17 research outputs found
Black hole binary formation and mergers in supermassive black hole discs
No full textFollowing the first gravitational wave (GW) detection in 2015, there have now been 90+ detections of merging black holes (BHs). In this thesis, BHs embedded in the gas discs of supermassive BHs within active galactic nuclei (AGN) are studied as a potential astrophysical source of these detections. In this channel, isolated BHs embed themselves in the AGN disc, encounter another in the disc, become bound via a complex interaction with the gas during the encounter, before gradually merging via gas driven inspiral. The binary formation and merger mechanism is modelled using both high resolution hydrodynamics and Monte-Carlo simulations.
Here, hydrodynamical simulations are performed of BH-BH interactions in AGN discs, showing that the energy of such encounters can be efficiently dissipated by the surrounding gas through a combination of accretion and gravitational drag, leaving behind a bound binary BH system (BBH). Binary capture of prograde and retrograde binaries can be successful in a range of AGN disc densities including cases well below that of theoretically predicted disc density. The majority of these captured binaries are then subsequently hardened by the surrounding gas. The eccentricity evolution depends strongly on the orbital rotation, where prograde binaries are governed by gravitational torques form their circumbinary mini-disc, with eccentricities being damped, while for retrograde binaries the eccentricities are excited to 0.9 by accretion torques. In certain cases, retrograde binaries can undergo a close periapsis passage which results in rapid merger via gravitational waves after only a few thousand binary orbits. Thus, the merger timescale can be far shorter than the AGN disc lifetime. These simulations support an efficient AGN disc merger pathway for BHs.
Exploring specifically the binary formation process using a large number of simulations over a range of impact parameters allowed further aspects of the mechanism to be quantified. A single range of impact parameters, typically of width binary Hill radii (depending on the AGN disk density) reliably leads to binary formation. The periapsis of the first encounter is the primary variable that determines the outcome of the initial scattering. The energy dissipated follows a power-law with the periapsis, where deeper encounters dissipate more energy. An analytic criterion for predicting the outcome of an encounter is constructed that utilises only the pre-encounter energy and impact parameter to determine the outcome of an encounter, with a reliability rate of >90\%. As the criterion is based directly on the simulations, it provides a reliable and physically motivated criterion for predicting binary scattering outcomes which can be used in population studies of BH binaries and mergers around AGN.
Using a simplified Monte Carlo approach, a population of BHs and their interactions in AGN are simulated using the aforementioned capture criterion showing the AGN channel can be a non-negligible contributor to the observed BH merger rate. The model suggests the merging mass distribution is flatter than the observed distribution, where the anticipated rate relative to the observed rate for low mass BBHs (50\msun). Due to the complexity of the AGN channel, several simplifying assumptions are made throughout this thesis. Further more detailed work is needed to assess the sensitivity of these observational predictions to these assumptions.0
In summary, the AGN channel remains an important mechanism for BH mergers, in particular for high mass BHs, with much work still to be done to further constrain its observational features
Black Hole Binary Formation in AGN Discs: From Isolation to Merger
No full textMotivated by the increasing number of detections of merging black holes by
LIGO-VIRGO-KAGRA, black hole (BH) binary mergers in the discs of active
galactic nuclei (AGN) is investigated as a possible merger channel. In this
pathway, BH encounters in the gas disc form mutually bound black hole binary
systems through interaction with the gas in the disc and subsequently inspiral
through gravitational torques induced by the local gas. To determine the
feasibility of this merger pathway, we present the first 3D global hydrodynamic
simulations of the formation and evolution of a stellar-mass BH binaries AGN
discs with three different AGN disc masses and five different initial radial
separations. These 15 simulations show binary capture of prograde and
retrograde binaries can be successful in a range of disc densities including
cases well below that of a standard radiatively efficient alpha disc,
identifying that the majority of these captured binaries are then subsequently
