The ionization mechanism of the 'Liner' galaxies Pictor A and PKS 1718 - 65

Astronomy and Astrophysics – Astrophysics

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Astronomical Spectroscopy, Galactic Nuclei, Gas Ionization, Photoionization, Seyfert Galaxies, Emission Spectra, Galactic Radiation, Spectral Line Width

Scientific paper

High-resolution optical spectra are presented of two galaxies with Liner (low-ionization nuclear emission-line region) nuclei, Pictor A and PKS 1718 - 65. Both emit strong forbidden O III 4363 A, but the ratio of this line with respect to forbidden OIII 5007 A is so high that it must be influenced by high electron density. Hence, these lines cannot be used to measure the electron density and test shock-heating versus photoionization models of Liners in a straightforward way. Nonetheless, Pictor A is shown to have a strong featureless optical continuum and He II 4686 A emission, making it the best case to date of a power-law photoionized Liner. The spectrum of PKS 1718 - 65 shows neither of these features, and thus is more like that of the prototype Liner, NGC 1052. If the Liners in such galaxies as these are photoionized rather than shock-excited, the ionizing spectrum must be more like a blackbody than a power-law spectrum.

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