Cosmic evolution of supermassive black holes: A view into the next two decades

TL;DR

This paper reviews the progress in understanding supermassive black hole evolution over the past two decades and discusses future prospects with upcoming observatories to explore SMBH formation, growth, and their role in galaxy evolution.

Contribution

It highlights the advancements made by X-ray surveys and outlines the key scientific questions and observational goals for the next two decades in SMBH research.

Findings

Significant progress in SMBH and galaxy connection understanding.

Future observatories will probe SMBH infancy at high redshifts.

Anticipated insights into obscured and low-luminosity AGNs.

Abstract

The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at $z>6$) and investigate the influence of their accretion on the formation of the first galaxies ($\S$ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at $z\sim$1-6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys ($\S$2.2 and $\S$2.3), enabling us to resolve the connection between SMBH growth and their environment.

Figures

Full text

Astro2020 Science White Paper

Cosmic evolution of supermassive black holes: A view into the next two decades

Thematic Areas:                    $\square$ Planetary Systems     $\square$ Star and Planet Formation       $\boxtimes$ Formation and Evolution of Compact Objects            $\boxtimes$ Cosmology and Fundamental Physics $\square$ Stars and Stellar Evolution   $\square$ Resolved Stellar Populations and their Environments              $\boxtimes$ Galaxy Evolution                $\square$ Multi-Messenger Astronomy and Astrophysics

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**Principal Authors:

**Francesca Civano1 (E-mail: fcivano@cfa.harvard.edu; Tel: +1 617-792-3190), Nico Cappelluti2, Ryan Hickox3, Rebecca Canning4,5

Co-authors:

James Aird 6, Marco Ajello 7, Steve Allen 4,5, Eduardo Bañados 8, Laura Blecha 9, William N. Brandt 10, Marcella Brusa 11, Francisco Carrera 12, Massimo Cappi 13, Andrea Comastri 13, Klaus Dolag 14,15, Megan Donahue 16, Martin Elvis 1, Giuseppina Fabbiano 1, Francesca Fornasini 1, Poshak Gandhi 17, Antonis Georgakakis 18, Kelly Holley-Bockelmann 19, Anton Koekemoer 20, Andrew Goulding 21, Mackenzie Jones 1, Sibasish Laha 22, Stephanie LaMassa 20, Giorgio Lanzuisi 13, Lauranne Lanz 3, Adam Mantz 4,5, Stefano Marchesi 7, Mar Mezcua 23, Beatriz Mingo 24, Kirpal Nandra 18, Daniel Stern 25,26, Doug Swartz 27, Grant Tremblay 1, Panayiotis Tzanavaris 28, Alexey Vikhlinin 1, Fabio Vito 29,30, Belinda Wilkes 1

**Abstract:

**The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at $z>6$) and investigate the influence of their accretion on the formation of the first galaxies ($\S$ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at $z\sim$1–6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys ($\S$2.2 and $\S$2.3), enabling us to resolve the connection between SMBH growth and their environment.

Endorsers:

