Characterising the effect of environment on galaxy evolution

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 <= 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

Environment from cross-correlations: connecting hot gas and the quenching of galaxies

The observable properties of galaxies are known to depend on both internal processes and the external environment. In terms of the environmental role, we still do not have a clear picture of the processes driving the transformation of galaxies. This may be due to the fact that these environmental processes depend on local physical conditions (e.g., local tidal force or hot gas density), whereas observations typically probe only broad-brush proxies for these conditions (e.g., host halo mass, distance to the N^th nearest neighbour, etc.). Here we propose a new method that directly links galaxies to their local environments, by using spatial cross-correlations of galaxy catalogues with maps from large-scale structure surveys (e.g., thermal Sunyaev-Zel'dovich [tSZ] effect, diffuse X-ray emission, weak lensing of galaxies or the CMB). We focus here on the quenching of galaxies and its link to local hot gas properties. Maps of galaxy overdensity and quenched fraction excess are constructed from volume-limited SDSS catalogs, which are cross-correlated with maps of tSZ effect from Planck and X-ray emission from ROSAT. Strong signals out to Mpc scales are detected for all cross-correlations and are compared to predictions from cosmological hydrodynamical simulations (the EAGLE and BAHAMAS simulations). The simulations successfully reproduce many, but not all, of the observed power spectra, with an indication that environmental quenching may be too efficient in the simulations. We demonstrate that the cross-correlations are sensitive to both the internal and external processes responsible for quenching. The methods outlined in this paper can be easily adapted to other observables and, with upcoming surveys, will provide a stringent direct test of physical models for environmental transformation

The GOGREEN survey: The environmental dependence of the star-forming galaxy main sequence at 1.0<z<1.51.0<z<1.5

We present results on the environmental dependence of the star-forming galaxy main sequence in 11 galaxy cluster fields at $1.0 < z < 1.5$ from the Gemini Observations of Galaxies in Rich Early Environments Survey (GOGREEN) survey. We use a homogeneously selected sample of field and cluster galaxies whose membership is derived from dynamical analysis. Using [OII]-derived star formation rates (SFRs), we find that cluster galaxies have suppressed SFRs at fixed stellar mass in comparison to their field counterparts by a factor of 1.4 $\pm$ 0.1 ($\sim3.3\sigma$) across the stellar mass range: $9.0 < \log(M_{*} /M_{\odot}) < 11.2$. We also find that this modest suppression in the cluster galaxy star-forming main sequence is mass and redshift dependent: the difference between cluster and field increases towards lower stellar masses and lower redshift. When comparing the distribution of cluster and field galaxy SFRs to the star-forming main sequence, we find an overall shift towards lower SFRs in the cluster population, and note the absence of a tail of high SFR galaxies as seen in the field. Given this observed suppression in the cluster galaxy star-forming main sequence, we explore the implications for several scenarios such as formation time differences between cluster and field galaxies, and environmentally-induced star formation quenching and associated timescales

The GOGREEN survey: Post-infall environmental quenching fails to predict the observed age difference between quiescent field and cluster galaxies at z>1

We study the star formation histories (SFHs) and mass-weighted ages of 331 UVJ-selected quiescent galaxies in 11 galaxy clusters and in the field at 11 has been driven by different physical processes than those at play at z=0

GOGREEN: a critical assessment of environmental trends in cosmological hydrodynamical simulations at z ~ 1

Recent observations have shown that the environmental quenching of galaxies at z ∼ 1 is qualitatively different to that in the local Universe. However, the physical origin of these differences has not yet been elucidated. In addition, while low-redshift comparisons between observed environmental trends and the predictions of cosmological hydrodynamical simulations are now routine, there have been relatively few comparisons at higher redshifts to date. Here we confront three state-of-the-art suites of simulations (BAHAMAS+MACSIS, EAGLE+Hydrangea, IllustrisTNG) with state-of-the-art observations of the field and cluster environments from the COSMOS/UltraVISTA and GOGREEN surveys, respectively, at z ∼ 1 to assess the realism of the simulations and gain insight into the evolution of environmental quenching. We show that while the simulations generally reproduce the stellar content and the stellar mass functions of quiescent and star-forming galaxies in the field, all the simulations struggle to capture the observed quenching of satellites in the cluster environment, in that they are overly efficient at quenching low-mass satellites. Furthermore, two of the suites do not sufficiently quench the highest mass galaxies in clusters, perhaps a result of insufficient feedback from AGN. The origin of the discrepancy at low stellar masses (⁠M∗≲1010 M⊙), which is present in all the simulations in spite of large differences in resolution, feedback implementations, and hydrodynamical solvers, is unclear. The next generation of simulations, which will push to significantly higher resolution and also include explicit modelling of the cold interstellar medium, may help us to shed light on the low-mass tension

Advances in the treatment of prolactinomas

Prolactinomas account for approximately 40% of all pituitary adenomas and are an important cause of hypogonadism and infertility. The ultimate goal of therapy for prolactinomas is restoration or achievement of eugonadism through the normalization of hyperprolactinemia and control of tumor mass. Medical therapy with dopamine agonists is highly effective in the majority of cases and represents the mainstay of therapy. Recent data indicating successful withdrawal of these agents in a subset of patients challenge the previously held concept that medical therapy is a lifelong requirement. Complicated situations, such as those encountered in resistance to dopamine agonists, pregnancy, and giant or malignant prolactinomas, may require multimodal therapy involving surgery, radiotherapy, or both. Progress in elucidating the mechanisms underlying the pathogenesis of prolactinomas may enable future development of novel molecular therapies for treatment-resistant cases. This review provides a critical analysis of the efficacy and safety of the various modes of therapy available for the treatment of patients with prolactinomas with an emphasis on challenging situations, a discussion of the data regarding withdrawal of medical therapy, and a foreshadowing of novel approaches to therapy that may become available in the future