Seminar by Ani Chiti, UChicago
The Milky Way is surrounded by dozens of ultra-faint (L < 10^5 L_sun) dwarf galaxies (UFDs) that are relics (~13 Gyr old) from the early universe, making them unique probes of the first stages of galactic evolution. Studies have been limited to stars within the core of these galaxies (<~2 half-light radii) due to the sparseness and faintness of their distant stars. I will present results from a novel technique that uncovered stars out to ~8 half-light radii in one UFD, Tucana II, that appear to be bound to the galaxy. These distant stars are, on average, more metal-poor than the central population and suggest that Tucana II, and perhaps other UFDs, plausibly were influenced by early, strong feedback episodes or a merger of first-generation galaxies as suggested by some simulations. In particular, the detailed chemical abundances in Tucana II indicate delayed chemical evolution, which is consistent with the system being formed by an early merger of two first galaxies that triggered star formation. Such distant stars also imply that Tucana II harbors a spatially extended, NFW-like dark matter halo (> 10^7 solar masses out to 1 kpc). These results suggest that key factors (e.g., most metal-poor stars, evidence of extended halos) in understanding the early evolution of these ancient galaxies lie in their outskirts and may have been missed by previous observational work. I will present initial results on another peculiar UFD that will test the formation mechanisms of these small-scale extended halos, and will conclude by introducing a recently approved survey using a newly installed narrow-band filter on the Dark Energy Camera that will generalize this work to a large area of the southern sky.
Host: Melissa Ness and Emily Cunningham