Scott Anderson 31, Vallia Antoniou 1,32, Manuel Aravena 33, Ashraf Ayubinia 34, Mitchell Begelman 35, Arash Bodaghee 36, Tamara Bogdanović 37, Rozenn Boissay-Malaquin 38, Angela Bongiorno 39, Peter Boorman 17, Stefano Borgani 40, Richard Bower 41, Laura Brenneman 1, Volker Bromm 42, Esra Bulbul 1, David Burrows 10, Claude Canizares 43, Chien-Ting Chen 27, Igor V. Chilingarian 1, Damian J. Christian 44, Benedetta Ciardi 15, Alison Coil 22, Thomas Connor 45, Paolo Coppi 46, Darren Croton 47, Akaxia Cruz 31, Raffaele DÁbrusco 1, Filippo D’Ammando 13, Romeel Davé 48, Gisella De Rosa 20, Ivan Delvecchio 49, Casey DeRoo 50, Kelly A. Douglass 51, Abraham Falcone 10, Xiaohui Fan 52, Claude-André Faucher-Giguère 53, Gary Ferland 54, K. E. Saavik Ford 55, William Forman 1, Antonella Fruscione 1, Keigo Fukumura 56, Massimiliano Galeazzi 2, David Garofalo 57, Massimo Gaspari 21, Lisseth Gavilan Marin 58, Jonathan Gelbord 59, Vittorio Ghirardini 1, Roberto Gilli 13, Omaira González Martín 60, Dale Graessle 1, Gian Luigi Granato 40, Paul J. Green 1, Richard Griffiths 61, Melanie Habouzit 62, Martin Haehnelt 63, Daryl Haggard 64, Nimish Hathi 20, Jeffrey Hazboun 65, Ralf K. Heilmann 43, Lars Hernquist 1, Julie Hlavacek-Larrondo 66, Phil Hopkins 25, David James 1, Manasvita Joshi 67, Philip Kaaret 50, Elias Kammoun 68, Nikita Kamraj 25, Margarita Karovska 1, Demosthenes Kazanas 28, Ildar Khabibullin 15,69, Dong-Woo Kim 1, Juna A. Kollmeier 45, Albert Kong 70, Alvaro Labiano 71, Claudia Lagos 72, George Lansbury 63, Erwin Lau 2, Denis Leahy 73, Bret Lehmer 74, Laura Lopez 75, Lorenzo Lovisari 1, Ray A. Lucas 20, Chelsea MacLeod 1, Kristin K. Madsen 25, W. Peter Maksym 1, Alberto Masini 3, Barry McKernan 55, Takamitsu Miyaji 76, Richard Mushotzky 77, Adam Myers 78, Qingling Ni 10, Joy Nichols 1, Kenichi Nishikawa 79, Emil Noordeh 4,5, Paul Nulsen 1, Anna Ogorzalek 4,5, Feryal Ozel 52, Fabio Pacucci 80, Frits Paerels 81, Antonella Palmese 82, Aaron B. Pearlman 25, Eric Perlman 83, Richard M. Plotkin 84, Stephanie Podjed 3, Meredith Powell 46, Francis A. Primini 1, Andrew Ptak 28, Mitchell Revalski 36, David Rosario 41, Bassem Sabra 85, Dhaka Sapkota 2, Gerrit Schellenberger 1, Dominic Sicilian 2, Lorenzo Sironi 81, Patrick Slane 1, Benny Trakhtenbrot 86, Virginia Trimble 87, Sara Turriziani 88, Reinout van Weeren 89, Cristian Vignali 11, Robert V. Wagoner 4,5, Devin Walker 3, Junxian Wang 34, Norbert Werner 90, Lacey West 74, J. Craig Wheeler 42, Daniel Wik 91, Benjamin Williams 31, Diana Worrall 92, Wei Yan 3, Guang Yang 10

1 Center for Astrophysics $|$ Harvard & Smithsonian, Cambridge, MA 02138

2 University of Miami, Coral Gables, FL 33124, USA

3 Department of Physics & Astronomy, Dartmouth College, Hanover, NH 03755, USA

4 Stanford University, Stanford, CA 94305, USA

5 Kavli Institute for Particle Astrophysics and Cosmology, Stanford 94305, USA

6 University of Leicester, University Rd, Leicester LE1 7RH, UK

7 Clemson University, Clemson, South Carolina, USA

8 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany

9 University of Florida, Gainesville, FL 32611, USA

10 The Pennsylvania State University, University Park, PA 16802, USA

11 Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy

12 Instituto de Fisica de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain

13 INAF - Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, I-40129 Bologna, Italy

14 Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany

15 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany

16 Michigan State University, East Lansing, MI 48824-2320, USA

17 Department of Physics & Astronomy, Faculty of Physical Sciences and Engineering, University of Southampton, Southampton, SO17 1BJ, UK

18 Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, D-85748 Garching bei München, Germany

