The intensities of the sulphur III lines and the ionization mechanisms in Liners

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Abstract
New near-infrared spectrophotometric observations show that the relative strengths of the sulphur emission lines [S III] λλ 9069, 9532 Å with respect to Hα constitute a powerful diagnostic to distinguish between shock and photoionization mechanisms in Liners. These observations strongly support the idea, already favoured on the basis of abundance arguments, that Liners and Seyferts form a continuous sequence, both types of objects being photoionized, but with the ionization parameter being lower in Liners. This photoionization could be due either to the presence of a central nonthermal source or to recent star formation.

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