hardened by the surrounding gas. The eccentricity evolution depends strongly on
the orbital rotation where prograde binaries are governed by gravitational
torques form their circumbinary mini-disc, with eccentricities being damped,
while for retrograde binaries the eccentricities are excited to > 0.9 by
accretion torques. In two cases, retrograde binaries ultimately undergo a close
periapsis passage which results in a merger via gravitational waves after only
a few thousand binary orbits. Thus, the merger timescale can be far shorter
than the AGN disc lifetime. These simulations support an efficient AGN disc
merger pathway for BHs.Comment: 27 pages, 28 Figures, accepted for publication in MNRA
Black hole merger rates in AGN: contribution from gas-captured binaries
No full textMerging black hole (BH) binaries in active galactic nucleus (AGN) discs formed through two-body scatterings via the ‘gas-capture’ process may explain a significant fraction of BH mergers in AGN and a non-negligible contribution to the observed rate from LIGO-VIRGO-KAGRA. We perform Monte Carlo simulations of binary BH formation, evolution, and mergers across the observed AGN mass function using a novel physically motivated treatment for the gas-capture process derived from hydrodynamical simulations of BH–BH encounters in AGN. Our models suggest that gas-captured binaries could result in merger rates of Gpc yr. Mergers from AGN are dominated by AGN with supermassive BH masses of , with 90 per cent of mergers occurring in the range . The merging mass distribution is flatter than the initial BH mass power law by a factor , as larger BHs align with the disc and form binaries more efficiently. Similarly, the merging mass ratio distribution is flatter therefore the AGN channel could explain high mass and unequal mass ratio detections such as GW190521 and GW190814. Using a simpler dynamical friction treatment for the binary formation process, the results are similar, where the primary bottleneck is the alignment time with the disc. The most influential parameters are the anticipated number of BHs and their mass function. Given the many uncertainties that remain in the AGN channel, we expect the true uncertainty extends beyond our predicted rates. None the less, we conclude that AGN remain an important channel for consideration, particularly for gravitational wave detections involving one or two high mass BHs
3D adiabatic simulations of binary black hole formation in AGN discs
No full textWe investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGNs), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering nine AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the mid-plane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the mid-plane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the supermassive black hole’s role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte Carlo simulations of AGN evolution
Gas meets Kozai:the influence of a gas-rich accretion disc on hierarchical triples undergoing von Zeipel-Lidov-Kozai oscillations
No full textActive galactic nuclei (AGNs) consist of a central supermassive black hole (SMBH) embedded in a region with both high gas and stellar densities: the gas is present as a thin accretion disc that fuels the central SMBH, while the stars form a dense, roughly isotropic nuclear star cluster. The binaries present in such a cluster could be considered naturally as triples, with the SMBH as a third object, and their dynamics also depend on the interaction with the gas-rich disc. In this paper, we study the evolution of such a binary on an inclined orbit with respect to the disc. The binary experiences both eccentricity excitation via the von Zeipel-Lidov-Kozai (ZLK) effect and drag forces from each time it penetrates the disc. We find that, as long as the inner binary remains in the ZLK regime, then the evolution of inner orbital separation can transition from a regime of gradual hardening to a regime of rapid softening as the outer orbital inclination decreases. As such binaries grow wider, their minimum pericentre distances (during ZLK oscillations) decrease. We show that a simple geometric condition, modulated by the complex ZLK evolution, dictates whether a binary expands or contracts due to the interactions with the AGN disc. Our results suggest that the interaction with gas-rich accretion disc could enhance the rate of stellar mergers and formation of gravitational wave sources, as well as other transients. The treatment introduced here is general and could apply, with the proper modifications, to hierarchical triples in other gas-rich systems
Hydrodynamic simulations of black hole evolution in AGN discs II: inclination damping for partially embedded satellites
