Oral Presentation
Chemical Evolution of Galaxies from z=0—5 with UniverseMachine
Presenter: Moka Nishigaki (SOKENDAI/NAOJ)
The main goal of this study is to understand the fraction of metals produced in galaxies that promptly re-enrich the interstellar medium (ISM) compared to metals that are ejected into the circumgalactic medium (CGM). We refer to this fraction as the ISM return fraction. Constraining the ISM return fraction helps us understand the baryon cycle, especially the feedback mechanisms involving outflows. We develop a novel empirical model that infers the average metallicity evolution of galaxies from redshift z~5 to z~0, which is enabled by recent JWST metallicity measurement and constraints on galaxy ISM masses. The key concept of this model is that the average ISM return fraction over time can be approximated by the fraction of metal mass in the ISM divided by the total metal mass produced and ejected into the ISM by stars. Anchored in the UniverseMachine framework (Behroozi+19), which links galaxy and halo properties, our model converts observational data on gas-phase ISM and CGM metallicities and galaxy ISM masses across z=0—5 into constraints on the ISM return fraction. Utilizing observational constraints on ISM metallicities from star-forming regions and damped Lyman-alpha systems, we develop the model with a realistic metal mixing approach that treats atomic and molecular gases individually, departing from the conventional assumption of well-mixed ISM. Our best-fitting model indicates that deeper potential well depths lead to more metals returning to the ISM, with no significant redshift evolution. We also find that the return fraction to the atomic phase is low, which indicates that the returning gas predominantly enriches the molecular phase, and that metals are not well-mixed within the molecular and atomic phases in the ISM.
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