Revisiting the Unified Model of Active Galactic Nuclei

TL;DR

This review discusses recent advances in the unified model of AGN, focusing on the origin and properties of the obscuring torus, with new insights from IR interferometry and hydrodynamical simulations.

Contribution

It synthesizes recent developments in understanding the AGN torus, including new models, observational techniques, and unresolved issues in the unification scheme.

Findings

IR interferometry challenges existing torus models

Mapping of ionization cones provides new insights

Uncertainty remains about the torus covering factor dependence

Abstract

This review describes recent developments related to the unified model of active galactic nuclei (AGN). It focuses on new ideas about the origin and properties of the central obscurer (torus), and the connection with its surrounding. The review does not address radio unification. AGN tori must be clumpy but the uncertainties about their properties are still large. Todays most promising models involve disk winds of various types and hydrodynamical simulations that link the large scale galactic disk to the inner accretion flow. IR studies greatly improved the understanding of the spectral energy distribution of AGNs but they are hindered by various selection effects. X-ray samples are more complete. A basic relationship which is still unexplained is the dependence of the torus covering factor on luminosity. There is also much confusion regarding "real type-II AGNs" that do not fit into a simple unification scheme. The most impressive recent results are due to IR interferometry, which is not in accord with most torus models, and the accurate mapping of central ionization cones. AGN unification may not apply to merging systems and is possibly restricted to secularly evolving galaxies.