Low-metallicity HII regions and galaxies

Our aims are to estimate the validity of empirical methods, such as R23, R23-P, log([NII]/H{alpha}) (N2), log[([OIII]/H{beta})/([NII]/H{alpha}] (O3N2), and log([SII]/H{alpha}) (S2), and to re-derive (or add) the calibrations of R23, N2, O3N2, and S2 indices for oxygen abundances on the basis of a large sample of galaxies with Te-based abundances. We determined the gas-phase oxygen abundance for a sample of 695 galaxies and HII regions with reliable detections of [OIII]4363, using the reliable and direct temperature-sensitive (Te) method of measuring metallicity.

We selected 531 star-forming galaxies from the SDSS-DR4 database with strong emission lines, including [OIII]4363 detected at a signal-to-noise ratio higher than 5{sigma}, as well as 164 galaxies and HII regions from the literature with Te measurements. The O/H abundances were derived from a two-zone model for the temperature structure, assuming a relationship between high ionization and low ionization species.