Relationship between the velocity dispersion in the ionsied gas and the shock-sensitive line ratios [NII]λ6584/Hα (upper left panel), [SII]λ6716,6731 Å/Hα (upper right panel), and [OI]λ6300 Å/Hα (lower left panel) measured for emission lines with a S/N >  3 (and S/N >  10 for the determination of the velocity dispersion as described in Sect. 2.2). Each point is colour-coded according to its distance from the galaxy centre, as indicated in the right lower panel.