Active Galactic Nuclei and their demography through cosmic time

TL;DR

This paper reviews the properties, diversity, and unification models of Active Galactic Nuclei (AGN), discusses complexities beyond simple models, and explores their evolution across X-ray and gamma-ray wavelengths.

Contribution

It provides a comprehensive review of AGN properties, challenges existing unification models with recent observations, and discusses AGN evolution in high-energy bands.

Findings

AGN properties can be explained by a small set of parameters

Unification models are insufficient to explain all observed complexities

Recent X-ray observations reveal multiple absorbers on different scales

Abstract

Active Galactic Nuclei (AGN) are highly energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, which present unique observational signatures covering the full electromagnetic spectrum (and more) over about twenty orders of magnitude in frequency. We first review the main AGN properties and diversities and show that they can be explained by a small number of parameters. We then discuss the so-called Unification Models for non-jetted AGN, according to which these sources are believed to have the same nuclear engine and circumnuclear matter, with the same geometry for the obscuring structure. This simplified scenario, however, cannot explain all the observed complexities, such as the presence of multiple absorbers on different physical scales, including recent X-ray observations of circumnuclear matter. Finally, we touch upon AGN evolution in the X-ray and $\gamma$-ray bands.