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Detecting dual active galactic nuclei (DAGNs) in observations and understanding theoretically which massive black holes (MBHs) compose them and in which galactic and large-scale environment they reside are becoming increasingly important questions as we enter the multimessenger era of MBH astronomy. This paper presents the abundance and properties of DAGN produced in nine large-scale cosmological hydrodynamical simulations. We focus on DAGN powered by AGN with |$L_{\rm bol}\geqslant 10^{43}\, \rm erg\, s^{-1}$| and belonging to distinct galaxies, i.e. pairs that can be characterized with current and near-future electromagnetic observations. We find that the number density of DAGN separated by a few to 30 proper kpc varies from |$10^{-8}$| (or none) to |$10^{-3} \, \rm comoving\, Mpc^{3}$| in the redshift range |$z=0\!-\!7$|⁠. At a given redshift, the densities of the DAGN numbers vary by up to two orders of magnitude from one simulation to another. However, for all simulations, the DAGN peak is in the range |$z=1\!-\!3$|⁠ , right before the peak of cosmic star formation or cosmic AGN activity. The corresponding fractions of DAGN (with respect to the total number of AGN) range from 0 per cent to 6 per cent. We find that simulations could produce too few DAGN at |$z=0$| (or merge pairs too quickly) compared to current observational constraints while being consistent with preliminary constraints at high redshift (⁠|$z\sim 3$|⁠). Next-generation observatories (e.g. Advanced X-Ray Imaging Satellite [AXIS]) will be of paramount importance to detect DAGN across cosmic times. We predict the detectability of DAGN with future X-ray telescopes and discuss DAGN as progenitors for future Laser Interferometer Space Antenna (LISA) gravitational wave detections. [ABSTRACT FROM AUTHOR]

We present an analysis showcasing how the Advanced X-ray Imaging Satellite (AXIS), a proposed NASA Probe-class mission, will significantly increase our understanding of supermassive black holes undergoing mergers—from kpc to sub-pc scales. In particular, the AXIS point spread function, field of view, and effective area are expected to result in (1) the detection of hundreds to thousands of new dual AGNs across the redshift range 0 < z < 5 and (2) blind searches for binary AGNs that are exhibiting merger signatures in their light curves and spectra. AXIS will detect some of the highest-redshift dual AGNs to date, over a large range of physical separations. The large sample of AGN pairs detected by AXIS (over a magnitude more than currently known) will result in the first X-ray study that quantifies the frequency of dual AGNs as a function of redshift up to z = 4 . [ABSTRACT FROM AUTHOR]

In the near future, projects like Laser Interferometer Space Antenna (LISA) and pulsar timing arrays are expected to detect gravitational waves from mergers between supermassive black holes, and it is crucial to precisely model the underlying merger populations now to maximize what we can learn from this new data. Here, we characterize expected high-redshift (z > 2) black hole mergers using the very large volume Astrid cosmological simulation, which uses a range of seed masses to probe down to low-mass black holes (BHs), and directly incorporates dynamical friction so as to accurately model the dynamical processes that bring black holes to the galaxy centre where binary formation and coalescence will occur. The black hole populations in Astrid include black holes down to |$\sim 10^{4.5} \, \mathrm{M}_\odot$|⁠ , and remain broadly consistent with the TNG simulations at scales |$\gt 10^6 \, \mathrm{M}_\odot$| (the seed mass used in TNG). By resolving lower mass black holes, the overall merger rate is ∼5× higher than in TNG. However, incorporating dynamical friction delays mergers compared to a recentring scheme, reducing the high- z merger rate mass-matched mergers by a factor of ∼2×. We also calculate the expected LISA signal-to-noise values, and show that the distribution peaks at high SNR (>100), emphasizing the importance of implementing a seed mass well below LISA 's peak sensitivity (⁠|$\sim 10^6 \, \mathrm{M}_\odot$|⁠) to resolve the majority of LISA 's gravitational wave detections. [ABSTRACT FROM AUTHOR]

