Synthetic Clones of the Most Distant Galaxies in the Universe

Abstract: 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 (${\rm [OIII]88\mu m/[CII]158\mu m} > 3$) that challenge galaxy formation models. To clarify this issue, we identify in the SERRA zoom-in simulations a synthetic analogue, Amaryllis, of these sources, and track its evolution from $z=16$ to $z=7$. During this period, Amaryllis grows from $\log(M_\star/M_\odot) \sim 7.4$ to $10.3$, linking super-early progenitors to the massive galaxy population at the end of reionization. At $z=11.5$, Amaryllis closely matches the observed properties of GS-z14-0, including $M_\star$, SFR, and the luminosity of FIR ([OIII]88$\mu$m) and UV (e.g. CIII]$1908$) lines. We find that high [OIII]/[CII] ratios appear during short, merger-driven starburst episodes, when low metallicity ($Z \sim 0.02\,Z_\odot$) and high ionization conditions ($U_{\rm ion} \sim 0.1$) 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. Despite such a dynamically violent environment, strikingly, Amaryllis develops a dynamically cold gaseous disk ($V/\sigma \sim 4-6$) as early as $z \sim 11$, while its stellar component remains dispersion-dominated down to $z\sim7$. The co-existence of ordered rotation and merger-driven disturbances in $z>10$ galaxies can explain the tentative disk signatures in GS-z14-0.