Divide metal enrichment by gas depletion in the closed-box model of galactic chemical evolution:
The logarithm appears on integrating from and :
With the gas fraction of a galaxy , this is
It gives at and increasing gas-phase metallicity as gas is consumed, independently of the time dependence of the star formation rate. For this closed initially star-free system, , so this initial-reservoir fraction equals the present gas-to-total baryonic fraction. That equality fails in an open system.

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