Can Dwarf Galaxies Tell Us How the Milky Way Formed?

Abstract

Dwarf galaxies are a key component in understanding the evolution of galaxies and provide tests on the standard cosmological $\Lambda$ Cold Dark Matter model. According to the formation hierarchy, dwarf galaxies are the pieces that create all galaxies. Our current understanding of dwarf galaxies faces discrepancies between theory and observations. These problems are mostly due to incompleteness in observational data and the low number of simulations available. Simulations are based on the physics that drive the Universe yet they are limited by resolution and our understanding of the Universe. Observations are limited by technology and brightness limitations.

The purpose of this project is to study the effect of last major merger modifications on the properties of satellites, such as the satellite luminosity function, the satellite ages and the quenching fraction, around Milky Way-like galaxies. We also show the effect that surface brightness limits have on the number of satellites and the total mass of satellites that a survey might see. To fulfil this, we use 8 cosmological hydrodynamic+N-body zoom-in simulations of Milky Way-mass galaxies. Five of those simulations are from a project called {\footnotesize VINTERGATAN-GM}, where the last major merger mass is modified to create five different simulations of identical final dynamical mass. For this project, these simulations provide an excellent investigation of the effect on the satellite population for the last major merger.

In our analysis, we find that the surface brightness limit greatly effects the number of satellite and the masses that surveys would see. Going form a limit of 26 mag/arcsec$^2$, the value of today's surveys, to a limit of 32 mag/arcsec$^2$ the number of satellites increased and the masses of already detected satellites increased. Using the {\footnotesize VINTERGATAN-GM} simulations, we show how the satellite luminosity function evolves. We see that the assembly history of the functions change with the mass of the last major merger, where the galaxy with the smallest merger evolves with a larger number of satellites than the galaxy with the largest merger.

By analysing the star formation histories of the simulations, we determined the ages of the satellites. We see an age trend from the smallest merger to the largest, where the oldest are the satellites around the galaxy with the largest merger. We also see the same trend in the quenching fraction where the older satellites are more quenched than the younger. We calculate the quenching fraction for all 8 simulations in order to compare with observations. We use results from two disagreeing large scale surveys, SAGA and ELVES, to compare with our results. Our results lie in between the two surveys with a quenching fraction of 29\%. This does not match with the known quenching fraction of the Milky Way and M31, which is higher than 90\%. This is one of the discrepancies in the studies of satellites, which will benefit from better observations, a larger sample of simulations, and further investigations of quantities such as gas, metallicity, structures, and other galactic effects.

Popular Abstract

Some of the most exotic objects in the universe are galaxies. Galaxies are observed on the night sky as majestic, colourful and beautifully structured objects, that are the pieces in the Universe puzzle. Galaxies are gravitationally bound systems of stars, gas, dust, and dark matter. Galaxies can only be observed through a telescope since they are far away and too faint to see with the naked eye.

Galaxies come in all shapes and sizes. The theory of evolution in the Universe states that smaller galaxies merge to create bigger galaxies. This has the implication that galaxies like our own Milky Way will have a population of dwarf galaxies (often called satellites) floating around them. The Milky Way has a dwarf galaxy population that effects the dynamics and the overall evolution of it. In this project, we ask if the satellite populations around host galaxies can tell us anything about the evolution of the Milky Way.

The field of galaxy formation and evolution is a growing field. Today, we see many discrepancies between theory and observations. That means our simulations are not showing the same results as observations. The simulations account for dark matter, baryons, stellar feedback and multiple other properties, but the lack of computational power and resolution is still hindering the completeness of models. Observations are also not free from faults. Observations are limited by technology in a way that depends on distance and brightness. When seeing satellites far away, the telescopes see only down to a certain brightness limit. In this thesis, we have shown that reaching fainter limitations in brightness, gives a greater number of satellites and more mass. This means that observations today are not seeing the full picture and lack data to make their analysis.

Luckily for us, there are multiple telescopes in preparation and simulations are always being improved that will solve the discrepancies in the future. The future will provide us with a better understanding of the Milky Way. In this thesis, we determine the effect that slight modifications of the same Milky Way-like simulation has on the properties of satellites around the main galaxy. We see that the modifications effect the number of satellites, the ages of the satellites, and the number of star forming satellites. These properties are all important in the theory of galaxy formation and evolution.

• author: Karlsdóttir, Þóra
• supervisor: Oscar Agertz
• organization: Astrophysics; Department of Physics
• course: ASTM32 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Dwarf Galaxy, Milky Way
• report number: 2023-EXA213
• language: English
• id: 9130735
• date added to LUP: 2023-06-28 10:58:52
• date last changed: 2023-08-30 15:00:05