Massive black holes in the centres of galaxies today must have grown by several orders of magnitude from seed black holes formed at early times. Detecting a population of intermediate mass black holes (IMBHs) can provide constraints on these elusive BH seeds. Here, we use the large volume cosmological hydrodynamical simulation Astrid , which includes IMBH seeds and dynamical friction to investigate the population of IMBH seeds. Dynamical friction is largely inefficient at sinking and merging seed IMBHs at high-z. This leads to an extensive population (several hundred per galaxy) of wandering IMBHs in large haloes at |$z\sim 2$|⁠. A small fraction of these IMBHs are detectable as HLXs, Hyper Luminous X-ray sources. Importantly, at |$z\sim 2$|⁠ , IMBHs mergers produce the peak of GW events. We find close to a million GW events in Astrid  between |$z=\rm{2\!-\!3}$| involving seed IMBH mergers. These GW events (almost all detectable by LISA) at cosmic noon should provide strong constraints on IMBH seed models and their formation mechanisms. At the centre of massive galaxies, where the number of IMBHs can be as high as 10–100, SMBH-IMBH pairs can form. These Intermediate mass ratio inspirals (IMRIs) and extreme mass ratio inspirals (EMRIs), will require the next generation of milli- |$\mu$| Hz space-based GW interferometers to be detected. Large populations of IMBHs around massive black holes will probe their environments and MBH causal structure. [ABSTRACT FROM AUTHOR]

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 Δ r ∼ 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 M halo > 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. [ABSTRACT FROM AUTHOR]

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]

Galaxy mergers produce pairs of supermassive black holes (SMBHs), which may be witnessed as dual quasars if both SMBHs are rapidly accreting. The kiloparsec (kpc)-scale separation represents a physical regime sufficiently close for merger-induced effects to be important1 yet wide enough to be directly resolvable with the facilities currently available. Whereas many kpc-scale, dual active galactic nuclei—the low-luminosity counterparts of quasars—have been observed in low-redshift mergers2, no unambiguous dual quasar is known at cosmic noon (z ≈ 2), the peak of global star formation and quasar activity3,4. Here we report multiwavelength observations of Sloan Digital Sky Survey (SDSS) J0749 + 2255 as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon (z = 2.17). We discover extended host galaxies associated with the much brighter compact quasar nuclei (separated by 0.46″ or 3.8 kpc) and low-surface-brightness tidal features as evidence for galactic interactions. Unlike its low-redshift and low-luminosity counterparts, SDSS J0749 + 2255 is hosted by massive compact disk-dominated galaxies. The apparent lack of stellar bulges and the fact that SDSS J0749 + 2255 already follows the local SMBH mass–host stellar mass relation, suggest that at least some SMBHs may have formed before their host stellar bulges. While still at kpc-scale separations where the host-galaxy gravitational potential dominates, the two SMBHs may evolve into a gravitationally bound binary system in around 0.22 Gyr.The authors report multiwavelength observations of SDSS J0749 + 2255, hosted by massive compact disk-dominated galaxies, as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon. [ABSTRACT FROM AUTHOR]

We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid  . Astrid  includes galaxy formation and black hole models recently updated with an MBH seed population between 3 × 104 h −1M⊙ and 3 × 105 h −1M⊙ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to 1.5 ckpc  h −1. We calculate the initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above 0.7 for most MBH pairs before the numerical merger. After approximating unresolved evolution on scales below |${\sim 200\, \text{pc}}$|⁠ , we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time (⁠|$\sim 500\, \text{Myr}$|⁠) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries (M BH < 2 M seed). As a result, only |$\lesssim 20{{\rm per \,cent}}$| of seed MBH pairs merge at z > 3 after considering both unresolved DF evolution and binary hardening. These z > 3 seed-mass mergers are hosted in a biased population of galaxies with the highest stellar masses of |$\gt 10^9\, {\rm M}_\odot$|⁠. With the higher initial eccentricity prediction from Astrid  , we estimate an expected merger rate of 0.3−0.7 per year from the z > 3 MBH population. This is a factor of ∼7 higher than the prediction using the circular orbit assumption. The Laser Interferometer Space Antenna events are expected at a similar rate, and comprise |$\gtrsim 60\,{\rm{per\,cent}}$| seed-seed mergers, |$\sim 30\,{\rm{per\,cent}}$| involving only one seed-mass MBH, and |$\sim 10\,{\rm{per\,cent}}$| mergers of non-seed MBHs. [ABSTRACT FROM AUTHOR]

