A Comparison of UV and Optical Metallicities in Star-forming Galaxies

Abstract

Our ability to study the properties of the interstellar medium in the earliest galaxies will rely on emission-line diagnostics at rest-frame ultraviolet (UV) wavelengths. In this work, we identify metallicity-sensitive diagnostics using UV emission lines. We compare UV-derived metallicities with standard, well-established optical metallicities using a sample of galaxies with rest-frame UV and optical spectroscopy. We find that the He2–O3C3 diagnostic (He ii λ 1640 /C iii ] λ 1906,1909 versus [O iii ] λ 1666 /C iii ] λ 1906,9 ) is a reliable metallicity tracer, particularly at low metallicity ( ), where stellar contributions are minimal. We find that the Si3–O3C3 diagnostic ([Si iii ] λ 1883 /C iii ] λ 1906 versus [O iii ] λ 1666 /C iii ] λ 1906,9 ) is a reliable metallicity tracer, though with large scatter (0.2–0.3 dex), which we suggest is driven by variations in gas-phase abundances. We find that the C4–O3C3 diagnostic (C iv λ 1548,50 /[O iii ] λ 1666 versus [O iii ] λ 1666 /C iii ] λ 1906,9 ) correlates poorly with optically derived metallicities. We discuss possible explanations for these discrepant metallicity determinations, including the hardness of the ionizing spectrum, contribution from stellar wind emission, and non-solar-scaled gas-phase abundances. Finally, we provide two new UV oxygen abundance diagnostics, calculated from polynomial fits to the model grid surface in the He2–O3C3 and Si3–O3C3 diagrams.