Origin and properties of dual and offset active galactic nuclei in a cosmological simulation at z=2

TL;DR

This study uses cosmological simulations to analyze the origins and properties of dual and offset active galactic nuclei at redshift 2, revealing differences in black hole mass, accretion, and triggering mechanisms.

Contribution

It provides the first detailed simulation-based analysis of dual and offset AGN properties and their formation mechanisms at high redshift.

Findings

Dual AGN have similar black hole masses and higher accretion rates in less massive progenitors.

Offset AGN typically have a more massive active black hole than its inactive partner.

Dual AGN accrete more gas from the intergalactic medium than offset AGN.

Abstract

In the last few years, it became possible to observationally resolve galaxies with two distinct nuclei in their centre. For separations smaller than 10kpc, dual and offset active galactic nuclei (AGN) are distinguished: in dual AGN, both nuclei are active, whereas in offset AGN only one nucleus is active. To study the origin of such AGN pairs, we employ a cosmological, hydrodynamic simulation with a large volume of (182Mpc)^3 from the set of Magneticum Pathfinder Simulations. The simulation self-consistently produces 35 resolved black hole (BH) pairs at redshift z=2, with a comoving distance smaller than 10kpc. 14 of them are offset AGN and nine are dual AGN, resulting in a fraction of (1.2 \pm 0.3)% AGN pairs with respect to the total number of AGN. In this paper, we discuss fundamental differences between the BH and galaxy properties of dual AGN, offset AGN and inactive BH pairs and investigate their different triggering mechanisms. We find that in dual AGN the BHs have similar masses and the corresponding BH from the less massive progenitor galaxy always accretes with a higher Eddington ratio. In contrast, in offset AGN the active BH is typically more massive than its non-active counterpart. Furthermore, dual AGN in general accrete more gas from the intergalactic medium than offset AGN and non-active BH pairs. This highlights that merger events, particularly minor mergers, do not necessarily lead to strong gas inflows and thus, do not always drive strong nuclear activity.