19 Physics & Astronomy Department, Vanderbilt University, PMB 401807, 2301 Vanderbilt Place, Nashville, TN 37235, USA

20 Space Telescope Science Institute, Baltimore, MD 21218, USA

21 Princeton University, Princeton, NJ 08544, USA

22 University of California San Diego, La Jolla, CA 92093, USA

23 Institute of Space Sciences (IEEC-CSIC) Campus UAB, Carrer de Can Magrans, s/n 08193 Barcelona, Spain

24 The Open University, Milton Keynes, UK

25 California Institute of Technology, Pasadena, CA 91125, USA

26 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

27 Marshall Space Flight Center, Huntsville, Alabama, USA

28 Goddard Space Flight Center, Greenbelt, MD 20771 USA

29 Pontifical Catholic University of Chile, Instituto de Astrofísica, Santiago, Chile

30 Chinese Academy of Sciences South American Center for Astronomy, China

31 University of Washington, Seattle 98195, USA

32 Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA

33 Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago, Chile

34 University of Science and Technology of China, 1129 Huizhou Ave, Baohe Qu, Hefei Shi, Anhui Sheng, China

35 University of Colorado, Boulder, CO 80309, USA

36 Georgia State University, Atlanta, GA 30302, USA

37 Georgia Institute of Technology, North Ave NW, Atlanta, GA 30332, USA

38 MIT Kavli Institute for Astrophysics and Space Research. Massachusetts Institute of Technology. 77 Massachusetts Avenue, 37-241. Cambridge, MA 02139, USA

39 INAF Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone, Italy

40 INAF - Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy

41 Department of Physics, Lower Mountjoy, South Rd, Durham DH1 3LE, UK

42 University of Texas at Autin, Texas, USA

43 Massachusetts Institute of Technology, Cambridge, MA 02139, USA

44 California State University Northridge, 18111 Nordhoff St, Northridge, CA 91330, USA

45 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA

46 Department of Physics, Yale University, New Haven, CT 06520, USA

47 Swinburne University of Technology, John St, Hawthorn VIC 3122, Australia

48 University of Edinburgh, EH8 9YL Edinburgh, UK

49 Service d’Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif-sur-Yvette, France

50 University of Iowa, Iowa City, IA 52242, USA

51 Department of Physics and Astronomy, University of Rochester, 500 Joseph C. Wilson Boulevard, Rochester, NY 14627, USA

52 Department of Astronomy/Steward Observatory, University of Arizona, Tucson, AZ 85721, USA

53 Northwestern University, Evanston, IL 60201, USA

54 University of Kentucky, Lexington, KY 40506, USA

55 Borough of Manhattan Community College, The City University of New York, 199 Chambers Street, New York, NY 10007

56 James Madison University, 800 S Main St, Harrisonburg, VA 22807, USA

57 Kennesaw State University, Kennesaw, Georgia, USA

58 NASA Ames Research Center, Moffett Field, CA 94035, USA

59 Spectral Sciences, Inc.

60 Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, México

61 University of Hawaii, Honolulu, HI 96822, USA

62 Center for Computational Astrophysics, 162 5th Ave, 10010, New York, NY, USA

63 Institute of Astronomy, University of Cambridge,Cambridge CB3 0HA, UK

64 McGill University, Montreal, QC H3A 2T8, Canada

65 University of Washington, Bothell, WA 98011, USA

66 Université de Montréal, Montréal, Quebec, Canada

67 Boston University, Boston, MA 02215, USA

68 University of Michigan, Ann Arbor, MI 48109, USA

69 Space Research Institute, Profsoyuznaya 84/32, Moscow 117997, Russia

70 Institute of Astronomy, National Tsing Hua University, Taiwan

71 Centro de Astrobiología (CAB, CSIC-INTA), 28850 Torrejón de Ardoz, Spain

72 International Centre for Radio Astronomy Research, University of Western Australia, 7 Fairway, Crawley, 6009, WA, Australia

