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In this review we have highlighted progress in identifying and characterizing obscured AGN, and have detailed the associated physical insights related to the accretion process onto SMBHs and its cosmological implications. Observationally, the combination of sensitive observations over a broad range of wavelengths (particularly the mid-IR and hard X-rays) have enabled a much improved census of the population of growing SMBHs in the past decade (Sections 2 and 3). Major recent advances include better understanding of the geometry and properties of the obscuring “torus” (Section 4.1), evidence for obscuration on host galaxy scales and connections to galaxy evolution (Sections 4.2, 4.3 and 5.1), direct measurements of the contribution of AGN to the hard CXB (Section 5.3), and the discovery of a (possibly large) population of very heavily obscured AGN that may be particularly important at high redshift (Section 5.2). These results suggest an important role for obscured accretion in the growth histories of SMBHs (both in the rapid evolution of massive galaxies, and in the early growth of “seed” SMBHs), and indicate that while the bulk of obscuration appears to occur in compact regions within the sphere of influence of the SMBH, obscuration occurs over a wide range of scales and physical conditions. In this final section we discuss how our understanding of obscured AGN wil be further improved with future observational facilities and developments in theoretical models, and suggest some open questions that provide particularly promising opportunities for progress.

6.1. Forecasts for future facilities

The coming decades will see an impressive array of new observational resources that will enhance our abilities to detect and characterize obscured AGN. Here we will discuss the prospects for a handful of upcoming or proposed facilities in each of the wavebands discussed in Section 2.

6.1.1. UV–Near-IR. Spectroscopy provides the most widely-applicable method for studying obscured AGN in the optical and NIR (Section 2.1.2). From the ground, new large-scale multi-object spectrographs in the optical and near-IR such as DESI 2. (DESI Collaboration et al., 2016), 4MOST 3 (de Jong et al., 2014), and Subaru PFS 4 (Takada et al., 2014) will identify and characterize huge numbers of obscured AGN, including rest-frame optical lines out to z ∼ 3–4. In addition, new surveys with integral field units such as SAMI 5 (Croom et al., 2012), CALIFA (Sánchez et al., 2012) 6, and MaNGA 7 (Bundy et al., 2015) are enabling detailed spatially resolved studies of NLR ionization and kinematics. From space, JWST 8 and WFIRST 9 will provide extremely sensitive infrared spectroscopy with high spatial resolution, allowing characterization of the emission features in rest-frame optical (which are critical for AGN diagnostics; Section 2.1) and also host galaxy properties of obscured AGN out to high redshift. Further into the future, the 30-m class telescopes such as GMT 10, TMT 11, and E-ELT 12 will perform high sensitivity observations of obscured AGN to high redshifts, and the LUVOIR 13 and HABEX 14 concept space missions would carry out sensitive optical spectroscopy and imaging of obscured AGN with extremely high angular resolution. Finally, LSST 15 will provide extremely deep, wide-field optical imaging that, while not able to efficiently select obscured AGN, will be powerful for the characterization of AGN host galaxies and identifying rare changing-look AGN (see Section 2.1.2).

6.1.2. X-rays. For statistical studies of obscured AGN, new generations of X-ray telescopes will enable deep, wide-area surveys resulting in very large samples of AGN. The eROSITA mission 16 will identify millions of AGN over the whole sky (Merloni et al., 2012), including a significant number of obscured sources (with follow-up spectroscopy from 4MOST), although its ability to probe the complete obscured AGN population will be limited by its energy response, which drops off more quickly at energies > 3 keV when compared to Chandra and XMM. Further into the future, the Athena mission 17 (Barcons et al., 2015) will enable sensitive X-ray imaging spectroscopy with sensitivity to ≈ 12 keV, and over a wide 40’ field of view. Athena surveys will reach, over a large area, a confusion-limited source detection flux limit approximately equal to that of the Chandra Deep Fields (e.g., Xue et al., 2011, Luo et al., 2017), and will allow extremely sensitive spectroscopy of faint and high-redshift AGN, providing a significant leap forward in the selection of distant heavily obscured and CT AGN. The Lynx concept mission 18 (with higher spatial resolution) would probe up to two orders of magnitude fainter with imaging and spectroscopy of obscured AGN out to the highest redshifts and providing constraints on SMBH seed models (see Section 5.2) and imaging of obscured AGN nuclei to better constrain the nature of obscuring and reflecting material (see Section 4.1). Among a number of proposed smaller X-ray missions, a particularly powerful tool for studying obscured AGN is HEX-P 19, which would build on the success of NuSTAR with higher-resolution imaging at E > 10 keV.

For detailed studies of individual obscured AGN, the upcoming XARM mission 20 will provide calorimeter observations with exquisite energy resolution to measure the strength and profile of the Fe Kα line, Compton shoulder, and other features to constrain the obscuring geometry. To best take advantage of XARMs capabilities, simultaneous observations at harder (> 10 keV) X-rays (for example with NuSTAR if it is still operational) are required to provide the broad energy coverage for constraining the strength and shape of the Compton-reflected continuum. Finally, we will soon see the emergence of X-ray polarization measurements, first with the IXPE mission 21 and potentially on longer timescales with XIPE 22. Polarization provides a unique capability for studying scattering and reflection, and so can offer new constraints on the geometry of the nuclear regions of obscured AGN (Marin et al., 2018).

