Properties and evolution of dual and offset AGN in the ASTRID simulation at z ∼ 2. Issue 2 (20th March 2023)

• Record Type: Journal Article
• Authors: Chen, Nianyi; Di Matteo, Tiziana; Ni, Yueying; Tremmel, Michael; DeGraf, Colin; Shen, Yue; Holgado, A Miguel; Bird, Simeon; Croft, Rupert; Feng, Yu
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 522:Issue 2(2023)
• Page Start: 1895
• Page End: 1913
• Publication Date: 2023-03-20
• Subjects: methods: numerical -- galaxies: active -- quasars: supermassive black holes; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stad834
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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.