Emission‐Line Diagnostics of the Central Engines of Weak‐Line Radio Galaxies

A handful of well-studied Weak-Line Radio Galaxies (WLRGs) have beentraditionally classified as Low Ionization Nuclear Emission-line Regions(LINERs), suggesting that these two groups of AGNs might be related. In thispaper, we present new optical emission-line measurements for twenty Weak-LineRadio Galaxies which we supplement with measurements for an additional fourfrom the literature. Classifying these objects by their emission-line ratios,we find that 50% of the objects are robustly classified as LINERs while anadditional 25% are likely to be LINERs. Photoionization calculations show thatthe Spectral Energy Distribution of the well-studied WLRG 3C 270 (NGC 4261) isable to produce the observed emission-line ratios, but only if the UV emissionseen by the narrow emission-line gas is significantly higher than thatobserved, implying A_V = 2.5-4.2 magnitudes along our line of sight to thenucleus. From the photoionization calculations, we find that the emission-linegas must have an ionization parameter between 10^-3.5 and 10^-4.0 and a widerange in hydrogen density (10^2-10^6 cm^-3) to reproduce the measuredemission-line ratios, similar to the properties inferred for the emission-linegas in LINERs. Thus, we find that properties of the emission-line gas as wellas the underlying excitation mechanism are indeed similar in LINERs and WLRGs.By extension, the central engines of accretion-powered LINERs and WLRGs, whichdo host an accreting black hole, may be qualitatively similar.Comment: 28 pages, 5 tables, 3 figures, accepted to Ap