Extreme Emission-Line Galaxies in SDSS. I. Empirical and model-based calibrations of chemical abundances

Abstract

Local star-forming galaxies show properties that are thought to differ from galaxies in the early Universe. Among them, the ionizing stellar populations and the gas geometry make the recipes designed to derive chemical abundances from nebular emission lines to differ from those calibrated in the Local Universe. A sample of 1969 Extreme Emission Line Galaxies (EELGs) at a redshift 0 𝑧 0.49, selected from the Sloan Digital Sky Survey (SDSS) to be local analogues of high-redshift galaxies, was used to analyze their most prominent emission lines and to derive total oxygen abundances and nitrogen-to-oxygen ratios following the direct method in the ranges 7.7 < 12+log(O/H) < 8.6 and -1.8 < log(N/O) < -0.8. They allow us to obtain new empirically calibrated strong-line methods and to evaluate other recipes based on photoionization models that can be later used for a chemical analysis of actively star-forming galaxies in very early stages of galaxy evolution. Our new relations are in agreement with others found for smaller samples of objects at higher redshifts. When compared with other relations calibrated in the local Universe, they differ when the employed strong-line ratio depends on the hardness of the ionizing radiation, such as O32 or Ne3O2, but they do not when the main dependence is on the ionization parameter, such as S23. In the case of strong-line ratios depending on [N ] lines, the derivation of O/H becomes very uncertain due to the very high N/O values derived in this sample, above all in the low-metallicity regime. Finally, we adapt the bayesian-like code HII-C -for the conditions found in this kind of galaxies and we prove that it can be used to derive within errors both O/H and N/O, in consistency with the direct method .

CLEAR: The Ionization and Chemical-Enrichment Properties of Galaxies at 1.1 < z < 2.3

“…To test if this potential bias affects the correlation, we used the [Ne III] λ3868 / [O II] ratio an as alternative tracer of gasphase metallicity. Multiple studies have demonstrated the correlation between [Ne III]/[O II] and 12 + log(O/H) (e.g., Maiolino et al 2008;Bian et al 2018;Pérez-Montero et al 2021;Sanders et al 2021) Figure 15 shows that the [Ne III]/[O II] ratios are similar in the stacked spectra of the low-and high-ionization samples. Comparison between Hβ equivalent width, EW, and gas ionization parameter,log q, for CLEAR galaxies in the high-ionization (log q > 7.8) and low-ionization subsamples (log q < 7.8).…”

“…Replacing R by n −2/3 H yields the relation that q increases with gas density as q ∼ n Sanders et al 2021). The models also include analogs of z ∼ 2 galaxies (Bian et al 2018), calibrations to SDSS at z ∼ 0 and higher redshifts (Maiolino et al 2008;Curti et al 2017;Pérez-Montero et al 2021).…”