New evidence for photoionization as the dominant excitation mechanism in Liners

Abstract

Optical spectrophotometry of the nearby QSO MR 2251-178 and the powerful radio sources PKS 1718-649 (a classical Liner) and Pictor A (a type 1 Seyfert galaxy with features of Liners) is presented. There is strong evidence that the emitting gas in these objects is photoionized by nonstellar radiation. In all three nuclei, on the other hand, the great strength of [O III] λ4363 relative to [O III] λ5007 implies T