The dust origin of the Broad Line Region and the model consequences for AGN unification scheme

TL;DR

This paper introduces a dust-driven wind model for the formation of the Low Ionization Line part of the Broad Line Region in AGNs, explaining observed size-luminosity scaling and predicting observable differences based on inclination and wavelength.

Contribution

It presents a simple physical mechanism for BLR formation based on dust formation in accretion disks, offering testable predictions and differentiating from previous models.

Findings

The model reproduces the observed size-luminosity scaling of the BLR.

Predicted line profile differences based on inclination angle.

Potential observational tests with current and future telescopes.

Abstract

We propose a very simple physical mechanism responsible for the formation of the Low Ionization Line part of the Broad Line Region in Active Galactic Nuclei. It explains the scaling of the Broad Line Region size with the monochromatic luminosity, including the exact slope and the proportionality constant, seen in the reverberation studies of nearby sources. The scaling is independent from the mass and accretion rate of an active nucleus. The mechanism predicts the formation of a dust-driven wind in the disk region where the local effective temperature of a non-illuminated accretion disk drops below 1000 K and allows for dust formation. We explore now the predictive power of the model with the aim to differentiate between this model and the previously proposed mechanisms of the formation of the Broad Line Region. We discuss the expected departures from the universal scaling at long wavelength, and the role of the inclination angle of the accretion disk in the source. We compare the expected line profiles with Mg II line profiles in the quasars observed by us with the SALT telescope. We also discuss the tests based on the presence or absence of the broad emission lines in low luminosity active galaxies. Finally, we discuss the future tests of the model to be done with expected ground-based observations and satellite missions.