PhD Defense Public Talk

Throughout cosmic history, ultraviolet (UV) emission provides one of the most accessible windows into the formation and evolution of galaxies. Measurements of the rest-frame UV continuum encode information about recent star formation, dust attenuation, chemical enrichment, and stellar population growth, making them a central tool for studying galaxies from cosmic noon through the first billion years after the Big Bang. This dissertation investigates the interpretation and evolution of galaxy UV spectral slopes across cosmic time, spanning the observational transition from the Hubble Space Telescope (HST) era to the transformative capabilities of the James Webb Space Telescope (JWST). By combining deep multi-wavelength imaging, spectroscopic observations, and advanced spectral energy distribution (SED) modeling, this work examines both what UV slopes reveal about galaxies and how the physical processes that shape UV light evolve with redshift.

The first portion of this dissertation focuses on galaxies at intermediate redshifts (z~2-4) observed with the HST UVCANDELS Survey. Using nonparametric SED reconstruction techniques with dense basis, these studies investigate the relationships between UV spectral slopes, dust attenuation, stellar mass assembly, and star formation histories during the peak epoch of cosmic star formation. This work establishes an empirical framework for interpreting UV continuum measurements in systems where stellar populations and dust are strongly coupled.

The dissertation then transitions into the JWST era and explores galaxies at increasingly earlier epochs. Using deep observations from NGDEEP, UV-faint galaxies at z~9-16 are modeled with the Bayesian SED-fitting framework bagpipes to characterize some of the earliest known galaxy populations. These analyses constrain stellar masses, star formation rates, dust attenuation, and UV continuum properties, and examine how observational limitations affect the interpretation of galaxy colors and inferred physical properties at the highest redshifts currently accessible.

A major component of this dissertation establishes robust methodologies for measuring the UV spectral slope in the JWST era at z>5. By combining spectroscopic and photometric observations of high-redshift galaxies with CEERS and RUBIES data, this work evaluates commonly adopted photometric estimators and demonstrates that photometric power-law fitting and SED-based approaches recover UV slopes more reliably than simplified single-color measurements. These results provide practical guidance for interpreting UV slopes and emphasize that the measurement methodology itself can significantly influence inferred trends in galaxy evolution.

This dissertation also presents two ongoing projects that extend these themes to larger statistical samples and new observational regimes. The first is a uniform analysis of approximately 18,000 galaxies spanning z~5-16 across multiple public JWST fields, using the UNICORN project and bagpipes SED modeling to study the evolution of UV continuum properties and galaxy populations throughout the first billion years of cosmic history. The second project uses joint spectroscopic and photometric modeling of galaxies at z>5 from the JWST CAPERS survey to compare observed and stellar UV spectral slopes. By decomposing stellar and nebular contributions to the emergent UV continuum, this work investigates how radiative processing, dust attenuation, and attenuation-curve evolution shape observed UV colors. These results suggest that UV slopes are best interpreted not as direct tracers of stellar populations alone, but as integrated probes of both stellar emission and the physical processes that transform light as it propagates through galaxies.

Collectively, the studies presented in this dissertation advance the interpretation of UV continuum measurements across a wide range of cosmic history. By connecting methodologies developed in the HST era with the unprecedented sensitivity and wavelength coverage of JWST, this work reframes UV spectral slopes as empirical tracers of both galaxy evolution and radiative transfer, providing new insight into the physical conditions governing galaxies from Cosmic Noon to some of the earliest stages of galaxy formation.