The abundances and properties of Dual AGN and their host galaxies in the EAGLE simulations

Yetli Rosas-Guevara, Richard Bower, Stuart McAlpine, Silvia Bonoli, Patricia Tissera

Published 2018-05-03Version 1

We look into the abundance of Dual AGN (active galaxy nuclei) in the largest volume hydrodynamical simulation from the EAGLE project. We define a Dual AGN as two active black holes with a separation below 30 kpc. We find that only 1 percent of AGN with $L_{\rm HX}\geq 10^{42}$ erg/s are part of an observable Dual AGN system at $z=0.8-1$. During the evolution of a typical binary black hole system, the rapid variability of the hard X-ray luminosity on Myr time-scales severely limits the detectability of Dual AGN. To quantify this effect, we calculate a probability of detection, $t_{\rm on}/t_{\rm 30}$, where $t_{\rm 30}$ is the time in which the two black holes were separated at distances below 30 pkpc and $t_{\rm on}$, the time that both AGN are visible (e.g. when both AGN have $L_{\rm HX}\geq 10^{42}$ erg/s) in this period. We find that the average fraction of visible Dual systems is 3 percent. The visible Dual AGN distribution as a function of black hole separation increases with small separations and it presents a pronounced peak at $20-25$ kpc. This shape can be understood as a result of the rapid orbital decay of the host galaxies after their first encounter. Looking at the merger history of the galaxies hosting a Dual AGN, we find that $75$ percent of the host galaxies have recently undergone or are undergoing a merger with stellar mass ratio $\geq 0.1$. Finally, we find that the fraction of visible Dual AGN with respect to the total AGN increases with redshift as found in observations.