Exploring the disk-jet connection from the properties of narrow line regions in powerful young radio-loud AGNs

TL;DR

This study links the properties of narrow-line regions in young radio-loud AGNs to their accretion disk states, suggesting that jet formation is associated with radiatively inefficient accretion flows lacking a strong big blue bump.

Contribution

It provides evidence that powerful jets in young radio-loud AGNs are connected to specific accretion disk conditions, particularly RIAFs, based on optical emission-line flux ratios.

Findings

Radio-loud AGNs show distinct emission-line flux ratios compared to radio-quiet AGNs.

Differences in line ratios are explained by variations in the ionizing spectral energy distribution.

Young radio-loud AGNs favor radiatively inefficient accretion flows without a strong big blue bump.

Abstract

We investigate the optical emission-line flux ratios of narrow-line regions, in order to determine whether the formation of 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{\sc i}]$\lambda$6300/[O{\sc iii}]$\lambda$5007 and smaller flux ratios of [O{\sc iii}]$\lambda$5007/[O{\sc iii}]$\lambda$4363 than radio-quiet (RQ) Seyfert 2 galaxies. Comparing the observed line ratios with photoionization models, it is found that the difference in the flux ratio of low- to high-ionization lines (e.g., [O{\sc i}]$\lambda$6300/[O{\sc iii}]$\lambda$5007) can be well understood by the difference in the spectral energy distribution (SED) of ionizing sources. Powerful young radio-loud (YRL) AGNs favor SED without a strong big blue bump, i.e., a radiatively inefficient accretion flow (RIAF), while RQ AGNs are consistent with the models adopting SED with a strong big blue bump, i.e., a geometrically thin, optically thick disk. These findings imply that the formation of powerful AGN jets requires the accretion disk with harder ionizing SED (i.e., a RIAF). We discuss the obscuring structure of YRL AGNs as a plausible origin of the difference in flux ratios of [O{\sc iii}]$ \lambda$5007/[O{\sc iii}]$\lambda$4363.