On the use of Sulphur as a tracer for abundances in galaxies

We present a methodology for the use of sulphur as global metallicity tracer in galaxies, allowing performing a complete abundance analysis using only the red-to-near infrared spectral region. We have applied it to a compilation of high-quality data split into two samples: HII regions (DHR) in spiral and irregular galaxies, and dwarf galaxies dominated by a strong starburst (HIIGal). Sulphur abundances have been derived by direct methods under the assumption of an ionisation structure composed of two zones: an intermediate one where S{++} is originated and a low ionisation one where S{+} is formed. Ionisation correction factors (ICF) have been calculated from the Ar{2+}/Ar{3+} ratio and are shown to correlate with the hardness of the radiation field. Only about 10% of the objects show S{3+} contributions to the total abundance larger than 30%. A good correlation exists between sulphur abundance and ionising temperature with low metallicity objects being ionised by hotter stars. No correlation is found between ionisation parameter and total S/H abundance. Most of the HIIGal objects show S/O ratios below the solar value and a trend for increasing S/O ratios with increasing sulphur abundances while DHR objects show S/O ratios larger than solar and a tendency for lower S/O ratios for higher metallicities. Finally, we present a calibration of the sulphur abundance through the S{23} parameter that remains single valued up to sulphur abundances well beyond the solar value. S{23} is independent of the ionisation parameter and only weakly dependent on ionising temperature.