Deriving the abundance distribution of HII galaxies using sulphur as tracer: Exploring the high-metallicity end

¹ Departamento de Física Teórica, Universidad Autónoma de Madrid, Madrid 28049, Spain
² CIAFF, Universidad Autónoma de Madrid, 28049 Madrid, Spain
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy

^⋆ Corresponding author; marta@langeber.es

Received: 9 May 2024
Accepted: 6 December 2024

Abstract

Aims. The main objective of this work is to derive the distribution of the metal content of HII galaxies using sulphur as an abundance tracer. This increases the metallicity range that can safely be reached.

Methods. We selected a sample of emission-line galaxies that we extracted from the SDSS-DR16. These objects have a redshift of z ≤ 0.04 so that the [S III] λ 9069 Å emission line and H_β equivalent widths that are higher than 10 Å in emission were included, and they are compact in appearance. We used the so-called direct method for objects with the electron-temperature-sensitive [S III] λ 6312 Å emission line, and an empirical method based on the S₂₃ parameter. The last provided an abundance calibration that monotonically increased up to at least the solar value, and can be applied based on the spectral range from 6000 to 9500 Å alone.

Results. We show that the bias that is introduced when the [O III] λ 4363 Å line is required restricts the sample to objects with an [O III] electron temperature higher than 10 000K, and their temperature distribution is then rather narrow. For objects with determinations of t_e[S III], the distribution is flatter and wider, which fits a more realistic scenario better. For these objects, we calculated the ionic abundances of sulphur and their ratios. In all cases, S²⁺ was found to be the dominant ion, with a contribution greatly exceeding that of S⁺. This fact justifies the adoption t_e[S III] as the dominant temperature throughout the nebula, although in 20% of the objects, an estimated ionization correction factor is required. For the objects in the sample that required the detection of the [O III] λ 4363 Å line (sample 3) and [S III] λ 6312 Å, the distribution abundances as traced directly by oxygen and sulphur appear to be very similar to each other. The median values are 12+log(O/H) = 8.1 and 12+log(S/H) = 6.4, which corresponds to an S/O ratio of log(S/O) = −1.7 that is close to the solar value (−1.5). However, when the restriction for weak temperature-sensitive lines is relaxed, the abundance distribution is wider and the median value is 12+log(S/H) = 6.6. When the S/O ratio is assumed to be constant, the median sulphur abundance value found here would imply a median value of the oxygen abundance of 12+log(O/H) = 8.3.

Conclusions. In summary, the abundance distributions traced by sulphur can reach reliable abundances up to the solar value at least and provide a more complete picture of the metallicity distribution of HII galaxies. The method presented here only involves the red part of the spectrum (between 6000 and 9600 Å), and the effect of reddening is weak there. Although the strong nebular [S III] lines shift beyond the far red spectral region for high-redshift objects, present-day infrared spectrographs can overcome this difficulty. Observations made with NIRSpec on board the JWST would be able to provide data for objects with redshifts between 0 and 4.24.