Separating the roles of metal-poor inflows and metal-enriched outflows in setting the ISM metallicity and effective yields of sub-Milky-Way galaxies

Supernova-driven galactic winds and cosmological gas accretion jointly regulate the chemical enrichment of low-mass galaxies, but their relative contributions have not been quantitatively separated. Simulations of 15 sub-Milky-Way galaxies in the CIELO-P7 suite show that outflows are metal-enriched while inflows are metal-poor, and that both processes lower the effective yield; the analysis that would attribute the deviations from closed-box chemical evolution to each process is explicitly deferred by the source paper and remains open. The problem is to quantify, per galaxy and at the sample level, how much of the present-day interstellar-medium metallicity (the low-mass end of the mass-metallicity relation) and of the effective-yield deficit of sub-Milky-Way galaxies is caused by dilution through metal-poor inflows versus metal removal through metal-loaded outflows.

Background

In the baryon cycle of galaxies, supernova (SN) feedback heats and accelerates interstellar gas, driving galactic winds that carry newly synthesized metals out of the interstellar medium (ISM) into the circumgalactic medium (CGM, the gas within the virial radius of the dark-matter halo) and beyond; cosmological gas accretion simultaneously supplies fresh, metal-poor fuel for star formation. The mass-metallicity relation (MZR) — the observed increase of gas-phase oxygen abundance with stellar mass, with dwarf galaxies markedly metal-poor — is thought to arise from this competition: outflows remove enriched gas more easily from shallow potential wells, while metal-poor inflows dilute the ISM. A classic diagnostic of gas exchange is the effective yield. In the closed-box model, a galaxy that forms stars from its initial gas reservoir with no inflows or outflows has metallicity $Z$ tied to the gas fraction $\mu \approx M_{\mathrm{g}}/(M_{\ast}+M_{\mathrm{g}})$ by $y_{\mathrm{eff}} = Z/\ln(1/\mu)$ , equal to the true stellar nucleosynthetic yield independently of galaxy mass. Measured effective yields below the closed-box value therefore signal gas exchange, and dwarfs generally show depressed $y_{\mathrm{eff}}$ . Crucially, however, both channels — dilution by metal-poor inflows and removal of enriched gas by outflows — lower $y_{\mathrm{eff}}$ in overlapping ways, so the effective yield (or any integrated quantity) alone cannot tell which process dominates; chemical-evolution fits to observed samples constrain essentially only the difference between inflow and outflow mass loading, a degeneracy that is acknowledged to be extremely difficult to break. CIELO-P7 is a suite of high-resolution chemo-dynamical cosmological zoom-in simulations (re-simulations of individual target haloes selected from a cosmological volume, jointly tracking dynamical and chemical evolution; Tissera et al. 2025) that follows central galaxies of $M_{\ast}\sim10^{8}$ – $10^{10}M_{\odot}$ with metal-dependent cooling and a subgrid prescription for SN feedback (a sub-resolution model for feedback processes below the simulation's resolution limit), enabling direct particle-level measurement of gas flows. Miranda et al. (2026) analyzed 15 of these central galaxies from $z=7$ to $0$ , using two dynamically motivated outflow definitions — unbound outflows (gas no longer gravitationally bound, measured in CGM shells) and expelled mass rates (gas tracked permanently beyond the virial radius) — together with inflow mass rates. They found that sub-Milky-Way (sub-MW, $M_{\ast}\lesssim10^{10}M_{\odot}$ ) outflows are metal-enriched relative to the ISM (median $Z_{\mathrm{out}}/Z_{\mathrm{ISM}}\approx1.5$ for $z<2$ ), that accreted gas is metal-poor (median $12+\log(\mathrm{O/H})\approx6.95$ for sub-MW systems, with 25th–75th percentiles $6.16$ – $7.28$ , well below the MZR), and that effective yields at low mass show a large diversity with no clear trend against baryonic mass. Both inflows and outflows contribute to lowering the effective yields, but the paper states verbatim: "The relative impact of inflows and outflows remains to be analysed."

Problem Statement

Quantitatively separate the relative impact of (i) dilution by metal-poor gas inflows and (ii) metal removal by SN-driven, metal-enriched outflows in setting the $z\approx0$ ISM metallicity (the low-mass end of the mass-metallicity relation) and the effective yields $y_{\mathrm{eff}} = Z/\ln(1/\mu)$ of sub-Milky-Way galaxies ( $M_{\ast}\lesssim10^{10}M_{\odot}$ ), in the CIELO-P7 simulation framework. The goal is a quantitative attribution, per galaxy and at the sample level, of the deviations from closed-box chemical evolution to each process, building on the measured inflow/outflow mass rates and flow metallicities of these galaxies reported by Miranda et al. (2026). This is the analysis explicitly deferred by the source paper with 'The relative impact of inflows and outflows remains to be analysed.'

References

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