Dynamics of thick discs around Schwarzschild-de Sitter black holes

TL;DR

This study investigates how a positive cosmological constant affects the dynamics and stability of constant angular momentum discs around Schwarzschild-de Sitter black holes, revealing that outer mass loss can suppress runaway instability.

Contribution

It provides the first qualitative analysis of how a cosmological constant influences disc stability and outflows in Schwarzschild-de Sitter spacetimes through hydrodynamical simulations.

Findings

Outer cusp mass loss can suppress runaway instability.

Positive cosmological constant introduces an outer cusp affecting disc dynamics.

Simulations show outflows significantly impact disc stability.

Abstract

We consider the effects of a cosmological constant on the dynamics of constant angular momentum discs orbiting Schwarzschild-de Sitter black holes. The motivation behind this study is to investigate whether the presence of a radial force contrasting the black hole's gravitational attraction can influence the occurrence of the runaway instability, a robust feature of the dynamics of constant angular momentum tori in Schwarzschild and Kerr spacetimes. In addition to the inner cusp near the black hole horizon through which matter can accrete onto the black hole, in fact, a positive cosmological constant introduces also an outer cusp through which matter can leave the torus without accreting onto the black hole. To assess the impact of this outflow on the development of the instability we have performed time-dependent and axisymmetric hydrodynamical simulations of equilibrium initial configurations in a sequence of background spacetimes of Schwarzschild-de Sitter black holes with increasing masses. The simulations have been performed with an unrealistic value for the cosmological constant which, however, yields sufficiently small discs to be resolved accurately on numerical grids and thus provides a first qualitative picture of the dynamics. The calculations, carried out for a wide range of initial conditions, show that the mass-loss from the outer cusp can have a considerable impact on the instability, with the latter being rapidly suppressed if the outflow is large enough.