Universal numerical convergence criteria for subhalo tidal evolution

TL;DR

This paper establishes universal criteria for numerical convergence in simulating subhalo tidal evolution, highlighting the importance of resolving the tidal radius to prevent artificial disruption or survival in cosmological simulations.

Contribution

It introduces the first comprehensive convergence criteria applicable across different simulation methods and subhalo properties, addressing the overmerging problem.

Findings

A universal force resolution criterion: at least 20 cells or softening lengths resolve the tidal radius.

A universal expression for discreteness noise affecting subhalo mass retention.

Up to 50% of subhalos in current simulations may be numerically unresolved.

Abstract

Dark matter subhalos and satellite galaxies in state-of-the-art cosmological simulations still suffer from the ``overmerging'' problem, where inadequate force and/or mass resolution cause artificially enhanced tidal mass loss and premature disruption. Previous idealized simulations addressing this issue have been restricted to a small subset of the subhalo orbital parameter space, and all assumed subhalos to be isotropic. Here, we present the first extensive simulation suite that quantifies numerical convergence in the tidal evolution of anisotropic subhalos under varying numerical resolutions and orbits. We report a universal force resolution criterion: the subhalo's instantaneous tidal radius must always be resolved by at least 20 cells in adaptive mesh refinement (AMR)-based simulations, or by 20 softening lengths (Plummer equivalent) in tree-based simulations, regardless of refinement details or subhalo physical properties such as concentration or velocity anisotropy. We also report a universal expression for the discreteness-noise-driven scatter in the bound-mass fraction of subhalos that depends only on the subhalo mass resolution at infall and the instantaneous bound mass fraction, agnostic of any further subhalo properties. Such stochastic discreteness noise causes both premature disruption and, notably, spurious survival of poorly mass-resolved subhalos. We demonstrate that as many as 50 percent of all subhalos in state-of-the-art cosmological simulations are likely to be either force and/or mass unresolved. Our findings advocate for adaptive softening or grid refinement based on the instantaneous tidal radius of the subhalo.