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 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 S2+ 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.