Loss cone refilling by flyby encounters--A numerical study of massive black holes in galactic centres

TL;DR

This study uses advanced N-body simulations with importance sampling to analyze how satellite flybys influence the refilling of the loss cone around supermassive black holes, impacting star disruption and merger rates.

Contribution

It introduces a new importance sampling scheme for N-body models to reduce noise and investigates satellite flybys' effects on loss cone refilling rates.

Findings

Flybys increase star flux into the loss cone by a factor of 3.

Poisson noise dominates the refilling rate enhancement.

Refilling rate enhancement is modest despite increased flux.

Abstract

A gap in phase-space, the loss cone (LC), is opened up by a supermassive black hole (MBH) as it disrupts or accretes stars in a galactic centre. If a star enters the LC then, depending on its properties, its interaction with the MBH will either generate a luminous electromagnetic flare or give rise to gravitational radiation, both of which are expected to have directly observable consequences. A thorough understanding of loss-cone refilling mechanisms is important for the prediction of astrophysical quantities, such as rates of tidal disrupting main-sequence stars, rates of capturing compact stellar remnants and timescales of merging binary MBHs. In this thesis, we use N-body simulations to investigate how noise from accreted satellites and other substructures in a galaxy's halo can affect the LC refilling rate. Any N-body model suffers from Poisson noise which is similar to, but much stronger than, the two-body diffusion occurring in real galaxies. To lessen this spurious Poisson noise, we apply the idea of importance sampling to develop a new scheme for constructing N-body realizations of a galaxy model, in which interesting regions of phase-space are sampled by many low-mass particles. We use multimass N-body models of galaxies with centrally-embedded MBHs to study the effects of satellite flybys on LC refilling rates. We find that although the flux of stars into the initially emptied LC is enhanced, but the fuelling rate averaged over the entire subhalos is increased by only a factor 3 over the rate one expects from the Poisson noise due the discreteness of the stellar distribution.