EXPLORING THE DISK-JET CONNECTION FROM THE PROPERTIES OF NARROW-LINE REGIONS IN POWERFUL YOUNG RADIO-LOUD ACTIVE GALACTIC NUCLEI

We investigate the optical emission-line flux ratios of narrow-line regions in order to determine whether the formation of active galactic nucleus (AGN) jets requires specific accretion conditions. We find that bright compact radio galaxies, which are powerful radio galaxies in the early stage of the jet activity, exhibit systematically larger flux ratios of [O I]λ6300/[O III]λ5007 and smaller flux ratios of [O III]λ5007/[O III]λ4363 than radio-quiet (RQ) Seyfert 2 galaxies. Comparing the observed line ratios with photoionization models, it is found that the difference in the flux ratio of low- to high-ionization lines (e.g., [O I]λ6300/[O III]λ5007) can be well understood by the difference in the spectral energy distribution (SED) of ionizing sources. Powerful young radio-loud (YRL) AGNs favor SED without a strong big blue bump (BBB), i.e., a radiatively inefficient accretion flow (RIAF), while RQ AGNs are consistent with the models adopting SED with a strong BBB, i.e., a geometrically thin, optically thick disk. These findings imply that the formation of powerful AGN jets requires the accretion disk with harder ionizing SED (i.e., an RIAF). We discuss the obscuring structure of YRL AGNs as a plausible origin of the difference in flux ratios of [O III]λ5007/[O III]λ4363.