No full textWe investigate the evolution of black holes on orbits with small inclinations () to the gaseous discs of active galactic nuclei (AGNs). We perform 3D adiabatic hydrodynamic simulations within a shearing frame, studying the damping of inclination by black hole-gas gravitation. We find that for objects with , where is the disc aspect ratio, the inclination lost per mid-plane crossing is proportional to the inclination preceding the crossing, resulting in a net exponential decay in inclination. For objects with , damping efficiency decreases for higher inclinations. We consider a variety of different AGN environments, finding that damping is stronger for systems with a higher ambient Hill mass: the initial gas mass within the BH sphere of influence. We provide a fitting formula for the inclination changes as a function of Hill mass. We find reasonable agreement between the damping driven by gas gravity in the simulations and the damping driven by accretion under a Hill-limited Bondi–Hoyle–Lyttleton prescription. We find that gas dynamical friction consistently overestimates the strength of damping, especially for lower inclination systems, by at least an order of magnitude. For regions in the AGN disc where coplanar binary black hole formation by gas dissipation is efficient, we find that the simulated damping time-scales are especially short with . We conclude that as the time-scales for inclination damping are shorter than the expected interaction time between isolated black holes, the vast majority of binaries formed from gas capture should form from components with negligible inclination to the AGN disc
Farming struggles and triumphs: investigating the impact of a unique working environment on farming family well-being
No full textThis research aimed to investigate the impact of the unique work lifestyle of farming on the well-being of Australian farming families. Past organisational research suggests that role conflict and interference have a significant impact on well-being (Carlson et al., 2000; Danes et al., 2000; Greenhaus & Beutell, 1985). Previous research by the current author suggested that the work-home environment of farming families is unique and therefore warranted further investigation due to the potential link between the working environment and the poor mental health status of farming families of Australia (McShane & Quirk, 2009; Page & Fragar, 2002). Due to the limited availability of contextually-specific scales of stress, coping, and inter-role conflict for Australian farming families, this research sought to identify the work and lifestyle determinants of well-being through the development of farming family specific scales of stress, coping, and the work-home interface. The theoretical framework for the development of the scales followed the procedure outlined by Streiner and Norman (1989) and consisted of 6 separate studies, involving a total of 474 participants.
The first study involved interviews with 53 farming family members across Queensland and New South Wales. Interview data was analysed using Grounded Theory and Content Analysis. From this process additional themes of commitment to farming, identification with farming, adaptive and maladaptive characteristics of intergenerational farming, and farming family values were generated from the interview content alongside stress, coping, and work-home interface themes. Generated items (519 items) were prepared for the Item Reduction study which included 13 potential farming family scales pertaining to stress, coping, role interference and completion, intergenerational farming, and buffering characteristics. This study asked farming family members (N=65) to rate items for importance and relevance to the individual, with calculated means and frequencies used to reduce the item set. Items were then formatted for an Expert Review Panel (N=11) who assessed the reduced item set (246 items; n=6 scales). Expert Panel feedback, face validity, and internal consistency were used to further reduce the total item pool to 100 items. A pilot study (N=14) identified additional items for removal and finalised the item set (95 items) for the validity (N=278) and reliability (N=53) studies. The items were distributed across 5 scales and assessed against criteria for factor analysis, criterion validity, discriminant validity, internal consistency, and test-retest reliability. The resulting scales included the Farming Family (FF) Role Impact Scale (18 items), the FF Stressor Scale (29 items), the FF Cope Scale (25 items), the FF Buffer Scale (12 items), and the Intergenerational Farming Impact Scale (11 items). The scales adequately satisfied validation and reliability criteria including exploratory factor analysis (loadings >.3), internal consistency (Cronbach alpha >.8), and test-retest reliability (rho>.6 for 4/5 scales).
