Physical origin of the Seyfert–LINER sulphur-to-oxygen offset

Determine whether the lower sulphur-to-oxygen ratios inferred for LINER regions relative to Seyfert nuclei arise from distinct ionising spectral energy distributions, different ionisation parameters, variations in gas geometry, or differences in dust depletion.

Background

The paper finds that LINER regions occupy an intermediate position between star-forming regions and Seyfert 2 nuclei in sulphur-to-oxygen ratio when analysed with the same AGN photoionisation framework. Recomputing the LINER sample with alternative AGN and post-asymptotic-giant-branch ionising spectra produces significantly different S/O distributions, demonstrating that the inferred abundance ratio is sensitive to the assumed ionisation structure.

The authors therefore leave unresolved whether the observed Seyfert–LINER difference reflects genuine physical distinctions in the ionising source or instead results from differences in ionisation parameters, gas geometry, or dust depletion. Resolving this issue is important for interpreting S/O as a diagnostic of the underlying ionising source and for determining whether LINER-specific photoionisation models are required.