UC Berkeley

The chemical composition of individual stars provide a fossil record of chemical evolution in a galaxy over cosmic time, encoding important galactic enrichment mechanisms, timescales, and nucleosynthetic pathways. Within the Milky Way, large spectroscopic surveys on modest telescopes have enabled precise chemical abundance measurements for millions of stars and have transformed our understanding of the formation and evolution of the Galaxy. Extending this progress beyond all but the nearest Milky Way satellite galaxies, however, is severely limited by the faintness and crowding of stars at these distances, which require large (6–10+ meter) telescopes and low-resolution spectrographs to achieve sufficient signal/noise (S/N). Recent advances in spectroscopic techniques have dramatically improved our ability to recover accurate and precise chemical abundances from low-resolution spectra. However, these techniques have not yet been extended to the extragalactic regime despite the wealth of data that has been collected in Local Group dwarf galaxies over the past decade. Moreover, with increasingly powerful and multiplexed spectroscopic facilities on the horizon, high-quality (albeit low-resolution) spectra will soon be accessible for millions of stars throughout the Local Group and out to several Mpc. These new datasets have the potential to reveal in unprecedented detail the stellar chemistry of the smallest and faintest galaxies in the Universe, which can in turn be used to investigate the physics of low-mass galaxy evolution. In this dissertation, I lay critical groundwork necessary to fully capitalize on the wealth of chemical information embedded in existing and future extragalactic resolved stellar spectra.

As new spectroscopic facilities are designed, commissioned, and begin amassing large datasets of resolved stellar spectra outside the Milky Way, it is imperative that we understand what chemical abundance information these observations contain. In Chapter 2 of this dissertation, I employ the Cramér-Rao Lower Bound (CRLB) to forecast the theoretical precision to which 41 existing and future spectrographs can measure chemical abundances in metal-poor, low-mass stars in Local Group galaxies. I demonstrate that even at low- to moderate-resolution, blue-optical spectroscopy with modest S/N enables the recovery of a dozen or more elements to a precision of <0.3 dex. Additionally, I find that high-resolution stellar spectra contain substantial chemical abundance information even at low S/N, which can be extracted via full-spectrum fitting techniques. Looking to the future, I show that with reasonable integration times JWST/NIRSpec and 30-m class telescopes can recover ~10 and 30 elements, respectively, throughout the Local Group and bulk metallicities ([Fe/H] and [Alpha/Fe]) for resolved stars out to several Mpc. This analysis is paired with the development and release of an open-source python package, Chem-I-Calc, that facilitates similar forecasts for additional spectrographs, stellar targets, and observing conditions relevant to the astronomical community at large.

In practice, achieving the precision forecasted by the CRLB is impeded by shortcomings of the stellar models used to analyze stellar spectra, which can introduce systematic biases and uncertainties into the measurement of chemical abundances, especially at low resolutions when absorption features are heavily blended. In Chapter 3, I perform a self-consistent analysis of archival Keck/HIRES spectra of low-metallicity stars in M15, which I convolve to lower resolutions in order to quantify the resolution dependence of systematics introduced by model-data mismatches. I demonstrate that systematic biases and uncertainties remain small (~0.1 dex) for 20 (9) elements down to R~10,000 (2500). This analysis illustrates the great promise of low-resolution spectroscopy for stellar chemical abundance measurements in extragalactic systems. As stellar models and spectroscopic fitting techniques improve, the viability of low-resolution resolved stellar spectroscopy will further expand.

Ultra-faint dwarf galaxies (UFDs) represent some of the oldest, lowest mass, most metal-poor, and dark-matter dominated systems, which makes them excellent laboratories to study stellar and galactic physics in the high-redshift Universe and at the faintest end of the galaxy luminosity function. In Chapter 4, I apply a one-zone analytic chemical evolution model within a hierarchical Bayesian framework to CaHK-based photometric metallicity measurements in the reionization-era UFD Eridanus II (Eri II). I present novel constraints on the underlying galaxy formation physics at z>7, finding that Eri II is well characterized by a short (~400 Myr) and inefficient burst of star formation with large supernova-driven gas outflows. The inferred scenario is consistent with the prevailing notion that low-mass galaxies struggle to both convert their gas content into stars and retain their gas reservoirs. Spectroscopic follow-up of stars in Eri II with JWST/NIRSpec will greatly improve the presented constraints. The framework introduced in this chapter can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the early Universe.

As a whole, this dissertation seeks to address a range of current and anticipated challenges in the field of extragalactic stellar archaeology, from the robust measurement of low-resolution stellar chemical abundance patterns to the interpretation of low-mass galaxy chemical evolution. Though considerable work remains in preparation for the next decade of extragalactic stellar spectroscopic observations and chemical evolution studies, this dissertation represents a substantial contribution to the field on which future work can build.