The low frequency of dual AGNs versus the high merger rate of galaxies: A phenomenological model

TL;DR

This paper presents a phenomenological model explaining the low observed frequency of dual AGNs despite high galaxy merger rates, emphasizing the conditions under which dual AGNs are triggered and predicting their redshift evolution.

Contribution

The model links galaxy merger rates and black hole activity to explain dual AGN frequency, aligning with observations and predicting a decline at higher redshifts.

Findings

Model reproduces observed dual AGN frequency and separation distribution.

Dual AGN occurrence is favored in gas-rich galaxy mergers within half-light radii.

Predicted decrease of dual AGNs at higher redshifts, with 0.02%-0.06% at z 0.5-1.2.

Abstract

Dual AGNs are natural byproducts of hierarchical mergers of galaxies in the LambdaCDM cosmogony. Recent observations have shown that only a small fraction (~ 0.1%-1%) of AGNs at redshift z<~ 0.3 are dual with kpc-scale separations, which is rather low compared to the high merger rate of galaxies. Here we construct a phenomenological model to estimate the number density of dual AGNs and its evolution according to the observationally-estimated major merger rates of galaxies and various scaling relations on the properties of galaxies and their central massive black holes. We show that our model reproduces the observed frequency and separation distribution of dual AGNs provided that significant nuclear activities are triggered only in gas-rich progenitor galaxies with central massive black holes and only when the nuclei of these galaxies are roughly within the half-light radii of their companion galaxies. Under these constraints, the observed low dual AGN frequency is consistent with the relatively high merger rate of galaxies and supports the hypothesis that major mergers lead to AGN/QSO activities. We also predict that the number of kpc-scale dual AGNs decreases with increasing redshift and only about 0.02%--0.06% of AGNs are dual AGNs with double-peaked narrow line features at redshifts of z 0.5-1.2. Future observations of high-redshift dual AGNs would provide a solid test for this prediction.