What broad emission lines tell us about how active galactic nuclei work

TL;DR

This review synthesizes current understanding of the broad-line region in active galactic nuclei, highlighting its geometry, dynamics, and role in accretion, supported by reverberation mapping and spectral analysis.

Contribution

It provides a comprehensive overview of BLR structure, dynamics, and their implications for black hole mass estimation, emphasizing a flattened, inflow-dominated geometry.

Findings

BLR has a high covering factor and flattened geometry.

Predominant motions are Keplerian with inflow, not outflow.

Black hole masses can be reliably estimated from BLR dynamics.

Abstract

I review progress made in understanding the nature of the broad-line region (BLR) of active galactic nuclei (AGNs) and the role BLRs play in the AGN phenomenon. The high equivalent widths of the lines imply a high BLR covering factor, and the absence of clear evidence for absorption by the BLR means that the BLR has a flattened distribution and that we always view it near pole-on. The BLR gas is strongly self-shielding near the equatorial plane. Velocity-resolved reverberation mapping has long strongly excluded significant outflow of the BLR and shows instead that the predominant motions are Keplerian with large turbulence and a significant net inflow. The rotation and turbulence are consistent with the inferred geometry. The blueshifting of high-ionization lines is a consequence of scattering off inflowing material rather than the result of an outflowing wind. The rate of inflow of the BLR is sufficient to provide the accretion rate needed to power the AGN. Because the motions of the BLR are gravitationally dominated, and the BLR structure is very similar in most AGNs, consistent black hole masses can be determined. The good correlation between these estimates and masses predicted from the bulge luminosities of host galaxies provides strong support for the similarity of AGN continuum shapes and the correctness of the BLR picture presented. It is concluded that although many mysteries remain about the details of how AGNs work, a general overall picture of the torus and BLR is becoming clear.