A halo of chemically primitive stars around an ancient dwarf galaxy. Online Seminar by Ani Chiti (MIT)

The Milky Way is surrounded by dozens of ultra-faint dwarf galaxies. These systems are the remnants of the earliest galaxies, and spectroscopy of their stars thereby reveals the elements produced by chemical evolution in a primitive, self-contained environment. Previous spectroscopic studies, however, had largely been limited to stars within the core of these galaxies (~2 half-light radii) due to the sparseness of their distant stars. In this talk, I will present the detection of extremely metal-poor stars ([Fe/H] < -3.0) out to 9 half-light radii in the Tucana II ultra-faint dwarf galaxy; the first detection of a population of stars outside the core region (~4 half-light radii) of any ultra-faint dwarf galaxy. These distant stars are, on average, more metal-poor than the central population (<[Fe/H]>=-3.18 vs. <[Fe/H]>=-2.71) and suppress the mean metallicity to <[Fe/H]> ~ -2.85. This difference is the first evidence of a metallicity gradient in an early galaxy remnant and suggests Tucana II, and perhaps other ultra-faints, plausibly were influenced by early, strong feedback episodes or a galactic merger. Such distant stars also imply that Tucana II harbors a massive, spatially extended dark matter halo. Collectively, these results suggest that key factors (e.g., most metal-poor stars, evidence of extended dark matter haloes) in understanding the early chemical evolution of ultra-faint dwarf galaxies lie in their outskirts and were missed by previous observational work. I will thereby also present an upcoming photometric survey of all southern-hemisphere ultra-faint dwarf galaxies to discover any spatially extended stellar populations, which our results suggest is necessary to understand the early evolution of these relic systems.