Chemical abundances in Seyfert galaxies X. Sulphur abundance estimates

Abstract

For the first time, the sulphur abundance relative to hydrogen (S/H) in the Narrow Line Regions of a sample of Seyfert 2 nuclei (Sy 2s) has been derived via direct estimation of the electron temperature. Narrow emission line intensities from the SDSS DR17 [in the wavelength range 3000 < 𝜆(Å) < 9100] and from the literature for a sample of 45 nearby (𝑧 < 0.08) Sy 2s were considered. Our direct estimates indicate that Sy 2s have similar temperatures in the gas region where most of the S + ions are located in comparison with that of star-forming regions (SFs). However, Sy 2s present higher temperature values (∼ 10 000 K) in the region where most of the S 2+ ions are located relative to that of SFs. We derive the total sulphur abundance in the range of 6.2 < ∼ 12 + log(S/H) < ∼ 7.5, corresponding to 0.1 -1.8 times the solar value. These sulphur abundance values are lower by ∼ 0.4 dex than those derived in SFs with similar metallicity, indicating a distinct chemical enrichment of the ISM for these object classes. The S/O values for our Sy 2 sample present an abrupt (∼ 0.5 dex) decrease with increasing O/H for the high metallicity regime [12 + log(O/H) > ∼ 8.7)], what is not seen for the SFs. However, when our Sy 2 estimates are combined with those from a large sample of star-forming regions, we did not find any dependence between S/O and O/H.