Newly improved ionization corrections for the neutral interstellar medium: Enabling accurate abundance determinations in star-forming galaxies throughout the universe

Studies measuring the chemical abundances of the neutral gas in star-forming galaxies (SFGs) require ionization correction factors (ICFs) to accurately measure their metal contents. In the work presented here, we calculate newly improved ICFs for a sample of SFGs. These new corrections include both the contaminating ionized gas along the line of sight and unaccounted for higher ionization stages in the neutral gas. We make use of recently acquired spectroscopic observations taken with the Cosmic Origins Spectrograph on board Hubble to measure column densities for Fe ii and Fe iii. Using the Fe iii/Fe ii ratios as well as other physical properties (i.e., log, N(H i), T, and Z), we generate ad hoc photoionization models with CLOUDY to quantify the corrections required for each of the targets. We identify a luminosity threshold of log40.75 erg s^-1 above which the values for nitrogen are relatively higher than those for the rest of the elements. This behavior indicates that, for the high UV luminosity objects, N ii is found in non-negligible quantities in the neutral gas, making these corrections critical for determining the true abundances in the interstellar medium. In addition, we calculate ICFs from a uniform grid of models covering a wide range of physical properties typically observed in studies of SFGs and extragalactic H ii regions. We provide the community with tabulated ICF values for the neutral gas abundances measured from a variety of environments and applicable to chemical studies of the high-redshift universe.