AGNBlackHoles · Evolution and merger of dynamical assembly black holes in AGN disks

Evolution and merger of dynamical assembly black holes in AGN disks

Several binary black hole (BBH) mergers have now been observed in gravitational waves (GWs) by aLIGO/VIRGO/ KAGRA. With an estimated merger rate of 16–38 Gpc3/yr, many hundreds of mergers are expected to be detected in the coming years. However, a central outstanding question still remains: What astrophysical mechanisms bring these observed BBHs to merger? To date, the following three distinct merger channels have been greatly discussed: i) Isolated Binary Channel: In this scenario, two stars are born and evolve together in an isolated binary until they collapse into black holes (BHs). During this evolution, their separation can vary by orders of magnitude through mass transfer at the end of stellar evolution. If the final BBH separation is small enough, they will be able to merge and appear as an observable GW source. ii) Dynamical Channel: This scenario mainly has two flavors: a BBH is assembled and brought to merger through a scattering process, involving a BBH directly interacting with an incoming single BH; a BBH is brought to merger with the aid of a third distant body through Lidov-Kozai (LK) oscillations. iii) AGN disk Channel: In this scenario BHs either form in the active galactic nuclei (AGN) accretion disks as a result of star formation, or captured by the disk from the surrounding nuclear star cluster through gas-friction. The resultant BH population then undergoes scattering processes, together with gas-friction, leading to the mergers of BBHs. The major question is: How do we observationally tell these channels apart? The observed BH mass spectrum, BBH orbital eccentricity, and BH spin directions will differ between the different channels. Therefore, resolving the observational signatures in the AGN channel plays an absolute key role in the blooming field of GW astrophysics. My proposed program aims at resolving how BBH evolve and merge in AGN disks under the influence of both gas, post-Newtonian (PN) effects and tertiary companion (such as the central supermassive BH). The main research objectives (RO) addressed by the planned research are briefly outlined below: (RO1) Modeling Black Hole Interactions in an AGN Disk: I will here study the dynamical evolution of BBHs in the AGN disk with the inclusion of both gas-friction, and PN-corrections and the perturbation of the tertiary companion. For this, I need to at least perform the following two main tasks: (i) modeling the formation of hierarchical triple/multiple systems, including the prescriptions for weak dynamical encounters leading to ‘dissipative-capture-formation’; (ii) developing a module for evolving these hierarchical systems on secular timescales; A N-body code or a secular code including the non-secular orbital effect has to be developed. In all these interactions, it is expected that PN-corrections will lead to major effects that have not yet been discussed in this context. I will use my extended knowledge on few-body dynamics to study not only these effects in AGN disks for the first time, but also what effects to the global potential from the AGN disk and the perturbation of the tertiary companion like the supermassive BH (SMBH). (RO2) Predicting Observables: I will here study the observable properties of the BBH evolution and mergers in the AGN disk taking into account the dynamics described in RO1. With the numerical framework mentioned above, I will be able to systematically study how the GW observables, i.e., mass spectrum, orbital eccentricity in LIGO/Virgo, and relative BH spin orientations, relate to the dynamical processes. A proper inclusion of post-Newtonian (PN) dynamics for describing the evolution of the BH systems in the AGN disk will give rise to an un-explored rich eccentricity distribution observable by LISA and LIGO/Virgo, similar to what is found in the cluster case. Similarly, a proper inclusion of secular LK-effects for all the hierarchical BH systems that are formed in the disk will also give rise to great changes in the current set of reported observables.

Data: CORDIS, © European Union