The [O III ] λ 5007 Equivalent Width as an Ionization-parameter Diagnostic across Cosmic Time

Abstract Nebular emission lines and stellar continua probe the conditions of H II regions. The optical emission line [O III ] λ 5007 is important because it is observable across a wide redshift range, enabling studies of interstellar medium (ISM) conditions across cosmic time. We investigate the equivalent width of [O III ] λ 5007 (EWO3) as an ionization-parameter diagnostic, providing an alternative to the [O III ] λ 5007/[O II ] λ 3727 (O32) ratio, which requires broad wavelength coverage and dust corrections that may not be available at high redshift. We employ the MAPPINGS photoionization code with input spectra from Binary Population and Spectral Synthesis​​​​​, comparing single-star and binary-inclusive stellar populations. We find that binary-inclusive populations sustain harder ionizing spectra and strong [O III ] λ 5007 emission to later ages than single-star populations. EWO3 is highly sensitive to the ionization parameter and exhibits systematic dependencies on metallicity, gas pressure, and stellar population age. While these dependencies broadly follow those of O32, EWO3 is less sensitive to gas pressure in the low-metallicity, high-ionization regime, making EWO3 well suited for studies of metal-poor and high-redshift galaxies. Comparing the models with observations, we find that local galaxies are reproduced by lower-pressure models, whereas intermediate- and high-redshift galaxies require higher gas pressures, indicating evolving ISM conditions across cosmic time. We present metallicity- and pressure-dependent calibrations of EWO3 as an ionization-parameter diagnostic. When validated against ionization parameters derived from O32, the log ( P / k ) = 6 calibration provides the best agreement for local galaxies, while the log ( P / k ) = 7 calibration best reproduces intermediate- and high-redshift galaxies.