Obscured Active Galactic Nuclei

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.

50 pages, 16 figures. Invited review to appear in Annual Reviews of Astronomy and Astrophysics, Volume 56. An animated version of the schematic of AGN spectral energy distributions shown in Figure 4 is available at http://www.dartmouth.edu/~hickox/hickox_alexander_AGN.php

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• Infrared spectroscopic confirmation of z~2 photometrically-selected obscured quasars
• UV-FIR SED modeling of AGN in IR-luminous galaxies up to z~2.5: Understanding the effects of torus models
• Galactic Winds across the Gas-Rich Merger Sequence II. Lyman Alpha Emission and Highly Ionized O VI and N V Outflows in Ultraluminous Infrared Galaxies
• A Peculiar Type~II QSO Identified via Broad-band Detection of Extreme Nebular Line Emission
• Update of the INTEGRAL/IBIS AGN catalogue: deeper on the Galactic plane and wider beyond
• WISE2MBH: A scaling-based algorithm for probing supermassive black hole masses through WISE catalogues
• Updated Picture of the Active Galactic Nuclei with Dusty/Dust-free Gas Structures and Effects of the Radiation Pressure
• Identifying type II quasars at intermediate redshift with few-shot learning photometric classification
• X-ray Redshifts for Obscured Active Galactic Nuclei with AXIS Deep and Intermediate Surveys
• Testing the Radio-Selection Method of Dual Active Galactic Nuclei in the Stripe 82 Field
• Black hole-galaxy co-evolution and the role of feedback
• NuSTAR observations of a heavily X-ray obscured AGN in the dwarf galaxy J144013+024744
• Distinguishing AGN from starbursts as the origin of double peaked Lyman-Alpha Emitters in the reionization era
• SDSS-IV MaNGA: Identification and Multiwavelength Properties of Type-1 AGN in the DR15 sample
• Properties of IR selected Active Galactic Nuclei
• Hot graphite dust in the inner regime of NGC 4151
• ALMA polarimetry of radio-quiet AGNs
• Modelling radio luminosity functions of radio-loud AGN by the cosmological evolution of supermassive black holes
• AGN anisotropic radiative feedback set by black hole spin
• A spectacular galactic scale magnetohydrodynamic powered wind in ESO 320-G030
• Physical Models for the Clustering of Obscured and Unobscured Quasars
• The Complex X-ray Obscuration Environment in the Radio Loud Type 2 Quasar 3C 223
• Gaia-assisted discovery of a detached low-ionisation BAL quasar with very large ejection velocities
• Probing the Extent of Fe K$\alpha$ Emission in Nearby Active Galactic Nuclei using Multi-Order Analysis of Chandra High Energy Transmission Grating Data
• Unravelling the nature of the dual AGN in the galaxy pair system IRAS 05589+2828 and 2MASX J06021107+2828382
• X-ray Analysis of AGN from the GALEX Time Domain Survey
• Machine learning applications for the study of AGN physical properties using photometric observations
• Ray-tracing simulations of the Soft X-ray Scattered Emission in obscured Active Galactic Nuclei
• The infalling elliptical galaxy M89: The chemical composition of the AGN disturbed hot atmosphere
• I Zw 1 and H0557-385: The Dusty Tori of Two High Eddington AGNs Observed in the MATISSE LM-Bands
• A Complete 16 micron-Selected Galaxy Sample at $z\sim 1$: Mid-infrared Spectral Energy Distributions
• Mid-Infrared Variability in Nearby Galaxies from the MaNGA Sample
• X-ray Properties of Optically Variable Low-mass AGN Candidates
• pop-cosmos: Insights from generative modeling of a deep, infrared-selected galaxy population
• X-ray properties of high-redshift Radio Loud and Radio Quiet Quasars observed by Chandra
• X-Ray Redshifts of Obscured Chandra Source Catalog AGN
• Detection of extremely low frequency gravitational wave using gravitational lens
• Polarization in broad emission lines of Active Galactic Nuclei
• Active Galactic Nuclei and their demography through cosmic time
• Multiwavelength Analysis of a Nearby Heavily Obscured AGN in NGC 449
• QSO MUSEUM III: the circumgalactic medium in Ly$\alpha$ emission around 120 $z\sim 3$ quasars covering the SDSS parameter space. Witnessing the instantaneous AGN feedback on halo scales
• AGN radiative feedback in the early growth of massive black holes
• X-ray Unveiling Events in a z~1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South
• Reconstructing AGN X-ray spectral parameter distributions with Bayesian methods II: Population inference
