New Insights on the Evolutionary Mechanisms of Local and Distant Galaxies

Abstract

Galaxies can be roughly characterised by falling into two distinct populations, blue, star-forming, disc dominated and red, quiescent elliptical bulge dominated systems. These populations are defined via multiple scaling relations such as the star-forming main sequence, Schmidt-Kennicutt relation and Molecular Gas Main Sequence. But what halts star-formation in the disc galaxies and causes them to turn into quiescent bulge dominated galaxies? This stopping of star-formation is the process of galaxy quenching. Exploring this is the target of my thesis. In the first part of my thesis I use a methodology of partial correlation coefficients and random forest regression to disentangle multiple inter-correlated galaxy properties enabling me to determine the intrinsic drivers of some of the key galaxy observables. I will start in Chapter 1 with an introduction to the field and the reasoning behind the research. Chapter 2 then introduces the methodology used and explains partial correlation coefficients and random forest regression. Then I start in Chapters 3 and 4 by exploring the drivers of the star-forming main sequence, the relationship between star-formation rate (SFR) and stellar mass ($M_$), finding that it is simply a by-product of the more fundamental Schmidt-Kennicutt relation, between SFR and molecular gas mass ($M_{H_2}$), and the Molecular gas main sequence, between $M_{H_2}$ and $M_$, on both global and spatially resolved scales. I show that this is the case throughout most of cosmic time. In Chapters 5 and 6, I then expand my analysis to investigating gas-phase metallicities and the real drivers of the mass-metallicity relation (MZR) and fundamental metallicity relation (FMR) on both resolved and global scales. I find evidence for a resolved version of the FMR and also that local metallicity depends on both local and global galactic properties, hence the global FMR is not simply a by-product of the local version. I also find that dilution alone cannot describe the FMR. I discover that gas-phase metallicity does actually depend on stellar mass and that this is not simply a proxy for the dynamical mass or gravitational potential. I suggest that the explanation for the MZR is via the effects of integrated metal production. In Chapter 7 I investigate the drivers of stellar metallicity for star-forming and passive galaxies in order to investigate quenching explicitly. I find that for star-forming galaxies the stellar metallicity is driven by stellar mass as expected, but that for passive galaxies it is driven by black hole mass, showing the effects of integrated AGN feedback in quenching the galaxies via starvation. In Chapter 8 I utilise the power of the new JWST to explore the properties of one of the earliest galaxies. I use an approach of spatially resolved photometry to explore the properties of the galaxy's disc and core components. I then use SED modelling to infer the stellar population properties and compare to more local analogues to explore the galaxy's role as a possible progenitor.