6.1.3. Mid-IR. Dramatic breakthroughs in the mid-IR studies of obscured AGN will come from JWST, which will greatly improve the sensitivity of mid-IR spectroscopy (for characterizing optically faint or X-ray faint AGN and identifying extremely weak or buried sources; see Section 2.3.2) and photometry (for identifying obscured AGN in faint, high-redshift galaxies; see Section 2.3.1). For extremely high angular resolution observations, the new MATISSE instrument 23 on the VLT Interferometer (with corresponding NIR imaging from the GRAVITY instrument 24) will enable superior imaging capabilities for follow up imaging studies of the resolved dust emission in nearby AGN (see Section 4.1).

6.1.4. Far-IR–radio. ALMA 25 will continue to make leaps forward in our understanding of obscured AGN; in particular, the development of new submm line diagnostics (e.g., Aalto et al. 2015, Imanishi, Nakanishi & Izumi 2016; See Section 2.4.1) will enable the identification of excitation due to AGN in even the most heavily buried systems, particularly with anticipated improvements in sensitivity with future ALMA upgrades. ALMA will also be key in constraining spatially-resolved properties of the molecular torus such as the outer radius, gas mass, and kinematics, as discussed in Section 4.1. Sensitive AGN diagnostic studies can also be performed with the LMT 26 and CCAT 27, which reach similar depths to ALMA over wider fields of view, although with lower angular resolution. From space, concept far-IR observatories such as OST 28 and SPICA 29 would provide sensitive, high-resolution observations of line emission to explore the connection between obscured AGN activity and star formation, in order to constrain obscuration on galaxy scales and test evolutionary models (Sections 4.3 and 5.1). The most exciting potential for future breakthroughs in the radio band comes from the SKA 30 and its prescursor radio telescopes including LOFAR 31, ASKAP 32, MeerKAT 33, MWA 34, and HERA 35. These observatories are moving toward a dramatic improvement in the sensitivity of deep radio surveys and will eventually allow us to probe, even out to z = 5–6 and beyond, faint radio luminosities for which non-jetted sources dominate the AGN number counts. In combination with deep optical and IR surveys, the SKA will enable vastly deeper and more powerful use of the “radio-excess” method, as well as (along with enhanced capabilities with VLBI) directly resolving compact bright compact cores. These will allow for the detection of AGN that are otherwise obscured or swamped by star formation processes (Section 2.4.2).

6.2. Prospects for theoretical models

Our understanding of obscured AGN will also see progress through advances in theoretical models, both on the scale of the accretion flow and the obscuring torus, and in the context of large-scale cosmological models of galaxy formation.

On small scales, our understanding of AGN obscuration has been improved by advances in hydrodynamical simulations that capture the complex, dynamic, and multi-scale nature of the AGN central engine. Better physical prescriptions for feedback (e.g., Hopkins et al., 2016), chemistry (e.g., Wada, Schartmann & Meijerink, 2016), and radiative transfer (e.g., Jud et al., 2017) all yield a more complete picture of the physical origin and observational signatures of obscuring material in AGN. Complementary to fully hydrodynamic models, there have also been improvements in phenomenological models of AGN tori (for example, including multi-phase gas consisting of both clumpy and smooth components; e.g., Siebenmorgen, Heymann & Efstathiou 2015, Stalevski et al. 2016) and the associated radiative transfer calculations that provide input for interpreting the observed AGN SEDs. The future will see additional connections between hydrodynamic and phenomenological models. For example simulations can provide a framework for realistic ranges of torus opening angles, cloud sizes and optical depths that can then be used to create phenomenological models for constraining the properties of the observed AGN tori.

On the largest scales, we can make use of galaxy formation models, for which one direct and immediate application will be to better understand the strong selection effects involved in identifying obscured AGN. As illustrated in Sections 1 and 2 and Figure 4, selection effects due to obscuration, host dilution, and physical changes in accretion present a complex multi-faced problem, and it can be challenging to reliably “invert” these effects from any given survey to recover the underlying cosmological AGN population. To address these issues, studies increasingly make use of forward modeling that begin with a known underlying galaxy and AGN population, and model their observational signatures in a range of wavebands. By adjusting the input parameters to match the observational data, it is possible to recover (with some assumptions) a reliable measure of the intrinsic AGN population. This approach has been used successfully in synthesis models that produce the CXB and the local SMBH mass function (e.g., Merloni & Heinz, 2008), while also fitting the evolution of the observed XLF. However, in most cases, these models have assumed empirical parameters for the XLF without explicit connection between the AGN and their host galaxies and dark matter halos. Given the importance of host galaxy dilution on AGN selection (see Section 2), and the potentially critical role played by large-scale structures in driving galaxy-SMBH evolution (see Section 5.1), more complete modeling of the AGN population in a cosmological context is warranted. Some studies have included SMBH accretion in cosmological hydrodynamic simulations (e.g., McAlpine et al., 2017, Weinberger et al., 2017), with some success modeling the observed distributions of SMBH mass and AGN luminosity. However, these models necessarily assume simplified sub-grid prescriptions for AGN accretion and feedback (see the discussion in Negri & Volonteri, 2017) and due to the computational expense, they are not able to explore a wide range of parameters for the underlying AGN population. Recently, models have added AGN to simulated galaxies that are drawn either from semi-analytic dark matter and galaxy formation simulations (e.g., Jones et al., 2017), or observed distributions in galaxy mass and star formation history (e.g., Weigel et al., 2017). These prescriptions allow for the flexibility to include in galaxies a population of AGN with wide range of underlying parameters, while also modeling the full multiwavelength properties of the AGN including the emission and/or obscuration from the host galaxy. This modeling yields insights into which AGN are not selected in multiwavelength surveys, and provide useful tools for recovering the full population of obscured AGN.

