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

Abstract We examine the dual (both BHs active) and offset (one BH active and in distinct galaxies) AGN population (comprising ∼ 2000 pairs at 0.5 kpc ≲ Δr < 30 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 ten from z = 4 ∼ 2. Compared with the full AGN population, duals are characterized by low MBH/M* ratios, high specific star-formation rates (sSFR) of ∼1 Gyr−1, and high Eddington ratios (∼0.05, double that of single AGN). Dual AGN are formed in major galaxy mergers (typically involving Mhalo < 1013 M⊙), 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 % of the bright, close duals would merge within ∼500 Myrs. 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 center of massive halos (Mhalo ∼ 1013 − 14 M⊙). 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 Δr ∼ 5 kpc for a few hundred Myrs.