Scales of well-being indicated that though farming families reported low levels of psychological distress and moderate levels of life satisfaction, the population was at risk of burnout. Cluster analytical and structural equation modelling techniques were used to identify those factors which contributed to well-being. Overall, results suggested that stressors were the leading contributor to poor well-being and role interference played a key role in psychological distress and work burnout. However the negative impacts of role interference and farming lifestyle stressors were tempered by positive coping styles and the sense of purpose, commitment and value associated with farming. Recommendations were made for policy and health promotion developers to consider the connection to farming as an asset to improve well-being and sustainability of the farming communities and industry
Black hole binaries in AGN accretion discs – II. Gas effects on black hole satellite scatterings
No full textThe black hole (BH) binaries in active galactic nuclei (AGN) are expected to form mainly through scattering encounters in the ambient gaseous medium. Recent simulations, including our own, have confirmed this formation pathway is highly efficient. We perform 3D smoothed particle hydrodynamics (SPH) simulations of BH scattering encounters in AGN discs. Using a range of impact parameters, we probe the necessary conditions for binary capture and how different orbital trajectories affect the dissipative effects from the gas. We identify a single range of impact parameters, typically of width ∼0.86−1.59 binary Hill radii depending on AGN disc density, that reliably leads to binary formation. The periapsis of the first encounter is the primary variable that determines the outcome of the initial scattering. We find an associated power law between the energy dissipated and the periapsis depth to be ΔE ∝ r−b with b = 0.42 ± 0.16, where deeper encounters dissipate more energy. Excluding accretion physics does not significantly alter these results. We identify the region of parameter space in initial energy versus impact parameter where a scattering leads to binary formation. Based on our findings, we provide a ready-to-use analytic criterion that utilizes these two pre-encounter parameters to determine the outcome of an encounter, with a reliability rate of >90 per cent. As the criterion is based directly on our simulations, it provides a reliable and highly physically motivated criterion for predicting binary scattering outcomes which can be used in population studies of BH binaries and mergers around AGN
Black Hole Binaries in AGN Accretion Discs II: Gas Effects on Black Hole Satellite Scatterings
No full textThe black hole (BH) binaries in active galactic nuclei (AGN) are expected to
form mainly through scattering encounters in the ambient gaseous medium. Recent
simulations, including our own, have confirmed this formation pathway is highly
efficient. We perform 3D smoothed particle hydrodynamics (SPH) simulations of
BH scattering encounters in AGN disks. Using a range of impact parameters, we
probe the necessary conditions for binary capture and how different orbital
trajectories affect the dissipative effects from the gas. We identify a single
range of impact parameters, typically of width binary Hill
radii depending on AGN disk density, that reliably leads to binary formation.
The periapsis of the first encounter is the primary variable that determines
the outcome of the initial scattering. We find an associated power-law between
the energy dissipated and the periapsis depth to be
with , where deeper encounters dissipate more energy. Excluding
accretion physics does not significantly alter these results. We identify the
region of parameter space in initial energy vs impact parameter where a
scattering leads to binary formation. Based on our findings, we provide a
ready-to-use analytic criterion that utilises these two pre-encounter
parameters to determine the outcome of an encounter, with a reliability rate of
>90\%. As the criterion is based directly on our simulations, it provides a
reliable and highly physically motivated criterion for predicting binary
scattering outcomes which can be used in population studies of BH binaries and
mergers around AGN.Comment: 20 pages, 19 figure
Disc Novae: Thermodynamics of Gas Assisted Binary Black Hole Formation in AGN Discs
No full textWe investigate the thermodynamics of close encounters between stellar mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGN), during which binary black holes (BBHs) may form. We consider a suite of 2D viscous hydrodynamical simulations within a shearing box prescription using the Eulerian grid code Athena++. We study formation scenarios where the fluid is either an isothermal gas or an adiabatic mixture of gas and radiation in local thermal equilibrium. We include the effects of viscous and shock heating, as well as optically thick cooling. We co-evolve the embedded BHs with the gas, keeping track of the energetic dissipation and torquing of the BBH by gas and inertial forces. We find that compared to the isothermal case, the minidiscs formed around each BH are significantly hotter and more diffuse, though BBH formation is still efficient. We observe massive blast waves arising from collisions between the radiative minidiscs during both the initial close encounter, and subsequent periapsis periods for successfully bound BBHs. These disc novae have a profound effect, depleting the BBH Hill sphere of gas and injecting energy into the surrounding medium. In analysing the thermal emission from these events, we observe periodic peaks in local luminosity associated with close encounters/periapses, with emission peaking in the optical/near-IR. In the AGN outskirts, these outbursts can reach 4% of the AGN luminosity in the IR band, with flares rising over 0.5-1year. Collisions in different disc regions, or when treated in 3D with magnetism, may produce more prominent flares.16 pages, 13 figures, accepted for publication in MNRA
