IAUS379: Dynamical Masses of Local Group Galaxies

We present kinematics and detailed chemical abundances of stars in the outskirts (out to ~8 half-light radii) of the Tucana II ultra-faint dwarf galaxy (< 10^5 Lsun; UFD) from high-resolution Magellan/MIKE spectroscopy. The Milky Way’s UFDs are “relic” galaxies (~13 Gyr old) from the early universe, making their stars unique probes of the first stages of galactic evolution. Previous spectroscopic studies had largely been limited to stars within the core of these galaxies (~2 half-light radii) due to the sparseness of their distant stars. This work presents the first high-resolution spectroscopy of a population of stars outside the core region (~4 half-light radii) of a UFD that show no obvious evidence of being unbound from the galaxy; these distant stars are not aligned with the orbital track of Tucana II and do not show evidence of a velocity gradient. The farther stars are, on average, more metal-poor than the central population, and their detailed chemical abundances provide clues to the formation of the outskirts of Tucana II. The metallicity difference between inner and outer stars suggests Tucana II, and perhaps other ultra-faints, plausibly were influenced by early, strong feedback episodes or a galactic merger as suggested by simulations (Tarumi et al. 2021). The alpha element abundances in Tucana II indicate some delayed chemical evolution, which is consistent with Tucana II being formed by an early merger of first galaxies that triggered star formation. The chemical abundances of these distant stars do not indicate that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in-situ stellar halo formation then other UFDs should show similar extended features. The kinematics and location of these distant stars also hint that Tucana II may harbor a spatially extended dark matter halo (> 10^7 solar masses out to 1 kpc). Our results suggest that key factors (e.g., most metal-poor stars, evidence of extended halos) in understanding the early evolution and dynamical state of these relic galaxies lie in their outskirts and may have been missed by previous observational work. We demonstrate that detailed spectroscopic studies of individual stars in such low surface brightness features are now possible.