Signatures of the first stars in relics of the first galaxies

Abstract

In the billion years following the big bang, the first stars and galaxies fundamentally transformed the universe. They polluted the universe with the first heavy elements, and they produced high energy photons that reionized the intergalactic medium. Understanding this early era is at the frontier of modern astrophysics and cosmology, but it takes place so long ago that direct observations are challenging. An alternate approach is to study local relics that have survived from ancient times. Indeed, the old dwarf galaxies that orbit our Milky Way galaxy today have descended from the first galaxies and contain stars that retain a remarkably detailed record of their past. In this thesis, I use the chemical abundances of stars in old dwarf galaxies to open a window into the era of first stars and galaxies. I begin by exploring theoretical models for interpreting the chemical signatures preserved in these relic stars. Then, I help run a suite of simulations that can trace the first star forming systems to their present day locations in the Milky Way. Finally, I use high-resolution spectroscopy to measure the chemical abundances of stars in three ultra-faint dwarf galaxies (Bootes II, Reticulum II, and Tucana II) that are relics from the early universe. The most important result is a serendipitous discovery with important implications for the origin of the elements. Seven of nine stars I observed 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 r-process abundances imply that the neutron-capture material in Reticulum II was synthesized in a single prolific event, likely a neutron star binary merger. This discovery has helped address a 60-year-old question about the astrophysical origin of r-process elements.