution Keck AO observations.

Findings

43% of the sample showed unresolved substructure consistent with offsets.

The method yields a higher detection rate than previous techniques.

Infrared observations reveal substructure missed or obscured in optical imaging.

Abstract

Accreting supermassive black holes (SMBHs) in galaxy mergers with separations $<$ 1 kpc are crucial to our understanding of SMBH growth, galaxy evolution, and SMBH binary evolution. Despite their importance, few are known, and most have been discovered serendipitously. In this work, we develop and test a method to systematically pre-select candidate advanced mergers likely to contain unresolved sub-kpc nuclear substructure constituting high-priority dual-AGN candidates for follow-up spectroscopy. By exploiting the survey area and astrometric precision of the Wide-field Infrared Survey Explorer (WISE) and the Sloan Digital Sky Survey (SDSS), we identified 46 nearby advanced mergers that have red WISE colors ($W_1-W_2>0.5$) indicative of accretion activity and significant sub-arcsecond offsets between their optical and infrared coordinates as measured by SDSS and WISE. We conducted high-resolution adaptive optics (AO) observations with the Keck NIRC2 camera in the $K_p$ band ($2.124 \mu m$, $\Delta\lambda = 0.351 \mu m$) to search for unresolved substructure suggested by the optical-to-infrared offsets. We find that 20/46 (43\%) of the sample shows substructure tracing the SDSS/WISE offset and unresolved by SDSS , representing a higher yield than previous pre-selection techniques such as double-peaked [O III] or hard X-ray selection. These results demonstrate that the SDSS/WISE offset method provides an efficient pathway for identifying late-stage mergers and dual-AGN candidates for spectroscopic confirmation. Archival optical Hubble Space Telescope (HST) imaging reveals that substructure identified with Keck is often missed in the optical or erroneously identified due to partial obscuration, underscoring the importance of infrared studies of late-stage mergers.