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

TL;DR

The paper tackles how extremely high FIR line ratios observed in the earliest galaxies can arise within standard cosmology. It leverages the SERRA zoom-in simulations to identify Amaryllis, a synthetic analog that evolves from $z=16$ to $z=7$, linking progenitors to the massive galaxy population at the end of reionization. The key findings are that merger-driven bursts produce transient, high $[\rm OIII]/[\rm CII]$ ratios under low metallicity and strong ionization, while a rotation-supported gas disk can form very early in a typical $\Lambda$CDM halo, accompanied by merger-driven outflows and dispersion-dominated stars. Collectively, these results illuminate the complex, nonequilibrium ISM and kinematic evolution of the first galaxies and provide a framework to interpret JWST/ALMA data with predictions for spatially resolved follow-ups.

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 ([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 $\log(M_\star/M_{\odot}) \sim 7.4$ 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 ($Z \sim 0.1\,Z_{\odot}$) and high ionization conditions ($U_{\mathrm{ion}} \sim 0.3$) 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 ($V/σ\sim 4$-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 $Λ$CDM halo, enabling disk formation even at cosmic dawn.

Figures (5)

• Figure 1: Overview of "Amaryllis," a digital twin of early galaxies. Upper-left panel: Merger history of Amaryllis. Each curve shows the stellar mass ($M_\star$) of the progenitors as a function of cosmic time ($t$), with the corresponding redshift ($z$) on the upper axis. The black line indicates the main galaxy, while each colored line represents a merging satellite. Solid lines mark the phases during which each system remains distinct, and dashed lines highlight the mergers. The observational measurements shown include massive $z>10$ galaxies, spectroscopically confirmed by JWST Ferrara+24, $z\sim6-9$ galaxies from CEERS and COSMOS Harikane+24_CEERS, and REBELS sources Dayal+22. Upper-right panel: SFH of Amaryllis throughout cosmic time, with vertical lines corresponding to minor (dotted) and major (dashed) merger events. Bottom panels: Composite images at four evolutionary stages, built from the stellar surface density ($\Sigma_\star$), gas surface density ($\Sigma_{\rm{gas}}$), and the Habing field intensity (G).
• Figure 2: DM halo structure of Amaryllis at $z=7.3$. We show a spherically averaged DM density profile (black points) fitted with a Burkert (solid red) and an NFW (dashed blue) profile.
• Figure 3: Temporal evolution of various Amaryllis properties: $L_{\rm{OIII}}/L_{\rm{CII}}$ (top panel) , the ionization parameter $U_{\rm{ion}}$ (second panel), gas metallicity, $Z$ (third panel), gas number density, $n$ (fourth panel), and star-formation variability, $\rm{SFR}_{3\,\rm{Myr}}/\rm{SFR}_{50\,\rm{Myr}}$ (bottom panel). The correlations between $L_{\rm{OIII}}/L_{\rm{CII}}$ and each other property are presented in Table \ref{['tab:correlation']}. The vertical dashed green lines correspond to major merger events, and the dotted red line indicates the snapshot with the highest [OIII]/[CII].
• Figure 4: Integrated kinematic diagnostics of Amaryllis during its post-merger starburst phase at $z \sim 10.4$. Main panel: Integrated [O $\rm III$]~$88\,\mu$m spectrum from a face-on orientation, highlighting a prominent narrow component tracing star-forming gas near systemic velocity, and a broad, blueshifted wing indicative of ionized outflows. The overlaid two-Gaussian fit separates these components, with residuals shown in the bottom subplot. The inferred outflow velocity is $v_{\rm outflow} \simeq 258\,\mathrm{kms}^{-1}$. Inset panel: Spatial distribution of [OIII] emission in the edge-on projection, revealing extra-planar ionized gas consistent with a large-scale outflow.
• Figure 5: Left panel: Evolution of mean circularity parameter, $\langle\epsilon_\mathrm{circ} \rangle= \langle J_z/J_\mathrm{circ} \rangle$, with cosmic time ($t$) and corresponding redshift ($z$) for Amaryllis. The horizontal dashed line indicates the disk threshold ($1/\sqrt{3}$, Simons+19), while the shaded region highlights the triple merger phase. Right panels: Velocity ($v$) and velocity dispersion ($\sigma$) maps of the gaseous disk in [C $\rm II$]~and [O $\rm III$]~emission, showcasing the early gaseous disk properties at $z\simeq 11$ in FIR emission lines observable with ALMA at these redshifts.