Dual AGN in the Horizon-AGN simulation and their link to galaxy and massive black hole mergers, with an excursus on multiple AGN

Abstract

The occurrence of dual active galactic nuclei (AGN) on scales of a few tens of kpc can be used to study merger-induced accretion on massive black holes (MBHs) and to derive clues on MBH mergers, using dual AGN as a parent population of precursors. We investigate the properties of dual AGN in the cosmological simulation H -AGN. We create catalogs of dual AGN selected with distance and luminosity criteria, plus sub-catalogs where further mass cuts are applied. We divide the sample into dual AGN hosted in different galaxies, on the way to a merger, and into those hosed in one galaxy, after the galaxy merger has happened. We find that the most luminous AGN in a pair has higher Eddington ratio and mass than the general AGN population, but that the relation between MBH and galaxy mass is similar to that of all AGN. The typical mass ratio of galaxy mergers associated to dual AGN is 0.2, with mass loss in the smaller galaxy decreasing the mass ratio as the merger progresses. The connection between dual AGN and MBH mergers is weaker. Between 30 and 80 per cent of dual AGN with separations between 4 and 30 kpc can be matched to an ensuing MBH merger. The dual AGN fraction increases with redshift and with separation threshold, although above 50 kpc the increase of multiple AGN limits that of duals. Multiple AGN are generally associated with massive halos, and mass loss of satellites shapes the galaxy-halo relation.

Massive Black Hole Binaries from the TNG50-3 Simulation: II. Using Dual AGNs to Predict the Rate of Black Hole Mergers

“…In comparison with Figure 5 of this paper, the CCFs in Volonteri et al (2021) are in general lower at all z dAGN . This is because the initial dAGN separations considered in Volonteri et al (2021, 30 − 50 kpc) are larger than that considered in this work (∼ kpc), so the evolution time is longer and CCFs are lower in their case.…”

“…This is because the initial dAGN separations considered in Volonteri et al (2021, 30 − 50 kpc) are larger than that considered in this work (∼ kpc), so the evolution time is longer and CCFs are lower in their case. As shown in Figure 5, the CCFs increase with increasing z dAGN , contrary to the results of Volonteri et al (2021). This is because we do not include those systems comprising one 'intergalactic' MBH in the analysis of this work (see § 3 of paper I).…”

“…In the paper by Volonteri et al (2021), dAGNs in the cosmological simulation Horizon-AGN are identified and related to the corresponding MBH binary mergers from the same simulation (Volonteri et al 2020), and the numerical relation between the dAGNs and MBHB mergers is studied. The right column of their Figure 11 illustrates the CCFs of dAGNs with 5 ∼ 10 kpc separations.…”

“…Thus, at high redshifts, when the gas reservoir is rich, some MBHs form in gas clouds that are too small to be identified as galaxies ('intergalactic' MBHs). These small 'intergalactic' MBHs can be captured by galaxies later on and shine as dAGNs if some stochastic accretion occurs (Volonteri et al 2021). According to the left column of Figure 11 of Volonteri et al (2021), dAGNs with one 'intergalactic' MBH have small mass ratios and occur frequently at high redshifts (20 ∼ 30% of all dAGNs at z = 2, 3).…”

“…These small 'intergalactic' MBHs can be captured by galaxies later on and shine as dAGNs if some stochastic accretion occurs (Volonteri et al 2021). According to the left column of Figure 11 of Volonteri et al (2021), dAGNs with one 'intergalactic' MBH have small mass ratios and occur frequently at high redshifts (20 ∼ 30% of all dAGNs at z = 2, 3). The evolution time of these systems is long and the resulting coalescence fractions are low due to the small mass ratios, which leads to low coalescence fractions of dAGNs at high redshifts as shown in the right column of their Figure 11.…”