We present the evolution of black holes (BHs) and their relationship with their host galaxies in Astrid , a large-volume cosmological hydrodynamical simulation with box size 250 h −1Mpc containing 2 × 55003 particles evolved to z = 3. Astrid statistically models BH gas accretion and active galactic nucleus (AGN) feedback to their environments, applies a power-law distribution for BH seed mass |$\, M_{\rm sd}$|⁠ , uses a dynamical friction model for BH dynamics, and executes a physical treatment of BH mergers. The BH population is broadly consistent with empirical constraints on the BH mass function, the bright end of the luminosity functions, and the time evolution of BH mass and accretion rate density. The BH mass and accretion exhibit a tight correlation with host stellar mass and star formation rate. We trace BHs seeded before z > 10 down to z = 3, finding that BHs carry virtually no imprint of the initial |$\, M_{\rm sd}$| except those with the smallest |$\, M_{\rm sd}$|⁠ , where less than 50 per cent of them have doubled in mass. Gas accretion is the dominant channel for BH growth compared to BH mergers. With dynamical friction, Astrid predicts a significant delay for BH mergers after the first encounter of a BH pair, with a typical elapse time of about 200 Myr. There are in total 4.5 × 105 BH mergers in Astrid at z > 3, ∼103 of which have X-ray detectable EM counterparts: a bright |$\, {\rm kpc}$| scale dual AGN with L X > 1043 erg s−1. BHs with |$\, M_{\rm BH}\sim 10^{7\!-\!8} \, M_{\odot }$| experience the most frequent mergers. Galaxies that host BH mergers are unbiased tracers of the overall |$\, M_{\rm BH}\!-\! M_*$| relation. Massive (⁠|$\gt 10^{11}\, M_{\odot }$|⁠) galaxies have a high occupation number (≳10) of BHs, and hence host the majority of BH mergers. [ABSTRACT FROM AUTHOR]

The launch of space-based gravitational-wave (GW) detectors (e.g. Laser Interferometry Space Antenna; LISA) and current and upcoming Pulsar Timing Arrays will extend the GW window to low frequencies, opening new investigations into dynamical processes involving massive black hole binaries (MBHBs) and their mergers across cosmic time. MBHBs are expected to be among the primary sources for the upcoming low-frequency (10−4–10−1 Hz) window probed by LISA. It is important to investigate the expected supermassive BH merger rates and associated signals, to determine how potential LISA events are affected by physics included in current models. To study this, we post-process the large population of MBHBs in the Illustris simulation to account for dynamical friction time delays associated with BH infall/inspiral. We show that merger delays associated with binary evolution have the potential to decrease the expected merger rates, with |$M_{\rm {BH}}\ \gt\ 10^6\ \mathrm{M}_\odot$| MBHBs (the lowest mass in Illustris) decreasing from ∼3 to ∼0.1 yr−1, and shifting the merger peak from z ∼2 to ∼1.25. During this time, we estimate that accretion grows the total merging mass by as much as 7x the original mass. Importantly, however, dynamical friction-associated delays (which shift the mergers toward lower redshift and higher masses) lead to a stronger signal/strain for the emitted GWs in the LISA band, increasing mean frequency from 10−3.1 to 10−3.4–10−4.0 Hz, and mean strain from 10−17.2 to 10−16.3–10−15.3. Finally, we show that after including a merger delay and associated M BH growth, mergers still tend to lie on the typical M BH– M * relation, but with an increased likelihood of an undermassive BH. [ABSTRACT FROM AUTHOR]

We introduce the Astrid  simulation, a large-scale cosmological hydrodynamic simulation in a |$250 \, h^{-1}\mathrm{Mpc}$| box with 2 × 55003 particles. Astrid contains a large number of high redshift galaxies, which can be compared to future survey data, and resolves galaxies in haloes more massive than |$2\times 10^9 \, \mathrm{M}_{\odot }$|⁠. Astrid  has been run from z  = 99 to 3. As a particular focus is modelling the high redshift Universe, it contains models for inhomogeneous hydrogen and helium reionization, baryon relative velocities and massive neutrinos, as well as supernova and AGN feedback. The black hole model includes mergers driven by dynamical friction rather than repositioning. We briefly summarize the implemented models, and the technical choices we took when developing the simulation code. We validate the model, showing good agreement with observed ultraviolet luminosity functions, galaxy stellar mass functions and specific star formation rates (SFRs). We show that the redshift at which a given galaxy underwent hydrogen reionization has a large effect on the halo gas fraction. Finally, at z  = 6, haloes with |$M \sim 2\times 10^9 \, \mathrm{M}_{\odot }$| which have been reionized have an SFR 1.5 times greater than those which have not yet been reionized. [ABSTRACT FROM AUTHOR]

In this work, we establish and test methods for implementing dynamical friction (DF) for massive black hole pairs that form in large volume cosmological hydrodynamical simulations that include galaxy formation and black hole growth. We verify our models and parameters both for individual black hole dynamics and for the black hole population in cosmological volumes. Using our model of DF from collisionless particles, black holes can effectively sink close to the galaxy centre, provided that the black hole's dynamical mass is at least twice that of the lowest mass resolution particles in the simulation. Gas drag also plays a role in assisting the black holes' orbital decay, but it is typically less effective than that from collisionless particles, especially after the first billion years of the black hole's evolution. DF from gas becomes less than |$1{{\ \rm per\ cent}}$| of DF from collisionless particles for BH masses >107 M⊙. Using our best DF model, we calculate the merger rate down to z  = 1.1 using an L box = 35 Mpc  h −1 simulation box. We predict ∼2 mergers per year for z > 1.1 peaking at z ∼ 2. These merger rates are within the range obtained in previous work using similar resolution hydrodynamical simulations. We show that the rate is enhanced by factor of ∼2 when DF is taken into account in the simulations compared to the no-DF run. This is due to |${\gt}40{{\ \rm per\ cent}}$| more black holes reaching the centre of their host halo when DF is added. [ABSTRACT FROM AUTHOR]

