UC Berkeley

Strong gravitational lensing of distant background galaxies into magnified multiple-images by foreground galaxy clusters presents a number of exciting opportunities for observational astrophysics. This thesis concerns two primary applications of this observed consequence of Einstein's general relativity. Using multiply-imaged galaxies as constraints, this thesis models the mass distributions of two galaxy clusters. Combining these mass models with ground- and space-based data we determine these clusters are likely recent mergers, and may be useful for detecting transient lensed stars. Leveraging the achromatic magnification provided by strong lensing, this thesis also studies the z=2.37 Sunburst galaxy, focusing on two young massive star clusters which are bright and resolved from other star-forming regions in the galaxy due to lensing. Photoionization modeling with joint photo-spectroscopic data of these clusters reveal they host nebulae with unusual abundance patterns; both showcasing heavy gas-phase nitrogen enrichment. We investigate the physical origin of these nebulae, and connect these unusual abundances to the puzzle of multiple stellar populations in globular clusters. This thesis also analyzes the z=1.78 SN H0pe, which is triply-imaged due to strong gravitational lensing. JWST observations of all three SN images confirmed the Ia typing, and enabled photometric and rare spectroscopic measurements of the two relative time delays. Synthesizing these measured delays with strong-lensing mass models yields a value for the Hubble-Lemaitre constant (H0). Crucially, the lensing magnification obtained from the standard candle augments this inference, breaking strong-lensing degeneracies. Following strict double-blinded protocols, these two measured time delays and three measured magnifications were compared to the predictions of seven independently constructed cluster strong-lensing models to measure H0 to $<10\%$ uncertainty. Finally, this thesis revisits the enigmatic, Earendel, a lensed star candidate in the z=6 Sunrise galaxy. Using JWST spectroscopy, we evaluate Earendel's likelihood as a star cluster rather an an individual star or binary. We find the spectrum to be highly consistent with a star cluster, and moreover find that Earendel, as well as another star cluster within the Sunrise, are consistent with the formation age-metallicity expectations for intermediate age star clusters in the first billion years of the universe.