What is the heavy element content of the Universe?
Chemical evolution of galaxies is a powerful probe of their assembly, and of the cycle of baryons between galaxies and the intergalactic medium. However, current measurements based on oxygen abundance are uncertain by a factor of 2, and many common diagnostics suffer from systematic offsets at high redshift. I will discuss efforts to establish the absolute gas-phase metallicity scale at low redshift and appropriate calibrations at high redshifts, including direct temperature-based measurements up to z~8. This in turn allows us to understand how the processes of accretion, star formation, and feedback govern the growth of galaxies across cosmic history. SOFIA plays a critical role as the only facility which can access key infrared spectral features for nearby galaxies.
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