73 University of Calgary, 2500 University Drive NW Calgary, AB T2N 1N4 Canada

74 University of Arkansas, Fayetteville, AR 72701

75 The Ohio State University, Columbus, OH 43212, USA

76 Instituto de Astronomía sede Ensenada, Universidad Nacional Autónoma de México, México

77 University of Maryland, College Park, MD 20742, USA

78 Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA

79 Department of Physics, University of Alabama in Huntsville, ZP12, Huntsville, AL 35899, USA

80 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands

81 Columbia University, New York, NY 10027, USA

82 Fermi National Accelerator Laboratory, Batavia, IL 60510, USA

83 Florida Institute of Technology, 150 W University Blvd, Melbourne, FL 32901, USA

84 University of Nevada, Reno, NV 89557, USA

85 Dept of Physics & Astronomy, Notre Dame University–Louaize, Lebanon

86 Tel-Aviv University, 69978 Tel-Aviv, Israel

87 University of California, Irvine, CA 92697, USA

88 Computational Astrophysics Laboratory – RIKEN, 2-1 Hirosawa, Wako, Saitama, Japan

89 Lorentz Institute, Leiden University, Niels Bohrweg 2,Leiden, NL 2333 CA, The Netherlands

90 MTA-Eötvös Loránd University, Hungary/Masaryk University, Czech Republic

91 University of Utah, Department of Physics and Astronomy, 115 S 1400 E, Salt Lake City, UT 84112, USA

92 University of Bristol, Tyndall Ave, Bristol BS8 1TL, UK

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1 Introduction

The past two decades have revealed strong links between the cosmic evolution of supermassive black holes (SMBHs), their host galaxies, and dark matter halos. In the local Universe, SMBH masses are tightly correlated with their host-galaxy bulge luminosity and stellar velocity dispersion (e.g., Kormendy & Ho 2013), while at higher redshifts, the global density of star formation and SMBH growth follow a common trend, rising from the epoch of reionization to $z\sim$ 2–3, then steeply declining to the current epoch. Feedback from growing SMBHs as active galactic nuclei (AGNs; see Laha et al. 2019, Tombesi et al. 2019) is critical for reproducing the observed populations of galaxies and their hot halos in models (e.g., McAlpine et al. 2018, Weinberger et al. 2018). Despite this progress, the nature of SMBH fueling and the interplay between SMBHs and their hosts remain unclear, particularly before the peak epoch of structure formation.

To uncover the co-evolution of SMBHs and galaxies, we must probe the full scope of cosmic time to detect and characterize large samples of AGNs and their hosts, including sources that are heavily obscured by gas and dust (e.g., Hickox & Alexander 2018). While AGNs can be identified and studied across a range of wavelengths (e.g., Padovani et al. 2017), a particularly powerful waveband for AGN selection is in the X-rays. At X-ray energies, the contamination from non-nuclear emission, mainly due to star formation processes, can be far less significant than in broad-band optical and IR observations. This white paper will focus on X-ray observations.

Over the past 20 years, Chandra and XMM-Newton X-ray surveys have successfully collected a clean and largely unbiased sample of AGNs ($\sim$60,000) up to $z=5$ (e.g., Brusa et al. 2010, Nandra et al. 2015, LaMassa et al. 2016, Civano et al. 2016, Luo et al. 2017, Chen et al. 2018, Masini et al. in prep.). These samples cover a broad range of luminosities (down to $L_{X}\sim 10^{44}$ erg s*-1* at $z=5$ and $L_{X}=10^{39}$ erg s*-1* at $z=0.5$; see Fig. 1), corresponding to BH masses of $10^{6}-10^{9}$ $M_{\odot}$, and include large numbers of both unobscured and obscured AGNs. In concert with multiwavelength data, X-ray observations have made a number of key discoveries: (1) massive ($>$109 M$\odot$) accreting SMBHs already existed at $z\sim$ 7.5, challenging seed formation models; (2) AGN number densities peak at $z\sim$ 2–3, with luminous sources peaking earlier in cosmic time and with most of the accretion being obscured; (3) correlations exist between SMBH accretion and galaxy properties, particularly with stellar/dark matter halo mass and (perhaps indirectly) star formation, although due to AGN stochasticity these can only be revealed in statistical samples.

These discoveries have opened up several exciting questions that are beyond the capabilities of current facilities and will need to be addressed by new observatories into the next two decades:

• •

How did the first SMBHs form and grow so rapidly in the early Universe?

• •

What is the complete census of growing SMBHs from cosmic dawn ($\bm{z\sim}$6) to the peak formation epoch ($\bm{z\sim}$2) and beyond?

• •

How does SMBH accretion influence the growth of galaxies and large-scale structures?

