title: Characterising the effect of environment on galaxy evolution
Creators: name: Kukstas, E
Publisher: name: Liverpool John Moores University
Year: 2020

description: Galaxies are not the `island Universes' they were once thought to be. Instead, they form a part of a larger structure called the `cosmic web' which consists of dark matter, gas, and stars in order of decreasing fraction of the total mass budget. As part of it, galaxies are both influencing the cosmic web and are influenced by it. There is strong evidence demonstrating that galaxies located in dense environments (such as group or clusters) exhibit suppressed star-formation rates, red colours, early-type morphologies, and older stellar populations than their counterparts in the general `field'. This feature is often referred to as environmental galaxy quenching and, while there are many possible processes proposed as being responsible for this transformation, the detailed understanding of how it takes place is still lacking. In this thesis I propose a new method of characterising the link between galaxies and the environment responsible for quenching them, as well as improving on existing methods of studying galaxy transformation in dense environments. Cosmological, hydrodynamical simulations are used extensively to understand and interpret the results. Further work is necessary to make them capture the complex physics in groups and clusters - I highlight some of the limitations. I begin by proposing a map-based method involving spatial cross-correlations between gas measures and a low-redshift galaxy survey (z &lt;= 0.15). This approach avoids the issues associated with membership assignment, and also directly links the underlying measure of environment and galaxy properties. I demonstrate that it can be applied to current observations from SDSS, Planck, and ROSAT surveys, yielding strong cross-correlation signals between gas pressure/density and galaxy density/quenched fraction. Hydrodynamical simulations, EAGLE and BAHAMAS, both reproduce the observed signal with some variation due to feedback implementations. The simulations can also be used to understand the measurements: I use BAHAMAS to demonstrate that most of the signal in Sunyaev-Zel'dovich effect -- quenched fraction cross-correlation originates from quenched satellites in groups and clusters. The same exercise shows that BAHAMAS over-quenches satellite galaxies. Next, I investigate the performance of three hydrodynamical simulation codes (BAHAMAS, EAGLE AGNdT9, TNG300) at z ~ 1. There is evidence to suggest that quenching mechanisms may be different at this regime relative to the nearby Universe. Simulations do not currently capture all the necessary processes of quenching at z ~ 0. If the processes change between the two epochs, there is a possibility that simulations perform better at higher redshifts. I make several predictions of stellar content in haloes as well as quenched fraction from all three simulations in preparation for observational counterparts from the GOGREEN survey. There is great variation in the predicted relations, demonstrating that models used in simulations are relatively unconstrained in their current form. Comparing to data that is currently available indicates that none of the three simulations fully capture quenching of galaxies in dense environments at z ~ 1. Further observations will be able to inform future implementations of feedback for better agreement. Galaxy membership assignment is, potentially, a big source of uncertainty in observations, especially at higher redshifts. Simple aperture combined with velocity cut methods are commonly employed in studies of group and cluster galaxies. I investigate the potential biases introduced from one such method, demonstrating that a relatively large number of contaminants is introduced. This severely affects the dominant type of galaxies and, subsequently, measured quenched fractions. I identify the main sources of contaminants and make suggestions on how to minimise them.