Dynamical encounters and black hole mergers in gaseous environments

Recent detections of gravitational waves (GWs) have shown evidence for a high rate of black hole (BH)-BH and neutron star (NS)-NS mergers in the Universe, finding 23.9+-14.9 Gpc^-3 yr^{-1} and 320 Gpc^-3 yr^{-1}. The great challenge is to understand the possible astrophysical mechanisms that may lead to mergers and how GW observations can discriminate between these channels. Existing theoretical models of the astrophysical origin of the observed events are currently either highly incomplete or in tension with data which hints at the need for major theoretical improvements of modeling GW source populations.

We move beyond the existing semianalytical methods to model mergers in AGN accretion disks and to make predictions for the GW source populations and possible electromagnetic signatures. We will develop 3D hydrodynamical simulations to investigate the most relevant physical processes in detail and develop a more accurate analytical model of the accretion disk taking into account the influence of the embedded compact objects on the disk. We will incorporate the results in our semianalytical models to make them more accurate. This will also pave the way to construct a fully self-consistent simulation of the coevolution of stars, stellar mass compact objects, and AGN accretion disks.
We move beyond the existing semianalytical methods to model mergers in AGN accretion disks and to make predictions for the GW source populations and possible electromagnetic signatures. We will develop 3D hydrodynamical simulations to investigate the most relevant physical processes in detail and develop a more accurate analytical model of the accretion disk taking into account the influence of the embedded compact objects on the disk. We will incorporate the results in our semianalytical models to make them more accurate. This will also pave the way to construct a fully self-consistent simulation of the coevolution of stars, stellar mass compact objects, and AGN accretion disks.

We focus on the following undertakings during the project.
1. Gas-capture binary formation Tagawa et al 2020 has pointed out that most of the merging binaries in AGN are those which form during single-single captures in gasous environments. We investigate this process with hydrodynamics simulations with massive point particles.
2. Disk crossing binaries- Investigate the effects of dynamical friction on a binary crossing an accretion disk to investigate the efficiency of the relaxation of the binary into the disk.
3. Binary inclination alignment- Examine how the binary orbital inclination evolves when the binary is embedded in the accretion disk, to determine the expected level of alignment between binary-single or binary-binary interactions.
4. Binary-single and binary-binary interaction in gas- Examine how the dynamical interactions are altered when the binary is embedded in the accretion disk.
5. Gap opening and accretion flow in multibody systems- Examine the angular momentum exchange between the gaseous disk and several objects embedded in the disk to determine the conditions to open an annular cavity in the disk. Examine how the conditions for opening the gap depends on the number of single objects and binaries if present in a narrow range of radii. Further, examine the flow across the gap onto the objects and into the inner region to feed the SMBH (e.g. Jiang+ 2014) if multiple objects are present in the gap.