# Modelling broad emission lines in active galactic nuclei

## TL;DR

This paper reviews the formation and physical modeling of broad emission lines in active galactic nuclei, highlighting current understanding and gaps in time-dependent modeling of the broad line region.

## Contribution

It synthesizes recent progress in modeling the physical conditions of the broad line region and discusses the fixed radii and density parameters based on dust sublimation and radiation pressure.

## Key findings

- Outer radius of BLR set by dust sublimation temperature
- Inner radius likely fixed by dust sublimation in disk atmosphere
- Local cloud density is universally fixed by radiation pressure confinement

## Abstract

Broad Emission Lines are the most characteristic features of Active Galaxies, but the mechanism of creating a medium able to emit these intense lines is not quite clear. Observations clearly indicate that the motion of the material is predominantly Keplerian, with traces of inflow and clear signatures of outflow, but this still does not point out whether the lines partially come from the disk surface, or exclusively from the circumnuclear material, and whether this material originates from the disk as a wind, or comes, at least partially, from outside. I review the basic scenarios for the formation of the Broad Line Region (BLR), and the recent progress in modelling the physical conditions in the emitting medium. The current state is the the outer radius of the BLR is fixed by the dust sublimation temperature in the medium exposed to the irradiation from the central source, the inner radius is likely fixed by the dust sublimation temperature in the atmosphere of the non-illuminated accretion disk, and the local cloud density is a universal number fixed by the radiation pressure confinement. The time-dependent aspects of the BLR formation, however, still wait for serious modelling effort matching the quality of the observational data.