The Impacts of Star Formation on Galaxy Evolution since the End of Cosmic Reionization

Stephenson, H. M. O. and Stott, John (2026) The Impacts of Star Formation on Galaxy Evolution since the End of Cosmic Reionization. PhD thesis, Lancaster University.

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Abstract

Star-forming galaxies at different epochs of the Universe, and those residing in a range of environments, exhibit significant variations in their physical properties and scaling relations, both among themselves and relative to quiescent galaxy populations. Connecting the evolutionary pathways followed by these populations is therefore crucial to understanding galaxy formation and evolution as a whole. In this thesis, we present an analysis of star-forming galaxies across ~11 billion years of cosmic history (0.2 < z < 6.1), spanning both field and galaxy cluster environments. We probe a wide range of galaxy properties and scaling relations using both space- and ground-based facilities, and a variety of observational techniques including slitless grism spectroscopy, wide-field broad-band imaging, and narrow-band emission-line selection. We begin by analysing the chemical evolution of star-forming field galaxies at Cosmic Noon (1.99 < z < 2.32), the epoch during which the cosmic star formation rate density was at its peak. Using strong emission-line calibrations applied to Hubble Space Telescope grism spectroscopy, we determine the gas-phase metallicities of galaxies in the Quasar Sightline and Galaxy Evolution (QSAGE) survey. This analysis allowed us to determine the scaling relations between gas-phase metallicity, galaxy stellar mass and star formation rate to probe the chemical evolution of Cosmic Noon galaxies relative to those in the local Universe. In Chapter 3, we study the spatial distribution of star-forming galaxies in massive clusters at 0.206 < z < 0.494 in the Cluster Lensing And Supernova survey with Hubble (CLASH). Utilising the wide-field imaging capabilities of the Subaru Prime Focus Camera, we investigated the phenomenon known as "anisotropic quenching" (also referred to as "angular conformity" or "angular segregation"), which describes angular dependence of satellite galaxy colour and passive fraction with respect to the major axis of the brightest cluster galaxy. This analysis was conducted to assess the impact of large-scale cosmic web structure on satellite populations in galaxy clusters. In the final science chapter, we analyse the multi-wavelength sizes of star-forming galaxies toward the end of the Epoch of Reionization (z=6.1) using data from the JWST Emission Line Survey (JELS), the first survey to perform narrow-band rest-optical emission-line selection at this epoch. We use these narrow- and broad-band size measurements to determine the size--mass relation of star-forming galaxies at z=6.1. The novel narrow-band imaging from JELS enables a direct comparison between the spatial extent of active star formation and that of established stellar populations, providing new insights into galaxy assembly during the first major epoch of galaxy formation. Since both in-situ star formation and mergers play key roles in galaxy growth, we additionally perform a close-pair analysis to estimate the merger fraction at this epoch.