New evidence for photoionization as the dominant excitation mechanism in Liners

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 Te > 50,000K, which is incompatible with photoionized low-density clouds. The dilemma vanishes if relatively dense clouds (ne ⪆ 10⁶ - 10⁷cm⁻³) exist in the narrow-line regions. This paper provides unambiguous evidence for such high densities: it is shown that in each object the width of forbidden lines increases with their critical density for collisional deexcitation over the range 10³ ⪉ ne(crit) ⪉ 10⁷cm⁻³.