The THESAN-ZOOM project: Mystery N/O more -- uncovering the origin of peculiar chemical abundances and a not-so-fundamental metallicity relation at $3<z<12$

We present an analysis of metallicities and chemical abundances at $3<z<12$ in the THESAN-ZOOM simulations. We find that smoothly curved gas-phase and stellar mass-metallicity relations (MZR) are already in place at $z\approx12$ and evolve slowly ($\sim$0.2 dex increase for gas, $\sim$0.4 dex increase for stars at a fixed stellar mass) down to $z=3$, governed largely by the efficiency with which galaxies retain their metals, rather than gas fraction. The canonical fundamental metallicity relation (FMR) survives in stars but breaks down and inverts for gas in low-mass galaxies ($M_\ast\lesssim10^{9}\mathrm{M_\odot}$) due to regular dilution by low-metallicity gas inflow. We find broad agreement of gas-phase N/O, Fe/O, and C/O with high-redshift observations, including the presence of nitrogen-rich galaxies (NRGs; $\log(\mathrm{N/O})>-0.6$) without the need for exotic yields in our chemical network. Instead, bursty star formation naturally generates order-of-magnitude excursions in N/O on $\lesssim$100 Myr timescales due to temporally differential galactic winds; after a starburst, stellar feedback expels gas, leaving a large population of asymptotic-giant-branch stars to dominate the enrichment of the relatively low-mass interstellar medium. NRGs lie below the main sequence and typically exhibit $\mathrm{EW}[H$β$]\lesssim40$ Å, in apparent tension with observed high-EW NRGs. This tension is reconciled if observed NRGs are in the initial stages of a subsequent starburst, illuminating previously enriched gas, which is supported by the finding of high SFR surface density nitrogen-rich giant molecular clouds.

💡 Research Summary

The paper presents a comprehensive analysis of metallicities and elemental abundance patterns in the early Universe (3 < z < 12) using the THESAN‑ZOOM suite of high‑resolution radiation‑hydrodynamic zoom‑in simulations. THESAN‑ZOOM builds on the large‑volume THESAN run, selecting individual regions for re‑simulation at 4×, 8×, and 16× finer spatial resolution (down to a baryonic particle mass of 1.4 × 10² M⊙). The simulations employ the AREPO‑RT code with on‑the‑fly M1 radiative transfer, non‑equilibrium thermochemistry for six hydrogen/helium species, and metal‑line cooling based on pre‑computed CLOUDY tables. A sophisticated multi‑channel stellar feedback model (photo‑ionisation, radiation pressure, stellar winds, supernovae, and an early‑feedback channel that disrupts dense clouds within a few Myr) regulates star formation and drives galactic outflows.

Key results on the mass‑metallicity relation (MZR) show that both gas‑phase and stellar metallicities already follow a smoothly curved relation at z≈12. From z≈12 to z=3 the gas metallicity at fixed stellar mass increases by ≈0.2 dex, while the stellar metallicity rises by ≈0.4 dex. The primary driver is the efficiency with which galaxies retain metals; gas fraction plays a secondary role. In low‑mass systems (M★ ≲ 10⁹ M⊙) the gas‑phase MZR flattens and the canonical fundamental metallicity relation (FMR) – a three‑parameter surface linking metallicity, stellar mass, and star‑formation rate (SFR) – breaks down and even inverts. This breakdown is attributed to continuous dilution by low‑metallicity inflows that dominate over the SFR‑dependent enrichment term in the gas phase, whereas the stellar FMR remains intact because stellar metallicities integrate over longer timescales.

The study also examines element‑by‑element abundance ratios. Simulated N/O, C/O, and Fe/O trends agree with recent JWST and ground‑based high‑z measurements. Notably, nitrogen‑rich galaxies (NRGs) with log(N/O) > ‑0.6 appear naturally without invoking exotic nucleosynthetic yields. The authors demonstrate that bursty star formation produces rapid, order‑of‑magnitude excursions in N/O on ≤100 Myr timescales. After a starburst, supernova‑driven winds expel the freshly produced α‑elements (O, Ne, etc.) while the interstellar medium (ISM) mass drops dramatically. The remaining low‑mass ISM is then enriched primarily by asymptotic‑giant‑branch (AGB) stars, which release nitrogen on 30–100 Myr timescales. Differential winds – preferentially removing supernova‑enriched gas – further amplify the N/O ratio. Consequently, NRGs occupy the region below the star‑forming main sequence and exhibit modest Hβ equivalent widths (EW ≲ 40 Å). This appears at odds with observed high‑EW NRGs; the authors reconcile the tension by proposing that observed NRGs are caught at the onset of a new starburst, where the previously nitrogen‑enriched gas is illuminated, producing high EW emission. Supporting evidence includes the detection of high SFR surface density, nitrogen‑rich giant molecular clouds in the simulations.

The paper also discusses the broader implications for the baryon cycle in the early Universe. Metal retention is shown to be mass‑dependent: deeper potential wells keep a larger fraction of metals, leading to the slow evolution of the MZR. Gas inflow rates, outflow mass‑loading factors, and the timing of AGB enrichment collectively shape the observed scatter in N/O and other ratios. The authors argue that the interplay of these processes, captured only in simulations with sub‑parsec resolution and explicit radiative feedback, is essential for interpreting the emerging JWST data set.

In summary, the THESAN‑ZOOM simulations reveal that (1) the MZR is already established by z≈12 and evolves modestly to z=3, (2) the gas‑phase FMR fails for low‑mass galaxies due to persistent inflow dilution, (3) nitrogen‑rich galaxies arise naturally from bursty star formation and differential winds without exotic stellar physics, and (4) the apparent discrepancy between simulated and observed EW(Hβ) in NRGs can be explained by the timing of successive starbursts. These findings provide a robust theoretical framework for the chemical evolution of galaxies in the first two billion years of cosmic history and set clear predictions for future JWST and ELT observations.