Amaryllis: a digital twin of the earliest galaxies in the Universe

Synergies between JWST and ALMA are unveiling a population of bright, super-early ($z>10$) galaxies, including systems like GS-z14-0 ($z=14.2$) and GHZ2 ($z=12.3$) with extreme FIR line ratios ([OIII] 88$$um / [CII] 158$$um $>3$) that challenge galaxy formation models. To address this, we identify a synthetic analog of these sources, “Amaryllis”, within the SERRA zoom-in simulations, and track its evolution from $z=16$ to $z=7$. During this period, Amaryllis grows from $\log (M_{\star }\mathrm{/}{M}_{\odot })\sim 7.4$ to $10.3$, linking super-early progenitors to the massive galaxy population at the end of reionization. At $z\sim 11.3$, Amaryllis closely matches the observed properties of GS-z14-0, including $M_{\star }$, SFR, and the luminosity of FIR ([OIII] 88$$um) and UV (e.g. CIII]$1908$) lines. We find that high [OIII]/[CII] ratios appear during short, merger-driven starburst episodes, when low metallicity ($Z\sim 0.1Z_{\odot }$) and high ionization conditions ($U_{\mathrm{i}\mathrm{o}\mathrm{n}}\sim 0.3$) push the ISM 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 ($V\mathrm{/}\sigma \sim 4$-6) by $z\sim 11$, while stars remain dispersion-dominated. This coexistence of ordered gas rotation and merger-driven disturbances occurs within a massive yet typical $\mathrm{\Lambda }$CDM halo, enabling disk formation even at cosmic dawn.