MEGATRON: Disentangling Physical Processes and Observational Bias in the Multi-Phase ISM of High-Redshift Galaxies

Now detected out to redshifts of $z\sim 14.5$, the rest-frame ultraviolet and optical spectra of galaxies encode numerous physical properties of the interstellar medium (ISM). Accurately extracting these properties from spectra remains a key challenge that numerical simulations are uniquely suited to address. We present a study of the observed ISM of galaxies in MEGATRON: a suite of cosmological radiation hydrodynamics simulations coupled to on-the-fly non-equilibrium thermochemistry, with multiple prescriptions for star formation/feedback and parsec-scale resolution; capable of directly predicting spectroscopic properties of early galaxies. We find that irrespective of feedback physics used, the ISM of high-redshift galaxies is denser, less metal enriched, and subject to higher ionization parameters and radiation fields compared to similar mass galaxies in the local Universe -- in agreement with interpretations of JWST observations. Using common observational techniques to infer bulk galaxy properties, we find that ISM gas density controls the slope of the mass-metallicity relation. Similarly, at the densities reached in some high-redshift galaxies, O32 becomes a density tracer rather than one of ionization parameter. This motivates the use of other line ratios like C43 and N43 to infer the ionization state of the gas. Finally, various feedback models populate different regions of strong-line diagnostic diagrams as the line ratios are sensitive to the feedback-modulated density-temperature structure of the ISM. Therefore, observed strong-line diagnostics can provide a strong constraint on the underlying physics of star formation and feedback in the high-redshift Universe.