2018MNRAS.474.1970B

The broad line region (BLR) in active galactic nuclei (AGNs) is composed of dense gas (∼10¹¹ cm^–3) on sub-pc scale, which absorbs about 30 per cent of the ionizing continuum. The outer size of the BLR is likely set by dust sublimation, and its density by the incident radiation pressure compression (RPC). But, what is the origin of this gas, and what sets its covering factor (CF)? Czerny & Hryniewicz (2011) suggested that the BLR is a failed dusty wind from the outer accretion disc. We explore the expected dust properties, and the implied BLR structure. We find that graphite grains sublimate only at T ≃ 2000 K at the predicted density of ∼10¹¹ cm^–3, and therefore large graphite grains (>=0.3 µm) survive down to the observed size of the BLR, R_BLR. The dust opacity in the accretion disc atmosphere is ∼50 times larger than previously assumed, and leads to an inflated torus-like structure, with a predicted peak height at R_BLR. The illuminated surface of this torus-like structure is a natural place for the BLR. The BLR CF is mostly set by the gas metallicity, the radiative accretion efficiency, a dynamic configuration and ablation by the incident optical-UV continuum. This model predicts that the BLR should extend inwards of R_BLR to the disc radius where the surface temperature is ≃2000 K, which occurs at R_in ≃ 0.18R_BLR. The value of R_in can be tested by reverberation mapping of the higher ionization lines, predicted by RPC to peak well inside R_BLR. The dust inflated disc scenario can also be tested based on the predicted response of R_BLR and the CF to changes in the AGN luminosity and accretion rate.