Observational methods towards constraining the chemical evolution of galaxies

Abstract

Understanding the array of physical processes that have shaped galaxy assembly remains one of the most fundamental pursuits in astrophysics. Gas in galaxies is enriched with heavy elements via stellar nucleosynthesis, but chemical abundances (``metallicity'') are also shaped by galaxy-scale processes including gas accretion, feedback-driven outflows, radial gas flows, interactions, and mergers. Metallicity measurements therefore afford one of our most powerful observational probes of galaxy evolution. In this thesis I explore the performance of observational methods for constraining (i) gas-phase metallicity in galaxies, and (ii) host dark matter halo masses of galaxies; the latter of which is critical to the physics of gas flows due to its contribution to the gravitational potential well of galaxies. A particular focus is the improved understanding of systematic uncertainties near instrumental limits, which will be vital to maximise the impact of surveys conducted with future facilities.

Galaxy clustering is an efficient approach for drawing statistical connections between galaxies and their host dark matter haloes, however traditional methods are challenging to apply at z > 2 where imaging survey volumes are limited. I instead apply a counts-in-cell approach to photometric z ~ 2 candidates from a random-pointing Hubble Space Telescope survey, showing mean counts of N > ~5 per field are capable of constraining the large scale galaxy bias. The James Webb Space Telescope will achieve comparable number counts out to z ~ 8, and thus a similar JWST survey could place novel constraints on the halo masses of galaxies in the epoch of reionization.

Global metallicities in low-mass galaxies afford important constraints on the impact of feedback-driven outflows on galaxy evolution. However at high-z, obtaining the requisite emission line measurements is observationally challenging. I use Keck/MOSFIRE spectroscopy to explore prospects for extending z ~ 1 - 2 metallicity measurements to lower masses. I find the dominant source of uncertainty arises from reduced number of emission lines as opposed to lower signal-to-noise, even at the detection limit. JWST/NIRSpec will revolutionise high-z metallicity studies due to the large suites of emission lines it will be able to assemble.

Electron temperatures (T_e) measured with auroral lines are an important baseline in metallicity studies. However the faintness of auroral lines has hitherto limited spatially resolved T_e studies. I report two separate studies based on mapping auroral lines in integral-field spectroscopy (IFS) of low-z galaxies. Measurements of auroral lines in the SAMI Galaxy Survey afford new insights into the effects of ionisation parameter variations on recovered metallicity gradients. Applying these principles to Keck/KCWI IFS data of an edge-on disk galaxy, I measure an extra-planar temperature gradient and present preliminary evidence for extra-planar metallicity variations.