We use the ASTRID cosmological hydrodynamic simulation to investigate the properties and evolution of triple and quadruple massive black hole (MBH) systems at z = 2–3. Only a handful of MBH tuple systems have been detected to date. In ASTRID, we find 4 per cent of the $M_{\rm BH}\gt 10^7\, M_\odot$ are in tuples with $\Delta r_{\rm max} \lt 200\, {\rm kpc}$. The tuple systems span a range of separations with the majority of the observable AGN systems at Δr ∼ 50–100 kpc. They include some of the most massive BHs (up to $10^{10} \, M_\odot$) but with at least one of the components of $M_{\rm BH} \sim 10^7 \, {\rm M}_{\odot }$. Tuples’ host galaxies are typically massive with $M_* \sim 10^{10-11} \, M_\odot$. We find that $\gt 10~{{\ \rm per\ cent}}$ massive haloes with Mhalo > 1013 M⊙ host MBH tuples. Following the subsequent interactions between MBHs in tuples, we found that in $\sim 5~{{\ \rm per\ cent}}$ of the triplets all three MBHs merge within a Gyr, and 15 per cent go through one merger. As a by-product of the complex multigalaxy interaction of these systems, we also find that up to $\sim 5~{{\ \rm per\ cent}}$ of tuples lead to runaway MBHs. In ASTRID, virtually all of the ultramassive black holes ($\gt 10^{10} \, M_\odot$) have undergone a triple quasar phase, while for BHs with $M_{\rm BH} \sim 10^9 \, M_\odot$, this fraction drops to 50 per cent.
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 MBH/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.
more » « less- NSF-PAR ID:
- 10408740
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 522
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 1895-1913
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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