Analysing and Comparing the Intra-Halo Region in Milky Way-like Simulations

Abstract

Dark matter has an important role in our Universe, especially in galaxy formation and evolution, but there are many theoretical models that attempt to describe it and to find out which one, if any, are correct we must test them. Testing a theoretical model of this nature at a galactic scale must and has been done using computational methods through simulations, but this has revealed more complicated problems. One of the more successful of the models is the cold dark matter (CDM) model, assuming non-relativistic dark matter velocities, but it still has problems, such as at smaller scales.

Testing models can be done in several different ways, such as changing the dark matter species i.e. particle type, but in this work the focus was on the effects of using slightly different baryonic physics on the distribution of matter, mostly stars, outside the disc and bulge in the so called intra-halo region (IHR). The light from the stars in this IHR, referred to as the intra-halo light (IHL) in this work, is expected to be correlated to the assembly history of the galaxy which in turn depends strongly on the properties of dark matter.

To test the baryonic physics in the IHR, an analysis and comparison of different galaxy simulations produced by the different codes ART-I, CHANGA, ENZO, GADGET-3, GEAR, GIZMO, and finally AREPO was done. All of these simulations were at a redshift of z=4, had the same initial conditions, very similar physics, and Milky Way-like mass. They were all produced and provided by the AGORA Collaboration assuming a Λ-CDM cosmological model with a matter density of Ω_m = 0.272, a vacuum density of Ω_Λ = 0.728, and a Hubble constant of H_0 = 70.2 km/s.

The first goal of this thesis was to produce radial profiles of the gas, stars, and dark matter belonging to the simulations from each code and compare these to similar ones produced by AGORA in order to both confirm their reproducibility and ensure that they were correct. The absolute majority of them had a strong resemblance, and many results were reproduced, but some discrepancies were found. While the exact source of the differences could not be properly discerned, they were most likely due to complications with the analysis tool yt.

The second goal was to isolate the IHR and again produce and compare radial profiles of the gas, stars, and dark matter but now only this region. This showed that the method of isolating the IHR using a transition radius based on the virial radius of the galaxy/satellite was effective in removing large quantities of matter but had, as expected, some problems with accuracy. Comparing the radial profiles revealed that most of them had a very strong similarity to the counterpart in the whole galaxy but with more noise. The source of the noise was most likely due to the decrease in sample size but could, for some of the properties, also be a characteristic of the IHR.

This work was partly intended as a preparatory work to in future assist in separating the effects of different baryonic physics from the change in dark matter properties which could hopefully reveal more about dark matter itself.

Popular Abstract

The Universe is a strange place that has asked many questions, we have answered some of them but many remain and even more are unknown. One of those we have actively begun to answer is the question of dark matter. How do we know it exists? How can we learn more? What is it? The first of these subquestions has already been answered. We know mark matter exists because some unknown invisible mass is required e.g. for the rotational velocities of matter around galaxies, such as our own Milky Way, to make sense and for the massive structures we observe, such as galaxies and galaxy clusters, to be stable. But to understand these phenomena we need to know more about dark matter, and as it seems to have a large role for galaxies the answer might lie in their formation and evolution.

But the question is then, how can we learn more? One of the problems with learning more about dark matter is that we have made many different theoretical models but we can not test them experimentally, as many other branches of physics can, and instead rely on observations and computational methods. Observations are however for the most part out of the questions as they are very limited both due to our current technology but also because we only see galaxies at one of their many phases. We have therefore turned towards computational methods, more specifically simulations.

