[O  <scp>iii</scp> ] line ratios and evolution of oxygen abundance with redshift using <i>JWST</i> –VLT–Keck observations

ABSTRACT We study high-z galaxies using James Webb Space Telescope observations and new atomic data, complemented by low-z observations from VLT and Keck, to examine a sample of 44 galaxies and analyse or re-analyse observed [O iii ] line ratios to derive electron temperatures, densities, and oxygen abundances. In addition to [O iii ] temperature diagnostics, [O ii ] and [S ii ] ratios are employed for density constraints and contour plots of physical conditions. Based on nebular temperature-abundance relations for H ii regions, we track the evolution of [12 + log(O/H)] with redshift. Our [O iii] atomic model incorporates recombination-cascade contributions to forbidden lines using new level-specific recombination rate coefficients and transition probabilities, together with collision strengths computed in earlier works. We find that individual galaxies show a large and systematic variation with electron temperature in the nebular range 5000–25 000K, and O-abundance down to 6.75 compared to the solar value 8.70. The oxygen abundances versus z display a broadly decreasing trend towards high-$z \sim 10$, with a best fit ranging from 8.25 to 7.50 from the present epoch at $z = 0$, generally consistent with previous works on metallicity-redshift variation. The present analysis employs a collisional-radiative-recombination model that also considers possible (e + O iv) –> O iii recombination-cascade contributions to [O iii ] forbidden lines, but is found to be negligible. We also explore AI Machine Learning models to predict and complement directly derived results, with preliminary simulations trained on observed flux ratios and PyNeb-simulated data sets that are promising but limited by current sample sizes. Future work may expand these data sets and further refine statistical models.