An extended halo around an ancient dwarf galaxy

Abstract

The Milky Way is surrounded by dozens of ultrafaint (<105 L☉) dwarf satellite galaxies1–3. They are the remnants of the earliest galaxies4, as confirmed by their ancient5 and chemically primitive6,7 stars. Simulations8–10 suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and feedback. However, the signatures of these events would lie outside their core regions11, where spectroscopic studies are challenging12. Here we identify members of the Tucana II ultrafaint dwarf galaxy out to nine half-light radii, demonstrating the system to be markedly more spatially extended and chemically primitive than previously found. The distant stars in this galaxy are, on average, extremely metal poor (≲1/1000 of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish an extended dark matter halo surrounding an ultrafaint dwarf galaxy out to 1 kpc, with a total mass of >107 M☉, consistent with a generalized Navarro–Frenk–White density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger10,11. We demonstrate that spatially extended stellar populations in ultrafaint dwarf galaxies13,14 are observable, opening up the possibility for detailed studies of the stellar halos of relic galaxies.