The Milky Way is surrounded by dozens of ultrafaint (<10⁵ L_☉) dwarf satellite galaxies^1,2,3. They are the remnants of the earliest galaxies⁴, as confirmed by their ancient⁵ and chemically primitive^6,7 stars. Simulations^8,9,10 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.

银河系周围环绕着数十个超微弱 (<10 ⁵  L _☉ ) 矮卫星星系^1,2,3。它们是最早的星系⁴的残余，正如它们古老的⁵和化学上原始的^6,7恒星所证实的那样。模拟^8,9,10表明这些系统形成于扩展的暗物质晕中，并经历了早期的星系合并和反馈。然而，这些事件的特征将位于其核心区域¹¹之外，在这些区域中，光谱研究具有挑战性¹². 在这里，我们确定了 Tucana II 超微弱矮星系的成员，半径为 9 个半光，表明该系统比以前发现的空间扩展和化学原始得多。平均而言，这个星系中遥远的恒星极少金属（≲太阳铁丰度的 1/1000），这表明 Tucana II 是已知最缺乏金属的星系。我们观察性地在超微弱矮星系周围建立了一个扩展的暗物质晕，其总质量 > 10 ⁷  M _☉，与广义的 Navarro-Frenk-White 密度分布一致。Tucana II 的扩展性质表明它可能经历了强烈的突发反馈，或者是早期星系合并的产物^10,11. 我们证明了在超微弱矮星系^13,14中空间扩展的恒星种群是可观察到的，这为详细研究遗迹星系的恒星晕提供了可能性。