Properties and evolution of dual and offset AGN in the ASTRID simulation at z ∼ 2.

We examine the dual [both black hole (BH) active] and offset (one BH active and in distinct galaxies) active galactic nucleus (AGN) population (comprising ∼ 2000 pairs at |$0.5\, \text{kpc}\lesssim \Delta r\lt 30\, \text{kpc}$|⁠) at z  = 2 ∼ 3 in the ASTRID simulation covering (360 cMpc)³. The dual (offset) AGN make up 3.0(0.5) per cent of all AGN at z  = 2. The dual fraction is roughly constant while the offset fraction increases by a factor of 10 from z  = 4 ∼ 2. Compared with the full AGN population, duals are characterized by low M _BH/ M _* ratios, high specific star formation rates (sSFR) of |$\sim 1\, \text{Gyr}^{-1}$|⁠ , and high Eddington ratios (∼0.05, double that of single AGN). Dual AGNs are formed in major galaxy mergers (typically involving |$M_\text{halo}\lt 10^{13}\, M_\odot$|⁠), with simular-mass BHs. At small separations (when host galaxies are in the late phase of the merger), duals become 2 ∼ 8 times brighter (albeit more obscured) than at larger separations. 80  per cent of the bright, close duals would merge within |$\sim 500\, \text{Myr}$|⁠. Notably, the initially less-massive BHs in duals frequently become the brighter AGN during galaxy mergers. In offset AGN, the active BH is typically ≳ 10 times more massive than its non-active counterpart and than most BHs in duals. Offsets are predominantly formed in minor galaxy mergers with the active BH residing in the centre of massive haloes (⁠|$M_\text{ halo}\sim 10^{13-14}\, \mathrm{M}_\odot$|⁠). In these deep potentials, gas stripping is common and the secondary quickly deactivates. The stripping also leads to inefficient orbital decay amongst offsets, which stall at |$\Delta r\sim 5\, \text{kpc}$| for a few hundred Myrs. [ABSTRACT FROM AUTHOR]