Obscured Active Galactic Nuclei

TL;DR

This review discusses the challenges of detecting obscured Active Galactic Nuclei (AGN), the techniques used across multiple wavelengths to identify them, and their significance for understanding cosmic evolution and SMBH growth.

Contribution

It provides a comprehensive overview of multi-wavelength methods for detecting obscured AGN, evaluates their effectiveness, and discusses implications for cosmology and future research.

Findings

Multiple techniques have varying reliability in identifying obscured AGN.

Obscured AGN constitute a significant portion of the total AGN population.

Understanding obscured AGN is crucial for insights into SMBH growth and cosmic evolution.

Abstract

Active Galactic Nuclei (AGN) are powered by the accretion of material onto a supermassive black hole (SMBH), and are among the most luminous objects in the Universe. However, the huge radiative power of most AGN cannot be seen directly, as the accretion is "hidden" behind gas and dust that absorbs many of the characteristic observational signatures. This obscuration presents an important challenge for uncovering the complete AGN population and understanding the cosmic evolution of SMBHs. In this review we describe a broad range of multi-wavelength techniques that are currently employed to identify obscured AGN, and assess the reliability and completeness of each technique. We follow with a discussion of the demographics of obscured AGN activity, explore the nature and physical scales of the obscuring material, and assess the implications of obscured AGN for observational cosmology. We conclude with an outline of the prospects for future progress from both observations and theoretical models, and highlight some of the key outstanding questions.