In the Simulations and Constructions of the Reionization of Cosmic Hydrogen (SCORCH) project, we compare analytical models of the hydrogen ionization fraction with radiation–hydrodynamic simulations. We derive analytical models of the mass-weighted hydrogen ionization fraction from the local ionization balance equations as a more accurate alternative to the widely adopted model based on the volume filling factor. In particular, our model has a recombination term quadratic in the ionization fraction, which is consistent with the two-body interaction nature of recombination. Then, we use the radiation–hydrodynamic simulations to study the clumping factors needed to solve the analytical equations and provide accurate fitting functions. We find that the ionized hydrogen clumping factors from our radiative transfer simulations are significantly different than those from other simulations that use a uniform photoionization background. In addition to redshift dependence, we also see the dependence of the ionized hydrogen clumping factor on ionization fraction, and we incorporate this into our fits. We calculate the reionization histories using our analytical models and clumping factors and compare with widely adopted models, and all of our models achieve <7% difference from simulation results, while the other models have >20% deviations. The Thomson optical depths from reionization calculated from our analytical models result in <5% deviation from simulations, while the previous analytical models have a >20% difference and could result in biased conclusions of the intergalactic medium reionization. [ABSTRACT FROM AUTHOR]

The bond-orientational order in molten salt LiF, KF, and KCl systems has been studied by molecular dynamics simulation. The bond spherical harmonics have been calculated for the instantaneous local structures in computer-generated configurations of the molten salts. A new method of bond spherical harmonics calculation with equal neighbor number N has been proposed. For the standard structures with different coordination number, their quadratic invariant Ql histograms with same N have been calculated and a model with parameter of square deviation have been suggested. The Ql histograms with N=4 and N=12 for some standard structures have been given and the qualitative or semiquantitative estimations of the bond-orientational orders in those three molten salt systems have been obtained. The bond angle probability distributions have also been calculated and taken as the basis for evaluating the usefulness of the new method. The results show that the local structures in molten salt systems are somewhat similar to tetrahedron while the icosahedron structure is hardly found. [ABSTRACT FROM AUTHOR]

Examines the regularities of formation of binary intermetallic compounds between transition and non-transition elements. Inequities of electronegativity; Prediction of the unknown binary alloy system; Test of the prediction ability of the model.

The pattern recognition methods and a four-parameter model, based on extended Miedema’s cellular model of alloy phases, are used to study the regularities of formation of ternary compounds between two transition elements (T, T′) and one non-transition element (N) (T-T′-N system). The criterion of formation can be expressed as some functions of ϕ (electronegativity), n (valence electron density in Wagner-Seitz cell), R (Pauling’s metallic radii) and Z (number of valence electrons in atom). [ABSTRACT FROM AUTHOR]

Using a four-parameter model based on extended Miedema’s cellular model of alloy phases and pattern recognition methods, the regularities of formation of ternary intermetallic compounds between non-transition metals have been investigated. The criterion of formation can be expressed as some empirical functions of ϕ (electronegativity), n (valence electron density in Wagner-Seitz cell), R (Pauling’s metallic radius) and Z (number of valence electrons in atom). [ABSTRACT FROM AUTHOR]

The calculations of clusters modeling the fluorine-doping and oxygen deficiency of YBa2Cu307 have been performed by the method of all-electron ab initio Hartree-Fock with self-consistent crystal field. Results show that in CuO planes electric charge significantly increases, the chemical valence of Cu decreases and the covalent bonding of Cu-O greatly weakens owing to oxygen deficiency, while the effect of F restores the local electronic structure of YBa2Cu3O7. The reported opinion that F occupied the oxygen vacancy in Cu-O chains seems disputable according to the calculated bonding characteristics [ABSTRACT FROM AUTHOR]

Gives information on the difficulty of solving problems completely about electronic configuration assignments of atomic energy levels. Information on artificial neural networks (ANNs); Details of experiments; Results and conclusions of experiments.

Examines the regularities of the formation and lattice distortion of perovskite-type compounds. Influence of the covalency of chemical bond and the ligand fields around ions; Exhibition of the outstanding ferroelectric modulation properties; Formation of polyhedral ionic cluster.