3.3. Liners and ULIGs
About 40% of all ULIGs have Liner type optical spectra (; Veilleux et al. 1999). Spectropolarimetric, near-infrared spectroscopic and MIR spectroscopic observations have been carried out in a number of such objects; none of them shows any obvious sign of an obscured BLR, suggesting that they are powered by starbursts (; ; ; ).
We have plotted on the diagnostic diagrams 5007 / H vs 6583 / H and 6300 / H all luminous and ultraluminous IR galaxies for which the required line ratios have been published (; Veilleux et al. 1999), excluding the Seyfert 1s for which the relative fluxes of the narrow component of the Balmer lines are generally not available. These plots are shown in fig. 4. The two objects with the strongest [O I] lines are PKS2338+03, a radio galaxy and NGC6240, a well known AGN (). Comparison with fig. 3 shows that in addition to low excitation H II regions and Seyfert 2s, there is a cluster of objects around 5007 / H ~ 1.6, 6583 / H ~ 0.8 and 6300 / H ~ 0.1; these are the objects which have been classified as Liners; however they seem to be different from the Liners in fig. 3 which are mostly associated with AGNs, suggesting that, indeed, they are not genuine AGNs.
Figure 4. Diagnostic diagrams for luminous and ultraluminous IR galaxies, excluding Seyfert 1s. The regions containing H II regions, Seyfert 2s, and classical Liners have been delineated as in fig. 3. H II regions are shown as crosses, Seyfert 2 as open circles. The black squares are the objects called ``Liners'' in the original papers (see text); they cluster in a region which is well outside the region occupied by the ``classical'' Liners, i.e. Liners associated with an AGN.