12:00–1:00 pm Zoom
An extended halo around an ancient dwarf galaxy
Ani Chiti (MIT)
The Milky Way is surrounded by dozens of ultra-faint dwarf galaxies. These systems are the remnants of some of the earliest galaxies as suggested by their ancient (~13 Gyr) and chemically primitive stellar populations. In this talk, I will present the detection of extremely metal-poor stars ([Fe/H] < -3.0) out to 9 half-light radii in one such galaxy, Tucana II. This is 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]>=-3.0 vs. <[Fe/H]>=-2.6) and suppress the mean metallicity to <[Fe/H]> ~ -2.77. 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 (> 10^7 solar masses out to 1 kpc). Collectively, these results suggest that key factors needed for understanding early galaxy formation and chemical evolution lie in the outskirts of these small, relic galaxies and may have been missed by previous observational studies. I will thereby also present an upcoming photometric survey of all southern-hemisphere ultra-faint dwarf galaxies that will reveal any spatially extended stellar populations, which our results suggest is necessary to understand the early evolution of these relic systems.