2025A&A...704A..39K

Synergies between JWST and ALMA are unveiling a population of bright, super-early galaxies (z > 10), including systems like GS-z14-0 (z = 14.2) and GHZ2 (z = 12.3) with extreme far-infrared (FIR) line ratios ([OIII] 88 μm/[CII] 158 μm > 3) that challenge galaxy formation models. To address this, we identified a synthetic analog of these sources, “Amaryllis”, within the SERRA zoom-in simulations and tracked its evolution from z = 16 to z = 7. During this period, Amaryllis grows from log(M_★/M_⊙)∼7.4 to 10.3, linking super-early progenitors to the massive galaxy population at the end of reionization. At z ∼ 11.3, Amaryllis closely matches the observed properties of GS-z14-0, including the M_★, star formation rate, and the luminosity of FIR ([OIII] 88 μm) and UV (e.g., CIII]1908) lines. We find high [OIII]/[CII] ratios during short, merger-driven starburst episodes, when low metallicity (Z ∼ 0.1 Z_⊙) and high ionization conditions (U_ion ∼ 0.3) push the interstellar medium far from equilibrium. These extreme FIR line ratios are thus transient and linked to major mergers that ignite strong ionized gas outflows. Strikingly, despite this dynamical activity, Amaryllis develops a rotation-supported gaseous disk (V/σ ∼ 4 - 6) by z ∼ 11, while stars remain dispersion-dominated. This coexistence of ordered gas rotation and merger-driven disturbances occurs within a massive yet typical Λ cold dark matter halo, enabling disk formation even at cosmic dawn.