The Influence of Initial Angular Momentum on the Orbital and Structural Evolution of Satellite Subhalos in a Milky Way-like Dark Matter Halo

Abstract

This thesis investigates how modifying the initial angular momentum (AM) condition of a Milky Way-like dark matter halo affects the orbital and structural properties of its satellite subhalos. This work uses a suite of five dark-matter-only (DMO) cosmological zoom-in simulations, the VINTERGATAN-GM-AM, in which the initial AM of the Lagrangian patch is modified using the genetIC code to obtain the following set of initial conditions relative to the fiducial simulation; AM_fid × {0.5, 1.05, 1.1, 1.2, 2.0}. The evolution is initialized at z = 99 and ends at z = 0, and we use ROCKSTAR to extract subhalo information for our analysis.
In order to proceed, we develop a method to identify, match and track three subhalo candidates around our central halo. Using this limited sample size, we provided an analysis of the evolution of four properties in subhalos - the specific orbital AM, the virial radius, the virial mass and the radial distance to the central halo.
In our results, it is found that the specific orbital AM is strongly sensitive to the modification of the main halo’s initial AM and that this sensitivity is persistent throughout the entire redshift range studied(z ∼ 6 to z = 0), with the ordering being set by the initial conditions. Moreover, the structural properties of the subhalos - the virial radius and mass - are shown to be largely insensitive to initial condition variations. Indeed, the virial radius of each run is shown to deviate by less than 10% compared to the reference run AM_fid× {1.1}. At low redshift, more significant deviations are observed and these are attributed to tidal stripping, occurring during orbital inspirals - an analysis of the radial distances show notable differences between simulation runs and the effect of the initial conditions modifications on the trajectory.
The results of this work show that modifying the initial AM condition leaves a persistent effect on the orbital properties of the subhalos. However, it provides evidence for little direct effect on the virial radius and virial mass evolution. Overall, the results present a physically consistent and coherent picture of the interplay between different dynamical processes in a purely gravitational environment, and are consistent with the predictions of Tidal Torque Theory (TTT) and the existing literature.

Popular Abstract

When we look at the night sky, not only do we see far into the cosmos - distant stars and galaxies - but also deep into the past. The reason for this is that light takes years to travel from the stars and galaxies to us. And in fact, not only are they far from us, but they continue to move further away from us. One may wonder, why are these stars so far from us and why do we even have objects such as stars and galaxies in what seems to be empty space - the answer lies at the beginning of time.
When the universe began, it looked nothing like what we have today. The matter in our universe that creates the flower out in the garden or the mountains in the Himalayas or the rocks on the moon, was originally spread out. One may think that all that we can see and touch is all the matter that exists in the universe, but there is actually a form of matter that is hidden to us which we have never been able to observe, but that we know exists because it is able to pull the matter that we can see through gravity. We are thus able to infer its existence. This matter also was spread out, but not perfectly, and in time, as the universe grew, these imperfections led to clumps of this invisible matter.
Studying dark matter in isolation is particularly powerful, as it only interacts through gravity. This means we are able to remove all the other ways in which regular matter interacts (electromagnetism, thermally, and so on). These clumps would later be homes to galaxies and mini-galaxies that orbit around them. These dark matter homes are called dark matter halos and our current understanding tells us that they are rotating. The goal of this project was to investigate how the rotation of the dark matter halo of a Milky Way sized galaxy would affect the movement and internal properties of the dark matter halos of the mini-galaxies orbiting around it. We call the dark matter halos of the mini-galaxies, subhalos. This is interesting to us because as our telescopes become more powerful, we are able to see these mini-galaxies better. These satellite galaxies have a lot of dark matter compared to regular matter, which makes them important probes for better understanding dark matter, which we know very little about. It is therefore important to understand how the properties of these subhalos are affected by the different physics involved -including changing the rotation of the central halo.
In our project, we did our investigation using computer simulations. There were five computer simulations of the universe with a zoom-in on a single Milky Way sized galaxy that were run with slightly different initial rotations. We developed an algorithm to select three subhalos and compared how their properties changed and evolved in each of the simulation runs.
What we found was that changing the initial rotation of the central halo had a long lasting impact on the orbit of the subhalo. A stronger rotation of the central halo means a larger orbital angular momentum throughout the evolution of the subhalo. We also found that the internal properties such as its size and mass are largely unaffected by this initial change. Most of the difference we observe can be explained by the differences in the trajectories of the subhalo orbits. When the subhalo gets close to the central halo, the latter pulls at the subhalos and rips parts of them away.
In this way, we were able to get a better understanding of how the properties of subhalos change when one changes the rotation of the central halo. As we look to the future, more powerful telescopes and specific space missions will provide us with better observations of these satellite galaxies, which will contribute to uncovering the mystery around dark matter.

• author: Chérel, Théo
• supervisor: Santi Roca Fàbrega; Corentin Cadiou
• organization: Astrophysics; Department of Physics
• course: ASTM32 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Dark Matter Halos, Angular Momentum, Subhalos, DMO, Simulations
• report number: EXA266
• language: English
• id: 9246665
• date added to LUP: 2026-08-10 12:22:37
• date last changed: 2026-08-10 12:22:37