This work focuses on using these galaxy simulations, more specifically galaxies similar to the Milky Way, and their evolution with the purpose of analysing the properties of its gas, stars, and dark matter inside its bulge, disc, and halo. This was done firstly to compare with the results from the group researchers that gave the data called the AGORA (Assembling Galaxies Of Resolved Astronomy) Collaboration. They focus on verifying that the computational methods that are being used for the simulation codes produce the same results independent on the approach, therefore ensuring that the physics make the difference and not the method. The other goal was to analyse the baryonic physics, meaning the physics involving normal atomic matter such as proton and neutrons, in the specific part surrounding many galaxies called the intra-halo light, which is the light the stars in the halo produce. The distribution of these stars are expected to depend on the properties of dark matter, but also the baryonic physics that is being considered. This was partly a preparatory work to in the future analyse how a change in the properties of dark matter changes the intra-halo light, but for this to be possible one must know how the baryonic physics also affects it, so they can be separated.

Popular Abstract (Swedish)

Universum är en underlig plats som har ställt många frågor, vissa som vi svarat på men många är kvar och ännu fler är okända. En av de som vi aktivt börjat besvara är frågan om mörk materia. Hur vet vi att det finns? Hur kan vi veta mer? Vad är det? Den första av dessa delfrågor har vi redan besvarat, vi vet att mörk materia finns för att det behövs någon osynlig massa för att, bland annat, rotationshastigheterna av materia runtom galaxer, så som vår egen Vintergatan, ska vara rimliga och de massiva strukturer som vi observerar, så som galaxer och galaxhopar, ska kunna vara stabila. Men för att förstå dessa fenomen så måste vi veta mer om mörk materia, och genom att den verkar ha en stor roll för galaxer så ligger kanske svaret i deras formation och utveckling.

Men frågan är då, hur kan vi veta mer? En av problemen med att lära oss mer om mörk materia är att vi har skapat många olika teoretiska modeller men vi kan inte dem experimentellt, så som många andra grenar av fysik kan, utan vi förlitar oss på observationer och beräkningsmetoder. Observationer är däremot för det mesta uteslutet då de är väldigt begränsade, både med vår teknologi men också för att vi ser endast galaxer i en av dess många faser. Därför har vi vänt oss mot beräkningsmetoder, mer specifikt simulationer.

Detta arbete fokuserade på att använda dessa simulationer av galaxer, mer specifikt Vintergatan-liknande galaxer, och deras utveckling i syfte att undersöka egenskaperna hos dess gas, stjärnor och mörka materia i dess bula, disk, och halo. Detta gjordes först för att jämföra med resultaten från gruppen forskare som gav datan vid namn the AGORA (Assembling Galaxies Of Resolved Astronomy) Collaboration. De fokuserar på att verifiera att de beräkningsmetoder som används för simulationskoderna producerar samma resultat oberoende på tillvägagångsätt, därmed säkra att det är fysiken som gör skillnad och inte metoden. Det andra målet var att undersöka den baryoniska fysiken, vilket är fysiken kring normal atomisk materia så som protoner och neutroner, i den specifika delen runt om många galaxer vid namn intra-halo regionen, som är den region där ljuset från stjärnorna i halon, som heter intra-halo ljuset, kommer ifrån. Fördelningen av dessa stjärnor är förväntade att bero på egenskaperna hos mörk materia, men de beror också på den baryoniska fysiken som användes. Detta är därmed delvis ett förberedande arbete för att i framtiden undersöka hur en ändring på den mörka materians egenskaper har på detta intra-halo ljus, men för att göra det så måste man veta hur den baryoniska fysiken också påverkar det, så man kan sära på dem.

• author: Lindh, Isak
• supervisor: Santi Roca Fàbrega; Tilly Evans-Hofmann; Oscar Agertz
• organization: Department of Physics; Astrophysics
• course: FYSK04 20241
• year: 2024
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: cosmology: theory, galaxies: evolution, galaxies: formation, methods: numerical, Astrophysics - Astrophysics of Galaxies, Astronomical simulations, Hydrodynamical simulations, Code comparison, Intra-halo light, IHL, Intra-halo region, IHR
• report number: 2024–EXA222
• language: English
• id: 9154955
• date added to LUP: 2024-05-29 07:55:50
• date last changed: 2024-05-29 07:55:50