https://arxiv.org/abs/1806.04680

For a PDF version of the article, click here.

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.

Keywords : active galaxies, AGN surveys, black holes, central torus, mergers, obscuration

Table of Contents

• INTRODUCTION
• A brief overview of obscured AGN
• The importance of identifying obscured AGN
• IDENTIFICATION OF OBSCURED AGN
• Selection of obscured AGN in the ultra-violet to near-infrared waveband
• Broad-band continuum techniques
• Spectroscopic techniques
• Selection of obscured AGN in the X-ray waveband
• Selection of obscured AGN in the mid-infrared waveband
• Broad-band continuum techniques
• Spectroscopic techniques
• Selection of obscured AGN at far-infrared-radio wavelengths
• Far-infrared-millimeter wavelengths
• Radio wavelengths
• Multi-wavelength identification and a comparison of selection methods
• THE DEMOGRAPHICS OF THE OBSCURED AGN POPULATION
• THE PHYSICAL NATURE OF OBSCURATION IN AGN
• The nuclear torus and the unified AGN model
• The basic torus properties are well-constrained
• The torus is clumpy
• The torus can have a range of covering factors, with dependence on AGN properties
• The torus is dynamic
• The torus may extend in the polar direction and to large scales
• Obscuration by nuclear starbursts
• Obscuration by galaxy-scale material
• IMPLICATIONS FOR OBSCURED AGN IN OBSERVATIONAL COSMOLOGY
• The evolutionary sequence and the SMBH-galaxy connection
• The evolution of obscured SMBHs at high redshift
• Obscured AGN, the cosmic X-ray background, and the radiative efficiency of black hole accretion
• CONCLUSIONS AND FUTURE PROSPECTS
• Forecasts for future facilities
• UV-Near-IR
• X-rays
• Mid-IR
• Far-IR-radio
• Prospects for theoretical models
• Outstanding questions
• What is the physical origin of the torus?
• What is the evolution of AGN obscuration and the connection to galaxy formation?
• How do we find the most heavily obscured AGN?
• What is the role of obscured accretion at the dawn of the first SMBHs?
• REFERENCES