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

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 λ1640 ${\rm{\mathring{\rm A} }}$ /C III] λ1906,1909 ${\rm{\mathring{\rm A} }}$ versus [O III] λ1666 ${\rm{\mathring{\rm A} }}$ /C III] λ1906,9 ${\rm{\mathring{\rm A} }}$ ) is a reliable metallicity tracer, particularly at low metallicity ( $12+{\mathrm{log}}_{10}({\rm{O}}/{\rm{H}})\leqslant 8$ ), where stellar contributions are minimal. We find that the Si3-O3C3 diagnostic ([Si III] λ1883 ${\rm{\mathring{\rm A} }}$ /C III] λ1906 ${\rm{\mathring{\rm A} }}$ versus [O III] λ1666 ${\rm{\mathring{\rm A} }}$ /C III] λ1906,9 ${\rm{\mathring{\rm A} }}$ ) 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 ${\rm{\mathring{\rm A} }}$ /[O III] λ 1666 ${\rm{\mathring{\rm A} }}$ versus [O III] λ 1666 ${\rm{\mathring{\rm A} }}$ /C III] λ 1906,9 ${\rm{\mathring{\rm A} }}$ ) 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.