Finding the most metal-poor stars in the Milky Way's dwarf galaxies

Ani Chiti (MIT)
Friday, October 25, 2019 - 12:15pm to 1:00pm

The Milky Way's dwarf galaxies are ideal self-contained systems for us to study early chemical evolution, test theories of galaxy formation on small scales, and understand the assembly history of the Milky Way. The ultra-faint dwarf galaxies (UFDs) are particularly interesting, since they are thought to be the existing analogs of the earliest galaxies formed. However, due to foreground Milky Way stars, the success rate of identifying UFD member stars is typically low, thus rendering the full characterization of UFDs very challenging. Many of these issues can be bypassed by screening stars based on metallicity, surface gravity, and Gaia DR2 proper motions to identify metal-poor giants as likely UFD members. Using SkyMapper photometry, I have developed a technique to derive precise and accurate metallicities and surface gravities that enables finding member stars with highly increased efficiency. As a result, I have discovered extremely metal-poor ([Fe/H] < -3.0) stars up to ~8 half light-radii from the center of the Tucana II UFD, solidifying Tucana II as the most metal-poor UFD. Based on modeling, these stars are likely not the result of the tidal disruption of the galaxy and are perhaps the first detection of a population of stars in the extended dark matter distribution of a UFD. Using purely public data from SkyMapper DR1.1 and Gaia DR2, I have also detected over 15 new very metal-poor stars in the Sagittarius dwarf galaxy (dSph), substantially expanding the prior sample of few such stars and confirming that even the most massive dwarf galaxies harbor very metal-poor stars. These results indicate that with efficient techniques, one can effectively probe the most metal-poor stellar populations of dwarf galaxies for the study of e.g., hierarchical merging, and for characterizing the ancient birthplaces of metal-poor stars found in the Milky Way today.

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