An extended halo around an ancient dwarf galaxy

The Milky Way is surrounded by dozens of ultrafaint (<10⁵ L_⊙) dwarf satellite galaxies^1-3. They are the remnants of the earliest galaxies⁴, as confirmed by their ancient⁵ and chemically primitive^6,7 stars. Simulations^8-10 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 merger^10,11. We demonstrate that spatially extended stellar populations in ultrafaint dwarf galaxies^13,14 are observable, opening up the possibility for detailed studies of the stellar halos of relic galaxies.