In this white paper, we will discuss these questions and also highlight how some future missions, such as ESA’s Athena (Nandra et al. 2013), and proposed concept missions, such as Lynx (The Lynx team 2018) and AXIS (Mushotzky et al. 2018), will help us address them.

2 Understanding the build up of SMBHs and galaxies

2.1 Black holes and galaxies at cosmic dawn

The ultimate origin of SMBHs is still unknown and remains one of the most pressing open questions in astrophysics. Optical and NIR surveys have revealed the existence of SMBHs with masses of 109 M*⊙* up to $z\sim$ 7.5 when the Universe was only 600 Myr old (e.g., Mortlock et al. 2011, Bañados et al. 2018). This discovery lead to the proposal of several BH seed formation models, including: (1) direct collapse of early pristine massive clouds (Begelman et al. 2006); (2) Pop III star remnants (Bromm & Loeb 2006); (3) coalescence of stellar mass BHs in star clusters (e.g., Devecchi & Volonteri 2009); and (4) primordial BHs formed during inflation (Hawking 1971).

A complete understanding of SMBH origins and their contributions to reionization demands both the census of massive, luminous AGNs at $z>6$ as well as direct measurements of the lower-mass seeds, forming and growing at $z=$ 10–15. The first can be observed by surveying wide areas (tens of deg2) at the equivalent of the faintest Chandra flux ($\sim 10^{-17}$ erg s*-1* cm*-2*), e.g. with the proposed Athena-WFI survey strategy (as in Fig. 1; Aird et al. 2013). The second requires extreme X-ray detection limits ($10^{-19}$ erg s*-1* cm*-2*), implying an observatory with both large effective area and sub-arcsecond angular resolution, to avoid source confusion, such as e.g. Lynx and AXIS.

A major challenge will be obtaining rest-frame optical counterparts for these X-ray sources, since these will be extremely faint at all wavelengths (e.g., Volonteri et al. 2017). In Fig. 2, we compare the limiting BH masses detectable in X-ray surveys with a flux limit range from $10^{-17}$ erg s*-1* cm*-2*, the Athena flux limit, to $10^{-19}$ erg s*-1* cm*-2*, the ultimate depth reachable with Lynx. AXIS, with comparable angular resolution to Lynx but a smaller effective area, will reach a depth between these two limits. Fig. 2 shows that the combination of Athena and WFIRST will provide X-ray and NIR information for growing seeds at $z\sim$ 5–8 and Lynx (or AXIS) will peer into the early Universe to $z\geq$ 10 and at lower mass, in concert with JWST.

While predictions for the number density of early BH seeds are still uncertain (Haiman et al. 2019), it is clear that combining a deep X-ray survey with sub-arcsecond positional accuracy with JWST and WFIRST data will return a sample of tens to thousands of X-ray selected SMBH seeds with confirmed counterparts (Fig. 1). With these samples, it will be possible to probe, for the first time, the luminosity function down to $L_{X}=10^{43}$ erg s*-1* at $z\sim$ 15, corresponding to $10^{4-5}$ M⊙ BHs. Progress will also be made by studying the fluctuations of the X-ray background and its counterparts (see e.g., Kashlinsky et al. 2018, Cappelluti et al. 2013, 2017), but the strongest constraints will come from direct detections of lower-mass seed BHs in the “first stars” epoch.

2.2 The full population of black holes from cosmic dawn to cosmic noon and the connection with the host galaxy

Beyond the origin of SMBHs, studying their growth from “cosmic dawn” at $z>$6 to “cosmic noon” at $z\sim$1–2 is essential for understanding their co-evolution with galaxies, in the epochs when the bulk of the Universe’s stars were formed. A recent breakthrough in our understanding of AGNs shows that they are not like light bulbs but instead “flicker” across a wide range of Eddington ratios on relatively short time scales ($<10^{5-6}$ years; Hickox et al. 2014). Thus, to understand the full AGN population, the full dynamic range in accretion rates (and thus the full luminosity function) needs to be probed across all of cosmic time. This approach has been critical for understanding the connection between BH and galaxy growth, the nature of AGN obscuration and the evolution of AGNs in large-scale structures (e.g., Chen et al. 2013, Yang et al. 2017, Powell et al. 2018).

