The Legacy of the First Galaxies: Exploring Ancient Stars in the Milky Way

Abstract

In the first several hundred million years after the Big Bang, the first stars and galaxies transformed the Universe. These ancient systems launched the creation of every galaxy we see today, including the Milky Way. Ever since, for the last 13 billion years, the Milky Way has grown through galaxy mergers. Several of these mergers were with other similarly-sized galaxies and possibly a hundred of these mergers were with small dwarf galaxies. The smallest dwarf galaxies accreted by the Milky Way, the ultra-faint dwarfs (UFDs), are relics of the first galaxies in the Universe and provide important insight into early galaxy formation and chemical enrichment. Currently, though, accreted UFDs are poorly understood and we lack ways to identify stars that accreted from UFDs. By utilizing a simulation suite of 35 Milky Way-mass galaxies forming, I find that chemical tagging with r-process elements and clustering in kinematic phase space can help us identify stars that accreted together from these dwarf galaxies. Kinematic clustering only identifies recently accreted UFDs, so we recommend chemical tagging as the more robust method to identify these stars. I also present an analytic model of collapsar enrichment that can self-consistently explain the observed scatter in r-process chemical elements of old stars. I am expanding on these studies with highly-resolved hydrodynamic simulations of the earliest dwarf galaxies, the Aeos simulations. The methodology and initial results for these simulations are also presented.