DM halo morphological types of MW-like galaxies in the TNG50 simulation: Simple, Twisted, or Stretched

TL;DR

This study classifies the shapes of dark matter halos in Milky Way-like galaxies from the TNG50 simulation into three types—Simple, Twisted, and Stretched—based on their principal axes behavior across radii, revealing diverse morphological features.

Contribution

It introduces a novel classification of DM halo shapes into three categories based on their principal axes behavior, with detailed analysis of their properties and comparison to observational data.

Findings

Approximately 32% simple halos with fixed axes

Twisted halos exhibit gradual rotational changes

Stretched halos show axis re-orientation with radius

Abstract

We present a comprehensive analysis of the shape of dark matter (DM) halos in a sample of 25 Milky Way-like galaxies in TNG50 simulation. Using an Enclosed Volume Iterative Method (EVIM), we infer an oblate-to-triaxial shape for the DM halo with the median $T \simeq 0.24 $. We group DM halos in 3 different categories. Simple halos (32% of population) establish principal axes whose ordering in magnitude does not change with radius and whose orientations are almost fixed throughout the halo. Twisted halos (32% of population), experience levels of gradual rotations throughout their radial profiles. Finally, stretched halos (36% of population) demonstrate a stretching in their principal axes lengths where the ordering of different eigenvalues change with radius. Subsequently, the halo experiences a "rotation" of $\sim$90 deg where the stretching occurs. Visualizing the 3D ellipsoid of each halo, for the first time, we report signs of re-orienting ellipsoid in twisted and stretched halos. We examine the impact of baryonic physics on DM halo shape through a comparison to dark matter only (DMO) simulations. This suggests a triaxial (prolate) halo. We analyze the impact of substructure on DM halo shape in both hydro and DMO simulations and confirm that their impacts are subdominant. We study the distribution of satellites in our sample. In simple and twisted halos, the angle of satellites' angular momentum with galaxy's angular momentum grows with radius. However, stretched halos show a flat distribution of angles. Overlaying our theoretical outcome on the observational results presented in the literature establishes a fair agreement.