DECIPHERING THE CHEMICAL ABUNDANCES OF THE FIRST GALAXIES AT COSMIC DAWN WITH JWST

Abstract

Astronomers have yet to discover the first generation of stars formed in the universe. These long theorized "Population III" stars would be pristine of heavy elements, or "metals", and likely supermassive, very different from stars we observe today. To find the first stars, we must push our observations as early as possible studying exceptionally small, low mass, metal-poor galaxies. In this thesis, I present an extensive study of MACS0647–JD, the brightest known galaxy within the first 500 million years of cosmic history, magnified by gravitational lensing and observed with cutting-edge JWST imaging and spectroscopy. JWST/NIRCam imaging resolves MACS0647–JD into two distinct components. Through analysis of their physical properties and star formation histories, I find that one clump is younger, while the other appears older, suggesting an early-stage galaxy assembly or merger. Using NIRSpec and MIRI spectroscopy, I measured a direct gas-phase metallicity of 13% solar and a carbon abundance of 60% solar, the first such measurements obtained in the first 500 million years. The elevated carbon abundance may be explained by enrichment from remnants of the first generation of stars. To identify even more primitive galaxies, I conducted a search for extremely metal-poor galaxy (EMPG) candidates using deep, pure-parallel JWST/NIRCam grism spectroscopy. I discovered seven EMPG candidates with metallicities below 2% solar. These galaxies appear to be undergoing starbursts, with high star formation rates (SFR~0.2-2 Msun/yr) and low stellar masses (log(M/Msun)~6.8-7.8). The continued search for distant galaxies, EMPGs, and first star candidates is ongoing, and will undoubtedly transform our understanding of galaxy formation and chemical evolution at cosmic dawn