Signatures of the First Stars in Relics of the First Galaxies

In the billion years after the big bang, the first stars and galaxies polluted the universe with the first heavy elements and produced high energy photons that reionized the intergalactic medium. Understanding this early era is at the frontier of modern astrophysics and cosmology, but direct observations are challenging. An alternate approach is to study the old dwarf galaxies that orbit our Milky Way galaxy today, which descended from the first galaxies and contain stars that retain a record of their past. I will first explore theoretical models to interpret the chemical signatures preserved in these relic stars, tracing them to their present day locations in the Milky Way. I then use high-resolution spectroscopy to measure the chemical abundances of stars in three ultra-faint dwarf galaxies. The key result is a serendipitous discovery with important implications for the origin of the elements. Stars in the galaxy Reticulum II display extremely enhanced r-process abundances 2-3 orders of magnitude higher than in other ultra-faint dwarf galaxies. The neutron-capture material in Reticulum II was thus synthesized in a single prolific event, likely a neutron star binary merger, addressing a 60-year-old question about the astrophysical origin of r-process elements.