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Clusters of galaxies are particularly interesting astrophysical systems, are the largest bound structures in the Universe, and contain fair sample of cosmic ingredients. Studies of cluster abundance as a function of mass and redshift were critical in establishing the standard model of cosmology. This dissertation presents results from X-ray imaging of massive distant (M > 10^14 M; 0:3 < z < 1.2) clusters, found via X-ray emission or Sunyaev-Zeldovich eff ect. This is the world's largest sample of massive galaxy clusters. We explore the radial and azimuthal profi les of the X-ray emitting gas and show that clusters are self-similar objects: their internal structure is largely independent of the cluster's mass or redshift, and the fractions of di fferent types of clusters does not change with redshift. We also present a new statistical technique for measuring a cluster's deviations from a perfect axisymmetric shape, which is especially useful in the case of low photon count observations of distant clusters. / Physics

Galaxy clusters are the most massive collapsed structures in the Universe and their properties offer a crucial insight into the formation of structure. High quality observational data is forthcoming with ongoing and upcoming surveys, but simulations are needed to provide robust theoretical predictions for comparison, as well mock data for testing observational techniques. Numerical simulations are now able to accurately model a range of astrophysical processes. This is highlighted in the BAHAMAS and MACSIS simulations, which have successfully reproduced the observed scaling relations of galaxy clusters. We use these simulations to quantify the impact baryons have on the mass distribution within galaxy clusters, as well as the bias in X-ray and weak lensing mass estimates. It is shown that baryons have only a minor affect on the spins, shape and density profiles of galaxy clusters and they have no significant impact on the bias in weak lensing mass estimates. When using spectroscopic temperatures and densities, the X-ray hydrostatic mass bias decreases as a function of mass, leading to a bias of ~40% for clusters with M_500 > 10^15 solar masses. In the penultimate chapter, we use the EAGLE and C-EAGLE simulations to construct more realistic mock cluster observations. The EAGLE simulations have been shown to successfully reproduce the properties of field galaxies and they are complemented by the C-EAGLE project, which extends this work to the cluster scale. We use these simulations to construct a cluster lightcone that accounts for the impact of uncorrelated large scale structure on cluster observables, including weak lensing mass estimates, the Sunyaev-Zel'dovich parameter and X-ray luminosity.

Chameleons are scalar fields coming from modied gravity theories and can be possible explanations for Dark Energy. They cause a fifth force and have a screening mechanism which allows this force to avoid solar system constraints. In this thesis, astrophysical consequences of the potential presence of the chameleon field will be studied. More precisely, the difference between the hydrostatical and the weak lensing mass of galaxy clusters due to the effect of the chameleon fifth force is discussed.

The cores of galaxy clusters contain the most massive galaxies in the Universe, the brightest cluster galaxies. These galaxies are unique compared to their counterpart galaxies outside of clusters as they have much brighter cores, and vast spatially- extended stellar envelopes. The theoretical picture of how they reached their huge masses relied on the idea of gradual stellar mass growth during the second half of the history of the Universe. However, recent observational evidence of highly-overdense protoclusters, the progenitors of these galaxies, demonstrates that some brightest cluster galaxies may have assembled within the first few billion years after the Big Bang – seemingly contradicting our theoretical predictions. I include my theoretical work that shows the short timescales over which these observed protoclusters trans- form into the brightest cluster galaxies and discuss the likelihood of finding these rare protoclusters in the early Universe. To push our understanding of the rapid evolution of these galaxies even further for- ward demands the use of numerical simulations due to the highly coupled, non-linear astrophysical processes that occur during the process. In this dissertation, I include improvements to our numerical models of hydrodynamical turbulence and supermas- sive black holes that I incorporated into a state-of-the-art hydrodynamical+gravity simulation code, in effort to provide the groundwork to improving our understanding of the build-up of the brightest cluster galaxies in the early Universe, and galaxy evolution in general. / Graduate

