ChemicalUniverseMachine. I. Uncovering the Cosmic Evolution of Metals in the Galaxy-Interstellar Medium-Circumgalactic Medium Ecosystem

We present an empirical chemical evolution model that explains the distribution of metals in the interstellar medium (ISM) and the circumgalactic medium (CGM) of galaxies based on the UniverseMachine and NeutralUniverseMachine models in the framework of ΛCDM structure formation. We parameterize the fractions of outflowing metals returned and mixed into the multiphase ISM of the star-forming regions (f_H2) and into the neutral gas regions (f_{H I}); metal production, transfer, and dilution are caused by star formation, galaxy mergers, and gas inflow from the intergalactic medium, respectively, with rates determined by the (Neutral)UniverseMachine models. Using a Markov Chain Monte Carlo algorithm, we explore the posterior distributions of metal return and mixing consistent with observed mass-metallicity relations in H ii regions (at 0 < z < 5), H i damped Lyα systems (at 1 < z < 4), and the CGM (at z = 0). We find that the fraction of metals present in the ISM, f_H2 + f_{H I}, increases with halo mass from ∼20% at 10¹⁰M_⊙to ∼80% at 10¹³M_⊙. These fractions increase mildly at higher redshifts, to ∼30% at 10¹⁰M_⊙and ∼80% at 10¹³M_⊙ at z = 5. Interestingly, there is no significant redshift evolution of f_H2 + f_{H I} at fixed circular velocity, suggesting that the metal distribution between the ISM and CGM is universally determined by the halo potential well depth. CGM metal enrichment is thus slow in high-z halos with deep potential wells. While f_H2 monotonically increases with halo mass, f_HI peaks at ∼10¹²−10¹³M_⊙, suggesting that reinfall may be inefficient in larger-mass halos.