2018ApJ...853..124J

The abundance of oxygen in galaxies is widely used in furthering our understanding of galaxy formation and evolution. Unfortunately, direct measurements of O/H in the neutral gas are extremely difficult to obtain, as the only O I line available within the Hubble Space Telescope (HST) UV wavelength range (1150-3200 Å) is often saturated. As such, proxies for oxygen are needed to indirectly derive O/H via the assumption that solar ratios based on local Milky Way sight lines hold in different environments. In this paper we assess the validity of using two such proxies, P II and S II, within more typical star-forming environments. Using HST-Cosmic Origins Spectrograph (COS) far-UV (FUV) spectra of a sample of nearby star-forming galaxies (SFGs) and the oxygen abundances in their ionized gas, we demonstrate that both P and S are mildly depleted with respect to O and follow a trend, log(P II/S II) = -1.73 ± 0.18, in excellent agreement with the solar ratio of log(P/S)_☉ =-1.71 ± 0.04 over the large range of metallicities (0.03-3.2 Z_☉) and H I column densities (log[N(H I)/cm^–2] =18.44-21.28) spanned by the sample. From literature data we show evidence that both elements individually trace oxygen according to their respective solar ratios across a wide range of environments. Our findings demonst-rate that the solar ratios of log(P/O)_☉=-3.28±0.06 and log(S/O)_☉=-1.57±0.06 can both be used to derive reliable O/H abundances in the neutral gas of local and high-redshift SFGs. The difference between O/H in the ionized- and neutral gas phases is studied with respect to metallicity and H I content. The observed trends are consistent with galactic outflows and/or star formation inefficiency affecting the most metal-poor galaxies, with the possibility of primordial gas accretion at all metallicities.