SIMBAD references

Detecting dual active galactic nuclei (DAGNs) in observations and understanding theoretically which massive black holes (MBHs) compose them and in which galactic and large-scale environment they reside are becoming increasingly important questions as we enter the multimessenger era of MBH astronomy. This paper presents the abundance and properties of DAGN produced in nine large-scale cosmological hydrodynamical simulations. We focus on DAGN powered by AGN with $L_{\rm bol}\geqslant 10^{43}\, \rm erg\, s^{-1}$ and belonging to distinct galaxies, i.e. pairs that can be characterized with current and near-future electromagnetic observations. We find that the number density of DAGN separated by a few to 30 proper kpc varies from $10^{-8}$ (or none) to $10^{-3} \, \rm comoving\, Mpc^{3}$ in the redshift range $z=0\!-\!7$. At a given redshift, the densities of the DAGN numbers vary by up to two orders of magnitude from one simulation to another. However, for all simulations, the DAGN peak is in the range $z=1\!-\!3$, right before the peak of cosmic star formation or cosmic AGN activity. The corresponding fractions of DAGN (with respect to the total number of AGN) range from 0 per cent to 6 per cent. We find that simulations could produce too few DAGN at $z=0$ (or merge pairs too quickly) compared to current observational constraints while being consistent with preliminary constraints at high redshift ($z\sim 3$). Next-generation observatories (e.g. Advanced X-Ray Imaging Satellite [AXIS]) will be of paramount importance to detect DAGN across cosmic times. We predict the detectability of DAGN with future X-ray telescopes and discuss DAGN as progenitors for future Laser Interferometer Space Antenna (LISA) gravitational wave detections.