Our ability to study the properties of the interstellar medium (ISM) 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 1640 / C III 1906,1909 vs. O III 1666 / C III 1906,1909) is a reliable metallicity tracer, particularly at low metallicity (12+log(O/H) < 8), where stellar contributions are minimal. We find that the Si3-O3C3 diagnostic (Si III 1883 / C III 1906,1909 vs. O III 1666 / C III 1906,1909) 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 1548,1550 / O III 1666 vs. O III 1666 / C III 1906,1909) 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.
@article{arxiv.2003.06925,
title = {A comparison of UV and optical metallicities in star-forming galaxies},
author = {Nell Byler and Lisa J Kewley and Jane R Rigby and Ayan Acharyya and Danielle A Berg and Matthew Bayliss and Keren Sharon and .},
journal= {arXiv preprint arXiv:2003.06925},
year = {2020}
}