Constraining quenching mechanisms at high redshift: the sizes, masses and star-formation histories of massive galaxies
Creators: name: Hamadouche, Massissilia Louisa
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: quenching mechanisms at high redshift
Constraining quenching mechanisms at high redshift
massive galaxies
star-formation histories
high-redshift Universe
bi-modality in the galaxy population
star-forming
quiescent galaxy population

description: Observations of the local and high-redshift Universe have revealed a clear bi-modality in the galaxy population. This is apparent in galaxy colours, morphologies and star-formation rates, with galaxies being categorised into two distinct populations; star-forming and quiescent. The existence of this bimodality means that one or more mechanisms must be able to cease, or quench, star-formation in galaxies. One of the key questions in extragalactic astronomy is to identify which quenching mechanisms are most important and on what timescales they operate. In this thesis, I use a combination of photometry and spectroscopy to examine the evolution of the quiescent galaxy population at high redshift in an attempt to piece together a coherent picture of their evolution over the past ten billion years of cosmic history. Some of the foundational results that have shaped our understanding of galaxy evolution are the relationships between galaxy stellar mass, size and age. In addition to the bi-modality observed up to z ∼ 2, it has also been shown that galaxies which formed earlier in cosmic time are more massive than their later counterparts, a phenomenon often referred to as “downsizing”. In this thesis, ultra-deep spectroscopic data along with HST and JWST imaging from large-scale surveys is used to study the sizes, masses and star-formation histories of massive galaxies from z = 0.25 to z = 2.25. Firstly, I present a study exploring the most massive galaxies at 0.6 &lt; z &lt; 1.3 using ultra-deep spectroscopy and photometry from the VANDELS and LEGA-C ESO public spectroscopic surveys. I investigate the relationships between galaxy stellar mass, physical size and age (using a well-known proxy for age predominant in quiescent galaxy spectra; the Dn4000 index), and demonstrate for the first time that downsizing is clearly evident in both our quiescent samples. I present a toy model to explain the size evolution of massive quiescent galaxies from z = 1.3 to z = 0.6 based on minor mergers. Next, I present the results of full spectral fitting of the VANDELS quiescent sample at 1.0 &lt; z &lt; 1.3, examining in further detail the relationship between stellar mass and age, this time using galaxy formation and quenching times. Upon further investigation into these trends, it becomes apparent that there is a sample of quiescent galaxies which appear to have experienced very short periods of star-formation, followed by abrupt quenching. These galaxies indicate higher levels of α-enhancement compared to the rest of the quiescent galaxies in my sample. This implies that there is an important mechanism at work which can end star-formation on very short timescales. One galaxy in my sample formed less than a billion years after the Big Bang, experiencing a short episode of star-formation before quickly becoming quiescent, based on star-formation and quenching timescales. Building on these key results at z ∼ 1, I investigate whether these trends can be seen at higher redshift using data obtained from the JWST PRIMER and JADES surveys. Due to the improved sensitivity and spatial resolution of JWST imaging, it is possible to examine the sizes and morphologies of star-forming and quiescent galaxies to lower stellar masses than previously possible. Over the redshift range 0.25 &lt; z &lt; 2.25, I find that low-mass quiescent galaxies in my sample exhibit Sérsic indices and sizes qualitatively consistent with these galaxies quenching by infall into cluster environments. Contrastingly, the sizes and morphologies of more massive quiescent galaxies are consistent with having quenched by internal feedback mechanisms. In summary, this thesis aims to provide a quantitative investigation of the evolution of quiescent galaxies over cosmic time. The unprecedented size and quality of the datasets used throughout this work have led to many important findings, enabling a deeper understanding of the properties of quiescent galaxies, and the overall formation and evolution of galaxies across time.