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 (ICF_ionized) and unaccounted for higher ionization stages in the neutral gas (ICF_neutral). 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[L_UV], 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 log[L_UV] ∼ 40.75 erg s^-1 above which the ICF_neutral values for nitrogen are relatively higher (ICF_neutral=0.05-0.7) than those for the rest of the elements (ICF_neutral ∼ 0.01). This behavior indicates that, for the high UV luminosity objects, N II is found in non-negligible quantities in the neutral gas, making these ICF_neutral 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.