AGN pairs: chance superpositions or black hole binaries?

TL;DR

This study evaluates whether observed high-velocity AGN pairs are due to chance superpositions or black hole binaries, finding that most are likely chance alignments, but some high-velocity cases may indicate binary systems.

Contribution

The paper uses cosmological simulations to estimate the likelihood of AGN pairs caused by chance superpositions versus binary black holes, providing constraints on their origins.

Findings

High-velocity AGN pairs up to ~2000 km/s are likely chance superpositions.

No pairs with velocity offsets >3000 km/s are predicted, aligning with observational limits.

Models cannot explain the largest velocity offsets, suggesting binary black holes are involved.

Abstract

Several active galactic nuclei (AGN) with multiple sets of emission lines separated by over 2000 km/s have been observed recently. These have been interpreted as being due to massive black hole (MBH) recoil following a black hole merger, MBH binaries, or chance superpositions of AGN in galaxy clusters. Moreover, a number of double-peaked AGN with velocity offsets of ~ a few 100 km/s have also been detected and interpreted as being due to the internal kinematics of the narrow line regions or MBH binary systems. Here we reexamine the superposition model. Using the Millennium Run we estimate the total number of detectable AGN pairs as a function of the emission line offset. We show that AGN pairs with high velocity line separations up to ~2000 km/s are very likely to be chance superpositions of two AGN in clusters of galaxies for reasonable assumptions about the relative fraction of AGN. No superimposed AGN pairs are predicted for velocity offsets in excess of ~3000 km/s as the required AGN fractions would violate observational constraints. The high velocity AGN pair numbers predicted here are competitive with those predicted from the models relying on MBH recoil or MBH binaries. However, the model fails to account for the largest emission line velocity offsets that require the presence of MBH binaries.