Dual Active Galactic Nuclei in Nearby Galaxies

TL;DR

This paper reviews the current understanding of dual active galactic nuclei (DAGN) in merging galaxies, discusses methods for identifying DAGN candidates, presents high-resolution radio observations, and analyzes star formation rates in DAGN hosts.

Contribution

It summarizes existing DAGN samples, describes identification techniques from optical and spectroscopic data, and provides new radio observations of DAGN candidates.

Findings

DAGN host galaxies do not have higher star formation rates than non-DAGN merging galaxies.

Methods based on optical emission lines and velocity dispersion are effective for identifying DAGN candidates.

Radio observations confirm the presence of DAGN in selected candidates.

Abstract

Galaxy mergers play a crucial role in the formation of massive galaxies and the buildup of their bulges. An important aspect of the merging process is the in-spiral of the supermassive black-holes (SMBHs) to the centre of the merger remnant and the eventual formation of a SMBH binary. If both the SMBHs are accreting they will form a dual or binary active galactic nucleus (DAGN). The final merger remnant is usually very bright and shows enhanced star formation. In this paper we summarize the current sample of DAGN from previous studies and describe methods that can be used to identify strong DAGN candidates from optical and spectroscopic surveys. These methods depend on the Doppler separation of the double peaked AGN emission lines, the nuclear velocity dispersion of the galaxies and their optical/UV colours. We describe two high resolution, radio observations of DAGN candidates that have been selected based on their double peaked optical emission lines (DPAGN). We also examine whether DAGN host galaxies have higher star formation rates (SFRs) compared to merging galaxies that do not appear to have DAGN. We find that the SFR is not higher for DAGN host galaxies. This suggests that the SFRs in DAGN host galaxies is due to the merging process itself and not related to the presence of two AGN in the system.