An ultimate goal for theoretical models of obscured AGN would be to perform a simulation over a dynamic range from the size of the galaxy or dark matter halo (> kpc) down to the accretion disk itself, covering all the relevant scales for gas dynamics and obscuration by gas and dust (Section 4). To date, computational limitations have meant that a full self-consistent treatment of the relevant physics over this range of scales has been out of reach. However, recent studies have performed 3D simulations of feeding, feedback, and obscuration from 0.1 pc to 100 pc scales (Hopkins et al., 2016), while simulations of SMBH feeding and feedback can be now performed on a full galaxy scale, with resolution as small as 3 pc (e.g., Gabor & Bournaud, 2013, Negri & Volonteri, 2017), so a complete treatment across the full range of scales may be on the horizon.

6.3. Outstanding questions

This review has highlighted just a fraction of the exciting observational and theoretical progress that has greatly enhanced our understanding of obscured AGN in recent years. Looking to build on these insights, we conclude with some key outstanding questions:

6.3.1. What is the physical origin of the torus? The structural properties of the AGN dusty torus (such as a clumpy geometry, and broad range of covering factors, and the existence of dust along the poles) are coming into sharper focus (see Section 4.1). However, there remain many possible explanations for its formation, including warped accretion disks, AGN-driven winds, and starburst disks. Uncovering the physical origin (or multiple origins) of the torus is an important goal in coming years.

6.3.2. What is the evolution of AGN obscuration and the connection to galaxy formation? Observations have uncovered hints of evolution with redshift of the fraction of AGN that are obscured (see Section 3), and suggest that some AGN are obscured by evolutionary processes in galaxies (see Section 5.1) that, along with galaxy gas fractions and merger rates, can depend strongly with redshift. A major future objective will be to accurately measure the distribution of AGN obscuration with redshift, luminosity, or Eddington ratio, and to nail down the fraction of obscuration that is produced by galaxy-scale processes as opposed to a nuclear torus. One important clue may come from SMBH mergers identified by the LISA gravitational wave observatory 36, which could place important constraints on galaxy and SMBH merger rates and provide signatures of obscured SMBHs that may not be identifiable from electromagnetic signatures.

6.3.3. How do we find the most heavily obscured AGN? Recent observations have begun to show evidence for a population of extremely heavily obscured AGN with NH ≫ 1025 cm−2. AGN at these levels of obscuration may exhibit few clear observational signatures (see Section 2) but might still contribute significantly to the global growth of SMBHs (see Section 5.3). Some of the most promising techniques to uncover these sources use long-wavelength observations, for which the opacity is the lowest. For example, submm line diagnostics (see Section 2.4.1) or high-resolution detections of radio cores (see Section 2.4.2) may be critical in uncovering this most heavily obscured AGN population.

6.3.4. What is the role of obscured accretion at the dawn of the first SMBHs? The rapid early growth of SMBHs from “seeds” to massive quasar engines is currently an active area of inquiry. Theoretical models, as well as the rapid drop-off in the observed AGN density at high redshift, raise the possibility that a large fraction of the earliest SMBH growth may have been heavily obscured (see Section 5.2). Uncovering this population will be a major motivation for upcoming observatories that seek to understand the ultimate origin of SMBHs.


The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.


R.C.H. acknowledges support from the National Science Foundation through AST grant number 1515364 and CAREER grant 1554584, from NASA through grants NNX15AU32H and NNX16AN48G, and from an Alfred P. Sloan Research Fellowship. D.M.A. acknowledges support from the Science and Technology Facilities Council (STFC) for support through grant ST/L00075X/1. We have made extensive use of NASA’s Astrophysics Data System and the arXiv. We are grateful to Cristina Ramos Almeida, Andrea Merloni, Michael DiPompeo, Lauranne Lanz, and Scientific Editor Luis Ho for valuable suggestions that improved the manuscript. Many of the ideas for the content of this review were developed during the “Hidden Monsters: Obscured AGN and Connections to Galaxy Evolution” workshop at Dartmouth College over August 8–12 2016 and the “Elusive AGN in the Next Era” workshop at George Mason University over June 12–15 2017. We would like to thank the workshop participants for stimulating talks and discussions!

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