<p>  Understanding the formation and evolution of galaxies is one of the most fundamental questions in modern astronomy. While it is widely accepted that galaxy formation needs to be understood in the context of cosmic structure formation of dark matter, a complex interplay of different physical processes that drive galaxy formation makes it challenging to elucidate how the large-scale environment of dark matter influences galaxies, particularly in their formative epoch (z > 2). </p> <p>    As the most luminous nebular emission arising from star formation,  Lyalpha provides a promising and effective tool to study the young universe and nascent galaxies.</p> <p>  At z>2, Lyalpha emission is redshifted into the visible window that is detectable by ground-based telescopes. Existing studies also suggest that strong Lyalpha-emitting galaxies represent a young and low-mass galaxy population and therefore are the best visible tracers of the large-scale structure of the distant universe. </p> <p>    In this thesis, I present two complementary studies designed to address these questions using Lyalpha emission as a cosmological tool. In Chapter 2, I investigate the kinematics and spatial distribution of the gas-phase interstellar and circumgalactic media using compact and diffuse Lyalpha emission in and around distant galaxies. I also carry out a comprehensive characterization of how Lyalpha properties correlate with other galaxy properties and the environment that galaxies reside in. In Chapter 3, I explore how Lyalpha-emitting galaxies trace the large-scale structure characterized by other means; I also conduct a detailed investigation of the distribution of different `types' of galaxies and H i gas  in  and around the most massive cosmic structure known to date. These investigations are informative in  building clear  expectations for the ongoing and upcoming experiments -- including the Legacy Survey for Space and Time, James Webb Space Telescope, Dark Energy Spectroscopic Instruments, and Hobby-Eberly Telescope Dark Energy eXperiment -- in obtaining a detailed picture of galaxy evolution in the context of their environments. </p>

Galaxy clusters, the most massive gravitationally bound objects in the universe, provide an important setting for exploring the structure and interactions of matter in the cosmos. When galaxy clusters merge, there is ample opportunity to examine interactions between densely-packed halos of luminous and dark matter; thus, understanding the dynamics of merging clusters provides insight into understanding properties of dark matter. This paper examines the galaxy cluster 1 RXS J0603.3+4214 (“Toothbrush Cluster”), incorporating information about the polarization of its associated radio relics into Monte Carlo simulations to constrain knowledge about its inclination angle, time since collision, and the velocity and separation distance between its subclusters. We find that the collision velocity, time since merger, and 3D separation between subclusters are well-constrained, which allows for more accurate analysis of the history of the merger. This type of constraint could be applied to a variety of merging systems. Additionally, this constraint may allow opportunity for exploring the validity of different models of dark matter.

Early-type galaxies (ETGs) are typically thought of as 'red and dead' with little to no star formation and old stellar populations. Their detailed kinematics measured locally suggest an interesting array of formation mechanisms and high-redshift observations are starting to reveal a two-phase evolutionary path for the most massive galaxies. In this thesis, I take a combined approach to studying the formation of ETGs. I look to distant quiescent galaxies in one of the densest regions of the early Universe and at the fossil record of a local galaxy to shed light on some of the unsolved mysteries of how ETGs evolved. Using the unique multiplexed instrument, the K-band Multi-Object Spectrograph (KMOS), the evolution of galaxies at both low and high redshift were studied as part of this thesis. I maximised the capabilities of this multi-integral field unit (IFU) near-infrared (NIR) instrument to study different aspects of ETG evolution. With 24 separate IFUs, many quiescent galaxies were efficiently observed in a massive high-redshift cluster as part of the KMOS Cluster Survey. Coupling KMOS spectroscopy with Hubble Space Telescope photometry, I studied the ages, kinematics, and structural properties of the galaxies. I then analysed the detailed properties of a massive local ETG with interesting kinematics, IC 1459. Coupling the NIR IFU data from KMOS with a large mosaic of optical data from the Multi-Unit Spectroscopic Explorer, I was able to study the spatially resolved kinematics, stellar populations, and initial mass function of the galaxy. The work presented in this thesis provides some interesting clues as to the formation of ETGs and possible diversity of their evolutionary paths.