Constraining jet production scenarios by studies of Narrow-Line-Radio-Galaxies

Abstract

We study a large sample of narrow-line radio galaxies (NLRGs) with extended radio structures. Using 1.4 GHz radio luminosities, $L_{1.4}$, narrow optical emission line luminosities, $L_{\oiii}$ and $L_{H_{\alpha}}$, as well as black hole masses $M_{BH}$ derived from stellar velocity dispersions measured from the optical spectra obtained with the Sloan Digital Sky Survey, we find that: (i) NLRGs cover about 4 decades of the Eddington ratio, $\lambda \equiv L_{bol}/L_{Edd} \propto L_{line}/M_{BH}$; (ii) $L_{1.4}/M_{BH}$ strongly correlates with $\lambda$; (iii) radio-loudness, ${\cal R} \equiv L_{1.4}/L_{line}$, strongly anti-correlates with $\lambda$. A very broad range of the Eddington ratio indicates that the parent population of NLRGs includes both radio-loud quasars (RLQs) and broad-line radio galaxies (BLRGs). The correlations they obey and their high jet production efficiencies favor a jet production model which involves the so-called 'magnetically choked' accretion scenario. In this model, production of the jet is dominated by the Blandford-Znajek mechanism, and the magnetic fields in the vicinity of the central black hole are confined by the ram pressure of the accretion flow. Since large net magnetic flux accumulated in central regions of the accretion flow required by the model can take place only via geometrically thick accretion, we speculate that the massive, 'cold' accretion events associated with luminous emission-line AGN can be accompanied by an efficient jet production only if preceded by a hot, very sub-Eddington accretion phase.