A New Discrete Dynamical Friction Estimator Based on $N$-body Simulations

TL;DR

This paper introduces a novel discrete dynamical friction estimator tailored for N-body simulations, addressing ambiguities in traditional models and demonstrating accurate results with minimal computational overhead.

Contribution

It presents a new sub-grid dynamical friction estimator based on the discrete nature of N-body simulations, improving accuracy and resolving issues in Chandrasekhar's formula.

Findings

Estimator agrees well with high-resolution simulations

Negligible additional computational cost

Addresses ambiguities in traditional models

Abstract

A longstanding problem in galactic simulations is to resolve the dynamical friction (DF) force acting on massive black hole particles when their masses are comparable to or less than the background simulation particles. Many sub-grid models based on the traditional Chandrasekhar DF formula have been proposed, yet they suffer from fundamental ambiguities in the definition of some terms in Chandrasekhar's formula when applied to real galaxies, as well as difficulty in evaluating continuous quantities from (spatially) discrete simulation data. In this work we present a new sub-grid dynamical friction estimator based on the discrete nature of $N$-body simulations, which also avoids the ambiguously-defined quantities in Chandrasekhar's formula. We test our estimator in the GIZMO code and find that it agrees well with high-resolution simulations where DF is fully captured, with negligible additional computational cost. We also compare it with a Chandrasekhar estimator and discuss its applications in real galactic simulations.