Merger-driven Fueling of Active Galactic Nuclei: Six Dual and Offset Active Galactic Nuclei Discovered with Chandra and Hubble Space Telescope Observations

TL;DR

This study uses Chandra and Hubble observations to identify and analyze dual and offset active galactic nuclei in galaxy mergers, revealing their properties, luminosities, and potential links to merger stages.

Contribution

It presents new multi-wavelength observations of 12 dual AGN candidates, discovering a new dual AGN system and providing insights into the luminosity and merger stage dependence of AGN activity.

Findings

Discovered a dual AGN with 2.2 kpc separation in a minor merger.

Major mergers host more luminous AGNs than single AGNs.

Confirmed dual AGNs have lower X-ray luminosity at fixed [O III] luminosity.

Abstract

Dual active galactic nuclei (AGNs) and offset AGNs are kpc-scale separation supermassive black holes pairs created during galaxy mergers, where both or one of the black holes are AGNs, respectively. These dual and offset AGNs are valuable probes of the link between mergers and AGNs but are challenging to identify. Here we present Chandra/ACIS observations of 12 optically-selected dual AGN candidates at z < 0.34, where we use the X-rays to identify AGNs. We also present HST/WFC3 observations of 10 of these candidates, which reveal any stellar bulges accompanying the AGNs. We discover a dual AGN system with separation of 2.2 kpc, where the two stellar bulges have coincident [O III] and X-ray sources. This system is an extremely minor merger (460:1) that may include a dwarf galaxy hosting an intermediate mass black hole. We also find six single AGNs, and five systems that are either dual or offset AGNs with separations < 10 kpc. Four of the six dual AGNs and dual/offset AGNs are in ongoing major mergers, and these AGNs are 10 times more luminous, on average, than the single AGNs in our sample. This hints that major mergers may preferentially trigger higher luminosity AGNs. Further, we find that confirmed dual AGNs have hard X-ray luminosities that are half of those of single AGNs at fixed [O III] luminosity, on average. This could be explained by high densities of gas funneled to galaxy centers during mergers, and emphasizes the need for deeper X-ray observations of dual AGN candidates.