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 ( \ion He2 \lambda 1640 Å / \ion C3 ] \lambda 1906,1909 Å \mathrm { vs . } [ \ion O3 ] \lambda 1666 Å / \ion C3 ] \lambda 1906,9 Å ) is a reliable metallicity tracer , particularly at low metallicity ( 12 + \log _ { 10 } ( \mathrm { O } / \mathrm { H } ) \leq 8 ) , where stellar contributions are minimal .
We find that the Si3-O3C3 diagnostic ( [ \ion Si3 ] \lambda 1883 Å / \ion C3 ] \lambda 1906 Å \mathrm { vs . } [ \ion O3 ] \lambda 1666 Å / \ion C3 ] \lambda 1906,9 Å ) 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 ( \ion C4 \lambda 1548,50 Å / [ \ion O3 ] \lambda 1666 Å \mathrm { vs . } [ \ion O3 ] \lambda 1666 Å / \ion C3 ] \lambda 1906,9 Å ) 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 .