Satellite galaxies, stellar streams and the nature of dark matter

Abstract

Dwarf galaxies are perfect candidates for studying dark matter due to their abundance in our Universe, low baryonic content and diverse appearance. They are considered as one of the most important probes of cosmological models, such as the most accepted ΛCDM, and are particularly important in the context of evolution of galactic environments. Satellite galaxies and their remnants can give us further information about the properties of their hosts’ galactic potentials, ultimately giving us clues on their morphological features, mass and possible formation history. Using the AGORA CosmoRun/CosmoRun2 simulations seven different code platforms were analysed by analysing 9 satellites created from the same initial conditions. First, the dwarf progenitor properties were extracted using Parallel-tempered MCMC samplers with Einasto and generalized Navarro-Frenk-White profiles on derived density and velocity dispersion profiles. Then, snapshots at the moment of remnant formation were identified, and cases when streams or elongated radial shells were formed were put under further study. In those specific cases, the dimensional parameters of streams were extracted and ultimately compared to progenitor properties across all codes. Results show that generally codes such as GEAR and AREPO-T formed more cored satellite dwarf halos, while all other codes produced less cored or even cuspy dwarf halos, the most cuspy ones being the ART-I candidates. Furthermore, comparisons with observations also showed that cored profiles are favored, and there is minimal tension with observations when the aforementioned codes are considered, although further thorough studies are needed both from observational surveys, and state-of-the-art cosmological zoom-in simulations on dwarf galaxies. Regarding streams, similar broad classification can be made across all codes, where GEAR and AREPO-T showed longer and wider streams, indicating that their structure is more diffuse and spread out compared to the remaining codes, while ART-I produced very compact and concentrated streams. In conclusion, GEAR and AREPO-T formed dwarf satellite galaxies more comparable to the currently available observational data, and their remnants were found to be more diffuse and extended compared to other contending codes in AGORA CosmoRun/CosmoRun2. However, higher resolution cosmological simulations are needed to acquire statistically robust conclusions, together with significantly more observational data from upcoming
surveys, such as ARRAKIHS and Euclid.

Popular Abstract

With the first observations of our Solar neighbourhood, there was evidence that there should be more matter contained inside
structures such as stellar clusters, compared to the luminous matter we see in them. This was determined by looking at how
fast stars orbit around the centers of these structures, compared to models with only the visible content. In order to support
the observed velocities, there must be significantly more matter—called dark matter—which cannot be seen with light. It does
not seem to interact with visible matter directly, but it can still interact through gravity. By our current understanding, it is
because of dark matter that we have grand astrophysical objects such as galaxies in our Universe—it can be thought of as a
vessel for holding visible matter together. A strong piece of evidence for this is dwarf galaxies—smaller cousins of galaxies like
the Milky Way—, which contain relatively few stars and little visible matter, yet remain gravitationally bound, making them
ideal candidates for understanding the nature of dark matter. Lately, they have become targets for large observational surveys.
Stellar streams are stream-like structures of stars that have been torn out of either globular clusters or dwarf galaxies that
orbit more massive galaxies. In recent decades, researchers have found such stellar streams around our, and even external,
galaxies in the Local Group, which is a group of galaxies situated in the vicinity of the Milky Way. Stellar streams form due to
the gravitational interaction between the main galaxy and the satellite dwarf galaxy or globular cluster. This means they can be
used to measure various properties of the main galaxy, such as its mass, its shape, and whether it has fine structures (for example,
spiral arms or other dwarf galaxies). Furthermore, they may also hold information about the dwarf galaxies or globular clusters
they originated from. This makes them particularly interesting to study, especially since they can also teach us more about the
dark matter contained in galaxies, and how it forms and evolves throughout cosmic timescales.
Due to difficulties and limitations posed by our observational capabilities, astrophysicists try to model these galactic systems
and the remnant structures they form after interactions to better understand the nature of our Universe. They test models using
computational simulations, which allow scientists to observe these interactions and analyse them in a thorough manner. By
comparing simulations to observed ”snapshots” of our Universe, this provides an opportunity to test how realistic the current
state-of-the-art models are, and in what ways they must be improved.
The main goal of this thesis is to analyse and compare dwarf galaxies and their remnants across seven large-scale simulations,
which use different intricate numerical techniques but aim to reproduce the same currently supported cosmological model,
called ΛCDM (Lambda cold dark matter). All simulations, albeit behaving in slightly different manners, had the same initial
conditions, meaning that the constituents in the whole artificially created Universe were at the exact same place at the moment
of creation. Across all simulations, nine satellite dwarf galaxies are chosen as comparison candidates, which were formed
from the same aforementioned initial conditions. First, properties of those dwarf galaxies are analysed at the same snapshots
of the simulations, mimicking observations, to understand their general structure and environment. Then, snapshots in the
simulations are found where the aforementioned dwarf galaxies were captured and stripped of their matter content due to
interactions with their host galaxies. In these special cases, the formed stellar streams are further analysed by looking at their
lengths and widths. Finally, the properties of the streams are compared to the properties of their progenitor dwarf satellite
galaxies. The results show that dwarf galaxies with profiles where the matter is more concentrated around the inner center
of their dark matter halo produce shorter, slimmer, and more compact streams. On the other hand, dwarf galaxies that had
more uniformly distributed matter in the central regions—imitating a core-like structure—form longer, wider, and more diffuse
remnants. Furthermore, it is also concluded that certain simulation codes forming dwarf galaxy profiles of the latter type imitate
the observed dwarf galaxies in our Local Group more closely. This project opens a new door in connecting the characteristics of
streams with properties of dwarf galaxies they have originated from, together with analysis methods that can be used on future
simulation results.

• author: Illés, Eduárd
• supervisor: Santi Roca Fàbrega; Oscar Agertz
• organization: Astrophysics; Department of Physics
• course: ASTM32 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: dwarf galaxies, stellar streams, cosmological zoom-in simulations, computational astronomy, galaxy formation, dark matter, extragalactic astronomy
• report number: 2025-EXA242
• language: English
• id: 9193022
• date added to LUP: 2025-06-26 13:40:21
• date last changed: 2025-06-26 13:40:21