• AGN -- host galaxy photometric decomposition using a fast, accurate and precise deep learning approach
• New insights on the central stellar population and gas outflow in NGC 1068 from YJH spectroscopy with SPHERE/VLT
• Constraints on dynamically-formed massive black holes in Little Red Dots from X-ray non-detections
• Supercritical dusty BH growth in the early Universe
• X-ray Sources in the SSA22 Chandra Field
• The Nature of LoBAL QSOs: II. HST/WFC3 Observations Reveal Host Galaxies Dominated by Mergers
• The $M_{\mathrm{BH}}-M_{\star }$ Relation of the hyperluminous Dust-obscured Quasars up to $z\sim 4$
• ALMA High-J CO Spectroscopy of High-Redshift Galaxies. II. 0.03" Resolution CO Kinematics Reveal Super-Eddington Accretion in a Dust-Obscured Galaxy at z=3.111
• The X-ray weakness of Little Red Dots and JWST-selected AGN: comparison with local AGN in different accretion regimes
• Exploring the physical properties of Type II Quasar candidates at intermediate redshifts with CIGALE
• A High-resolution Far-infrared Survey to Probe Black Hole-Galaxy Co-evolution
• Multi-Epoch Optical Spectroscopy Variability of the Changing-Look AGN Mrk 883
• BASS XLV: Quantifying AGN Selection Effects in the Chandra COSMOS-Legacy Survey with BASS
• AGN feeding along a one-armed spiral in NGC 4593: A study using ALMA CO(2-1) observations
• Reconstructing AGN X-ray spectral parameter distributions with Bayesian methods I: Spectral analysis
• PEARLS: Discovery of Point-Source Features Within Galaxies in the North Ecliptic Pole Time Domain Field
• Exploring the X-ray-radio connection for AGN via measurements of the multi-dimensional luminosity function
• The viewing angle in AGN SED models, a data-driven analysis
• The major merger-active galactic nucleus connection up to the cosmic noon
• AGN obscuration in optical and X-rays: Host properties and the interplay of nuclear and galactic gas and dust in a combined SDSS-XMM sample
• BASS. XLIV. Morphological preferences of local hard X-ray selected AGN
• How black hole activity may influence exoplanetary evolution in our Galaxy
• Identifying Compton-thick active galactic nuclei in the COSMOS. II. Searching among mid-infrared selected AGNs
• Mid-infrared Variability-based AGN Selection using the Multi-epoch Photometric Data from WISE
• Flavor Matters, but Matter Flavors: Matter Effects on Flavor Composition of Astrophysical Neutrinos
• ULISSE: Determination of star-formation rate and stellar mass based on the one-shot galaxy imaging technique
• Obscured and unobscured X-ray AGNs I: Host galaxy properties
• Self-consistent population synthesis of AGN from observational constraints in the X-rays
• Interstellar Medium in Extremely High Star-Formation Regions: A Prospect of Observations on the Millimetron Space Observatory
• MIDIS: Quantifying the AGN component of X-ray-detected galaxies
• A broadband X-ray study of a sample of AGNs with [OIII] measured inclinations
• A non-uniform metal distribution in the ram-pressure stripped circumgalactic medium of M86
• BASS XXX: Distribution Functions of DR2 Eddington-ratios, Black Hole Masses, and X-ray Luminosities
• Fueling, Evolution, and Diversity of AGN in Dwarf Galaxies: Insights from Star Formation and Black Hole Scaling Relations
• Chandra X-ray Observatory study of the X-ray emission of PKS 0023-26 and comparison with recent ALMA results
• Preliminary census of galaxies in the LISA localisation volume: I. Searching for LISA candidate massive black hole binary merger hosts using Sloan Digital Sky Survey photometry
• X-ray emission of radio-loud quasar SDSS J121426.52+140258.9: independent variations between optical/UV and X-ray emission
• X-ray sources in the 1.75 Ms Ultra Narrow Deep Field observed by XMM-Newton
• The Dyson Minds 2025 Workshop: SETI around Black Holes
• Spectroscopic confirmation of dual and offset quasars from the Subaru HSC-SSP program
• Identifying AGN host galaxies by Machine Learning with HSC+WISE
• Constraining Kerr supermassive black hole properties using gravitational waves from inspiraling stellar-mass binary black holes
• Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations
• Evidence of Heavy Obscuration in the Low-Luminosity AGN NGC 4941
• Dynamically close galaxy pairs from the unWISE survey: Testing the merger-AGN-star formation connection
• The optical-infrared relation for active galactic nuclei: The role of contaminations