# Isolated dark matter deprived galaxies in hydrodynamical simulations: real objects or artefacts?

## TL;DR

This study investigates isolated dark matter deprived galaxies in major hydrodynamical simulations and finds they are artifacts caused by boundary condition bugs rather than real objects.

## Contribution

It identifies and explains the origin of artificial dark matter deprived galaxies in simulations, revealing a rare bug in boundary condition implementation.

## Key findings

- Dark matter deprived galaxies are artifacts, not real objects.
- These artifacts result from boundary condition bugs in simulations.
- The bugs are caused by rare conditions at simulation box edges.

## Abstract

We searched for isolated dark matter deprived galaxies within several state-of-the-art hydrodynamical simulations: Illustris, IllustrisTNG, EAGLE, and Horizon-AGN and found a handful of promising objects in all except Horizon-AGN. While our initial goal was to study their properties and evolution, we quickly noticed that all of them were located at the edge of their respective simulation boxes. After carefully investigating these objects using the full particle data, we concluded that they are not merely caused by a problem with the algorithm identifying bound structures. We provide strong evidence that these oddballs were created from regular galaxies that get torn apart due to unphysical processes when crossing the edge of the simulation box. We show that these objects are smoking guns indicating an issue with the implementation of the periodic boundary conditions of the particle data in Illustris, IllustrisTNG, and EAGLE, which was eventually traced down to be a minor bug occurring for a very rare set of conditions.