Emission-line diagnostics of the central engines of weak-line radio galaxies

A handful of well-studied weak-line radio galaxies (WLRGs) have been traditionally classified as low-ionization nuclear emission-line regions (LINERs), suggesting that these two groups of active galactic nuclei (AGNs) might be related. In this paper, we present new optical emission-line measurements for 20 WLRGs, which we supplement with measurements for an additional four from the literature. Classifying these objects by their emission-line ratios, we find that 50% of the objects are robustly classified as LINERs, while an additional 25% are likely to be LINERs. Photoionization calculations show that the spectral energy distribution of the well-studied WLRG 3C 270 (NGC 4261) is able to produce the observed emission-line ratios, but only if the UV emission seen by the narrow emission line gas is significantly higher than that observed, implying A(V) = 2.5 - 4.2 mag along our line of sight to the nucleus. From the photoionization calculations, we find that the emission-line gas must have an ionization parameter between 10(-3.5) and 10-(4.0) and a wide range in hydrogen density (10(2) - 10(6) cm(-3)) to reproduce the measured emission-line ratios, similar to the properties inferred for the emission-line gas in LINERs. Thus, we find that properties of the emission-line gas as well as the underlying excitation mechanism are indeed similar in LINERs and WLRGs. By extension, the central engines of accretion-powered LINERs and WLRGs, which do host an accreting black hole, may be qualitatively similar.