Metallicity. Shown is the upper limit for the metalicity in FFB star-forming clouds, assuming full mixing between the outflowing metals Z_sn​ = ​1 Z_⊙ and inflowing gas Z_in​ = ​0.1 Z_⊙ (blue) or 0.02 Z_⊙ (orange) in Eq. (42). The metalicity is computed for halos at the FFB threshold mass. The bottom and top bounds of the shaded areas represent ϵ_max = 0.2 and unity, respectively. Despite the mixing with supernova-enriched gas from earlier generations, the metallicity remains low and close to Z_in, at Z​ ∼ ​0.1 Z_⊙ or below. Shown for reference are tentative observations (Curti et al. 2023, 2024; Jones et al. 2023; Hu et al. 2024; also cf. Arellano-Córdova et al. 2022), which suggest low but not negligible metalicities in the same ball park, though these galaxies are not necessarily in the FFB regime.