Theoretical astrophysics and plasma physics at RPC

Abstract:

The matter power spectrum, $P(k)$ , is one of the fundamental quantities in the study of large-scale structure in cosmology. Here, we study its small-scale asymptotic limit, and show that for cold dark matter in $d$ spatial dimensions, $P(k)$ has a universal $k^{-d}$ asymptotic scaling with the wave number $k$ , for $k\gg k_{\mathrm{nl}}$ , where $k_{\mathrm{nl}}^{-1}$ denotes the length scale at which nonlinearities in gravitational interactions become important. We propose a theoretical explanation for this scaling, based on a nonperturbative analysis of the system’s phase-space structure. Gravitational collapse is shown to drive a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and nonlinear time scales are balanced, and this balance dictates the $k$ dependence of the power spectrum. A parallel is drawn to Batchelor turbulence in hydrodynamics, where large scales mix smaller ones via tidal interactions. The $k^{-d}$ scaling is also derived by expressing $P(k)$ as a phase-space integral in the framework of kinetic field theory, which is analyzed by the saddle-point method; the dominant critical points of this integral are precisely those where the time scales are balanced. The coldness of the dark-matter distribution function—its nonvanishing only on a $d$ -dimensional submanifold of phase space—underpins both approaches. The theory is accompanied by 1D Vlasov-Poisson simulations, which confirm it.

Abstract:

The frequency of compact object interactions in AGN discs is naturally tied to the number of objects embedded within it. We investigate the evolution of black holes in the nuclear stellar cluster on inclined orbits to the AGN disc by performing adiabatic hydrodynamical simulations of isolated black hole disc crossings over a range of disc densities and inclinations . We find radiation dominates the pressure in the wake that forms around the BH across the full inclination and disc density range. We identify no well defined steady state wake morphology due to the thin geometry of the disc and the vertical exponential density drop off, where the wake morphology depends on the vertical depth of the transit within the disc. The inclination damping relative the pre-transit inclination behaves as a power law in and the ambient Hill mass as . The drag on the BH is dominated by the gravity of the wake for the majority of our inclination range until accretion effects become comparable at , where is the disc aspect ratio. At low inclinations () the wake morphology becomes more spherical, leading to a regime change in the inclination damping behaviour. Our results suggest that the inclination damping time-scale is shorter than expected from only episodic Bondi–Hoyle–Lyttelton accretion events during each transit, implying inclined objects may be captured by the AGN disc earlier in its lifetime than previously thought.

Abstract:

We 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.