The Impact of Star Formation and Feedback Recipes on the Stellar Mass and Interstellar Medium of High-Redshift Galaxies

We introduce MEGATRON, a new galaxy formation model for cosmological radiation hydrodynamics simulations of high-redshift galaxies. The model accounts for the non-equilibrium chemistry and heating/cooling processes of $\geq 80$ atoms, ions, and molecules, coupled to on-the-fly radiation transfer. We apply the model in a cosmological setting to the formation of a $10^9\ {\rm M_{\odot}}$ halo at $z=6$, and run 25 realizations at pc-scale resolution, varying numerous parameters associated with our state-of-the-art star formation, stellar feedback, and chemical enrichment models. We show that the overall budget of feedback energy is the key parameter that controls star formation regulation at high redshift, with other numerical parameters (e.g. supernova clustering, star formation conditions) having a more limited impact. As a similar feedback model has been shown to produce realistic $z=0$ galaxies, our work demonstrates that calibration at $z=0$ does not guarantee strong regulation of star formation at high-redshift. Interestingly, we find that subgrid model variations that have little impact on the final $z=6$ stellar mass can lead to substantial changes on the observable properties of high-redshift galaxies. For example, different star formation models based on, e.g. density thresholds or turbulence inspired criteria, lead to fundamentally distinct nebular emission line ratios across the interstellar medium (ISM). These results highlight the ISM as an important resource for constraining models of star formation, feedback, and galaxy formation in the JWST era, where emission line measurements for $>1,000$ high-redshift galaxies are now available.

💡 Research Summary

The authors present MEGATRON, a new galaxy‑formation framework built on the RAMSES‑RTZ code, designed specifically for cosmological radiation‑hydrodynamics simulations of high‑redshift galaxies. MEGATRON couples a comprehensive non‑equilibrium chemistry network—including more than 80 atomic, ionic, and molecular species (e.g., H I–II, He I–II, C I–VI, N I–VII, O I–VI, Fe I–XI, H₂, CO)—to on‑the‑fly multi‑frequency radiative transfer. By doing so, the model can predict the thermodynamic state of both ionized and neutral gas, as well as a wide array of nebular emission lines, without resorting to post‑processing.

The study focuses on the formation of a $10^{9},M_{\odot}$ halo at $z=6$. Using pc‑scale resolution (1–20 pc) the authors run 25 realizations, each varying a set of sub‑grid parameters: (i) total feedback energy budget (supernova energy, radiation pressure, photo‑heating), (ii) degree of supernova clustering, (iii) star‑formation criteria (simple density threshold versus turbulence‑inspired models), (iv) numerical resolution, and (v) sub‑grid clumping factors for H₂ and CO formation. The chemistry is split into a “minimal cooling model” where only metal ions contributing >1 % to the equilibrium cooling curve are followed in non‑equilibrium; the rest are treated in collisional ionization equilibrium. Dust is modeled empirically with a broken power‑law dust‑to‑gas ratio as a function of metallicity, and only cells cooler than $10^{6}$ K contain dust.

Key results:

1. Feedback energy dominates star‑formation regulation. Increasing the total injected feedback energy by a factor of two reduces the final stellar mass at $z=6$ by roughly 50 %. Other parameters—such as supernova clustering or the exact density threshold for star formation—have a comparatively modest effect on the total stellar mass.
2. Calibration at $z=0$ does not guarantee regulation at high redshift. The same feedback model that reproduces realistic $z=0$ galaxies fails to suppress star formation sufficiently in the early Universe, highlighting the need for redshift‑dependent calibration.
3. ISM diagnostics are highly sensitive to sub‑grid choices. While different star‑formation recipes produce nearly identical final stellar masses (differences <10 %), they generate markedly distinct interstellar‑medium conditions. Emission‑line ratios such as