Observations across a wide range of wavelengths are essential for this work. Optical and IR imaging and spectroscopy can detect luminous AGNs outshining their host galaxies and can characterize the hosts of weak or obscured AGNs, while far-IR–mm observations are critical for constraining star formation and dust masses. However, due to high penetrating power and contrast with host galaxy light, X-ray observations are required for probing the full range of SMBH masses and Eddington ratios. The most recent measurements of the luminosity function probe the X-ray luminosity range $10^{42}$–$10^{43}$ erg s*-1* to $z=$ 2 and $10^{45}$–$10^{46}$ erg s*-1* to $z=$ 5 (e.g., Ueda et al. 2014, Miyaji et al. 2015, Aird et al. 2015, Tasnim Ananna et al. 2019). In the next few years, eROSITA (Merloni et al. 2012) will constrain the bright end of the luminosity function ($>10^{45}$ erg s*-1*) with $\sim 10^{6}$ AGNs over a broad redshift range (Fig. 1).

Furthermore, it is critical to probe deeper, below the “knee” of the luminosity function at the earliest epochs ($z>$ 3), requiring large numbers of very faint sources (many thousands compared to the few hundreds available now; see Marchesi et al. 2016, Vito et al. 2018). The first step will be to cover large areas down to the flux limits of the current deepest X-ray survey (the Chandra Deep Field-South). This will be feasible with a large effective area telescope (e.g., Athena), identifying tens of thousands of distant AGNs over tens of deg2, to a flux limit currently available only in pencil-beam (0.1 deg2) fields (Fig. 1). The next step will be to probe to significantly fainter fluxes ($\sim$ 10*-19* erg s*-1* cm*-2*), constraining the X-ray luminosity and Eddington ratio distributions at least an order of magnitude fainter. Such deep flux limits require both high effective area and angular resolution (e.g., as proposed for Lynx; Fig. 1). As a complementary approach, it will be possible to use X-ray stacking techniques with high-resolution X-ray data (e.g., Vito et al. 2016, Fornasini et al. 2018) to push to even fainter average fluxes ($\sim$ 10*-20* erg s*-1* cm*-2*).

Counterpart identifications will be readily available by combining data from next generation space facilities (JWST, WFIRST, Euclid) as well as ground based 30m telescopes (ELT, GMT, TMT), allowing us to explore the distribution of AGN accretion rates as a function of key host properties (i.e., star formation rate, stellar mass, morphology), and perform statistical analyses of clustering to probe the connection between AGN and their host environment (e.g., Mendez et al. 2016, Allevato et al. 2016, Plionis et al. 2018, Yang et al. 2018).

Finally, reaching the very faintest luminosity limits, we will probe down into the galaxy population with emission dominated by X-ray binaries and hot gas (L${}_{X}<10^{41}$ erg s*-1*) as well as group and cluster-sized halos at $z\sim$ 2. High spatial resolution observatories can distinguish point sources above this diffuse background, enabling simultaneous measurements of AGN properties and a direct measure of how AGNs populate large-scale structures (see inset in Fig. 1 and Mantz et al. 2019). Following up halos detected with CMBs-4 and Athena, Lynx will build on the legacy of Chandra and will map the triggering of SMBHs in the build-up of large scale structure out to $z\sim$ 4. In lower-redshift systems, high-resolution X-ray imaging and spectrocopy will directly probe the effects of AGN feedback on the hot gas atmospheres in their host galaxies.

2.3 Uncovering the “hidden” black hole population

A significant number of AGNs are “hidden” by dust that only X-ray light can penetrate. The majority of this population are obscured by a dusty “torus” in close proximity to the SMBH, but dust at larger radii in the host galaxy can also shield the AGN from view.

