Papers › Properties and Evolution of Dual and Offset AGN in the ASTRID Simulation at z ∼2
Properties and Evolution of Dual and Offset AGN in the ASTRID Simulation at z ∼2
Nianyi Chen, Tiziana Di Matteo, Yueying Ni, Michael Tremmel, Colin DeGraf, Yue Shen, A. Miguel Holgado, Simeon Bird, Rupert Croft, Yu Feng
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We examine the dual (both BHs active) and offset (one BH active) 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(2.2)% 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 a low M_(BH)/M_* ratio, a high specific star-formation rate (sSFR) of ∼1 Gyr⁻¹, and a high Eddington ratio (∼0.05, double that of single AGN). The dual AGN are formed in major galaxy mergers (typically involving Mₕₐₗₒ<10¹³ M_⊙), with BHs that have similar masses. At small separations (when their 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 these bright, close duals merge in the simulation within ∼500 Myrs. Notably, the initially less-massive BH in duals frequently becomes the brighter AGN during the galaxy merger. 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.
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