Observability of Dual Active Galactic Nuclei in Merging Galaxies

TL;DR

This study uses high-resolution simulations to explore when dual active galactic nuclei are detectable during galaxy mergers, revealing that simultaneous activity is rare and occurs mainly at small separations.

Contribution

It provides new insights into the conditions and timing of dual AGN activity during galaxy mergers using detailed hydrodynamical simulations.

Findings

Dual AGN activity peaks at small separations (<1-10 kpc).

Simultaneous AGN activity is rare, matching observational data.

Detectable dual AGN fraction is a few percent.

Abstract

Supermassive black holes (SMBHs) have been detected in the centers of most nearby massive galaxies. Galaxies today are the products of billions of years of galaxy mergers, but also billions of years of SMBH activity as active galactic nuclei (AGNs) that is connected to galaxy mergers. In this context, detection of AGN pairs should be relatively common. Observationally, however, dual AGN are scant, being just a few percent of all AGN. In this Letter we investigate the triggering of AGN activity in merging galaxies via a suite of high resolution hydrodynamical simulations. We follow the dynamics and accretion onto the SMBHs as they move from separations of tens of kiloparsecs to tens of parsecs. Our resolution, cooling and star formation implementation produce an inhomogeneous, multi-phase interstellar medium, allowing us to accurately trace star formation and accretion onto the SMBHs. We study the impact of gas content, morphology, and mass ratio, allowing us to study AGN activity and dynamics across a wide range of relevant conditions. We test when the two AGN are simultaneously detectable, for how long and at which separations. We find that strong dual AGN activity occurs during the late phases of the mergers, at small separations (<1-10 kpc) below the resolution limit of most surveys. Much of the SMBH accretion is not simultaneous, limiting the dual AGN fraction detectable through imaging and spectroscopy to a few percent, in agreement with observational samples.