At low-redshifts, around a third of the AGN population are known to be obscured even to hard X-rays (e.g., Risaliti et al. 1999). Dedicated observing campaigns have been focusing on some of the extreme cases (e.g., NGC 1068 with $N_{H}\sim 10^{25}$ cm*-2*; e.g. Bauer et al. 2015; see Fig.3). However, little is known about these sources at higher redshifts: only a few 100 candidates have been detected thus far (e.g., Brightman et al. 2014, Lanzuisi et al. 2018) and the evolution of the obscured fraction is poorly constrained (e.g., Liu et al. 2017, Zappacosta et al. 2018). Pioneering work by NuSTAR, operating above 20 keV, is now uncovering some of these concealed sources (Civano et al. 2015, Lansbury et al. 2017, Masini et al. 2018) but still with low numbers of photons. Theoretical models predict phases of high obscuration during gas-rich galaxy mergers when feedback from AGN may play an important role in regulating galaxy growth (e.g., Hopkins et al. 2005, Blecha et al. 2018). It is thus critical to uncover and properly model these hidden AGNs, even at the highest redshifts, to understand the connection between the obscured growth of SMBHs, their host galaxies, and the connection with mergers (which can in turn be probed directly with future space-based gravitational wave observatories, e.g. LISA).

Reprocessed IR emission offers one way of detecting such AGNs (e.g., Stern et al. 2005, Alexander et al. 2008, Gandhi et al. 2009, Goulding et al. 2011). Unfortunately, this suffers from both contamination and dilution by the host galaxy. High sensitivity and spatial resolution NIR observations with JWST and WFIRST will begin to uncover these low-to-medium redshift AGNs buried within their host emission. We can directly observe reprocessed AGN emission in the high-energy X-rays, via the ‘Compton hump’ at 20 – 40 keV and a ubiquitous iron emission line at 6.4 keV. At z$>$2, the Compton hump is redshifted to low-energy X-rays, making the complex spectrum of these sources accessible by soft X-ray telescopes (Fig. 3).

Probe missions in the soft and hard X-rays (e.g., STROBE-X and HEX-P, see Hickox et al. 2019) will inform us about the most luminous obscured AGNs out to $\sim$2. Only the leap in sensitivity over Chandra and XMM-Newton combined with a wide field of view, e.g. such as with Athena, will push the census of heavily obscured AGNs beyond the peak epoch of activity out to $z=$4. We will be able to perform unprecedented spectral modeling, even at high redshifts, uncovering the physics of extremely obscured AGN, at levels currently available only for local sources (Fig. 3; e.g., Georgakakis et al. 2013). At even higher redshifts, the combination of effective area and spatial resolution will enable a mission such as Lynx to both uncover the least luminous of these hidden SMBHs and move toward a complete census of SMBH growth across cosmic time.

3 Our vision for an extragalactic survey program into the 2030s

This white paper outlines a very exciting range of science that can be completed with an extragalactic survey program in the next two decades, demanding a substantial leap forward in observing capabilities across a wide range of wavelengths. A full picture of SMBH growth and galaxy evolution requires a tiered, coordinated survey of all components: starlight in the rest-frame visible and NIR (planned or proposed facilities include WFIRST, JWST, LSST, HabEx, LUVOIR), gas and dust in the mid- and far-IR, microwaves, radio as well as sensitive optical/NIR spectroscopy and X-ray imaging (JWST, OST, ALMA, ngVLA, SKA, CMB-S4, 30m class telescopes, XRISM and Athena), and the signatures of SMBH growth, most powerfully probed in X-rays, IR, radio, and GW (Athena, Lynx, AXIS, JWST, OST, SKA, LISA). X-ray observations will produce the biggest breakthroughs in uncovering the complete AGN population. In the next 5 years, Chandra Source Catalog 2 and eROSITA AGNs will be spectroscopically characterized by SDSS-V, 4MOST and DESI. Afterwards, Athena will provide a huge increase in effective area and field of view, allowing us to reach the current faintest X-ray flux levels in the deepest Chandra observations but over a vastly larger area (tens of deg2). Lynx would then provide another enormous leap forward with exquisite angular resolution, allowing us to push more than an order of magnitude fainter and directly resolve and study the first BHs in the early Universe. (AXIS would make a comparable leap in resolution but with a more modest yield in high-redshift AGNs.) We urge the Astro2020 Decadal Survey Committee to consider the compelling astrophysical questions about the cosmic evolution of SMBH growth outlined here and endorse a plan to address them in the next two decades.

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