Newly Improved Ionization Corrections for the Neutral Interstellar Medium: Enabling Accurate Abundance Determinations in Star-forming Galaxies throughout the Universe

摘要

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 log[ ]?～?40.75 erg s~(?1) above which the values for nitrogen are relatively higher ( ?0.7) 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.