The Future Landscape of High-Redshift Galaxy Cluster Science

TL;DR

The paper discusses the future of high-redshift galaxy cluster science, emphasizing multi-wavelength observations, upcoming facilities, and the potential for new insights into galaxy evolution and structure formation during peak cosmic activity.

Contribution

It highlights the importance of multi-wavelength data and future observational facilities for advancing high-redshift galaxy cluster research.

Findings

Expansion of cluster catalogs into high-redshift regimes

Enhanced understanding of thermodynamic structures in early clusters

Improved galaxy evolution and structure formation insights

Abstract

Modern galaxy cluster science is a multi-wavelength endeavor with cornerstones provided by X-ray, optical/IR, mm, and radio measurements. In combination, these observations enable the construction of large, clean, complete cluster catalogs, and provide precise redshifts and robust mass calibration. The complementary nature of these multi-wavelength data dramatically reduces the impact of systematic effects that limit the utility of measurements made in any single waveband. The future of multi-wavelength cluster science is compelling, with cluster catalogs set to expand by orders of magnitude in size, and extend, for the first time, into the high-redshift regime where massive, virialized structures first formed. Unlocking astrophysical and cosmological insight from the coming catalogs will require new observing facilities that combine high spatial and spectral resolution with large collecting areas, as well as concurrent advances in simulation modeling campaigns. Together, future multi-wavelength observations will resolve the thermodynamic structure in and around the first groups and clusters, distinguishing the signals from active and star-forming galaxies, and unveiling the interrelated stories of galaxy evolution and structure formation during the epoch of peak cosmic activity.

Figures

Full text

Astro2020 Science White Paper

The Future Landscape of High-Redshift Galaxy Cluster Science

Thematic Areas: $\bullet$ Cosmology and Fundamental Physics  $\bullet$ Galaxy Evolution

Abstract:

Modern galaxy cluster science is a multi-wavelength endeavor with cornerstones provided by X-ray, optical/IR, mm, and radio measurements. In combination, these observations enable the construction of large, clean, complete cluster catalogs, and provide precise redshifts and robust mass calibration. The complementary nature of these multi-wavelength data dramatically reduces the impact of systematic effects that limit the utility of measurements made in any single waveband. The future of multi-wavelength cluster science is compelling, with cluster catalogs set to expand by orders of magnitude in size, and extend, for the first time, into the high-redshift regime where massive, virialized structures first formed. Unlocking astrophysical and cosmological insight from the coming catalogs will require new observing facilities that combine high spatial and spectral resolution with large collecting areas, as well as concurrent advances in simulation modeling campaigns. Together, future multi-wavelength observations will resolve the thermodynamic structure in and around the first groups and clusters, distinguishing the signals from active and star-forming galaxies, and unveiling the interrelated stories of galaxy evolution and structure formation during the epoch of peak cosmic activity.

Principal Author:

Name: Adam B. Mantz Institution: Kavli Institute for Partical Astrophysics and Cosmology, Stanford University Email: amantz@stanford.edu Phone: +1 650 498 7747

Co-authors: Steven W. Allen1,2,3, Nicholas Battaglia4, Bradford Benson5,6, Rebecca Canning1,3, Stefano Ettori7, August Evrard8, Anja von der Linden9, Michael McDonald10

Endorsers: Muntazir Abidi11, Zeeshan Ahmed2, Mustafa A. Amin12, Behzad Ansarinejad13, Robert Armstrong14, Camille Avestruz5, Carlo Baccigalupi15,16,17, Kevin Bandura18,19, Wayne Barkhouse20, Kaustuv moni Basu21, Chetan Bavdhankar22, Amy N. Bender23, Paolo de Bernardis24,25, Colin Bischoff26, Andrea Biviano27, Lindsey Bleem23,5, Sebastian Bocquet28, J. Richard Bond29, Stefano Borgani27, Julian Borrill30, Dominique Boutigny31, Brenda Frye32, Marcus Brüggen33, Zheng Cai34, John E. Carlstrom35,5,23, Francisco J Castander36, Anthony Challinor37,11,38, Eugene Churazov128,129, Douglas Clowe39, J.D. Cohn40, Johan Comparat41, Asantha Cooray42, William Coulton37,38, Francis-Yan Cyr-Racine43,44, Emanuele Daddi45, Jacques Delabrouille46, Ian Dell’antonio47, Shantanu Desai130, Marcel Demarteau23, Megan Donahue48, Joanna Dunkley49, Stephanie Escoffier50, Tom Essinger-Hileman51, Giulio Fabbian52, Dunja Fabjan27,53, Arya Farahi54, Simon Foreman29, Aurélien A. Fraisse49, Luz Ángela García55, Massimo Gaspari49, Martina Gerbino23, Myriam Gitti56, Vera Gluscevic57, Anthony Gonzalez57, Krzysztof M. G/’orski58, Daniel Gruen3,1, Jon E. Gudmundsson59, Nikhel Gupta60, Tijmen de Haan30, Lars Hernquist61, Ryan Hickox127, Christopher M. Hirata62, Renée Hložek63,64, Tesla Jeltema34,65, Johann Cohen-Tanugi66, Bradley Johnson67, William C. Jones49, Kenji Kadota68, Marc Kamionkowski69, Rishi Khatri70, Theodore Kisner30, Jean-Paul Kneib71, Lloyd Knox72, Ely D. Kovetz73, Elisabeth Krause32, Massimiliano Lattanzi74, Erwin T. Lau75, Michele Liguori76, Lorenze Lovisari61, Axel de la Macorra77, Silvia Masi24,25, Kiyoshi Masui10, Benjamin Maughan78, Sophie Maurogordato79, Jeff McMahon, Brian McNamara80, Peter Melchior49, James Mertens81,82,29, Joel Meyers83, Mehrdad Mirbabayi84, Surhud More85, Pavel Motloch29, John Moustakas86, Tony Mroczkowski87, Suvodip Mukherjee88, Daisuke Nagai89, Johanna Nagy63, Pavel Naselsky, Federico Nati, Laura Newburgh89, Michael D. Niemack4, Andrei Nomerotski90, Emil Noordeh1, Paul Nulsen61, Michelle Ntampaka61,91, Naomi Ota92, Lyman Page49, Antonella Palmese6, Mariana Penna-Lima93, Francesco Piacentini24, Francesco Piacentni24,25, Elena Pierpaoli94, Andrés A. Plazas49, Levon Pogosian95, Etienne Pointecouteau96, Abhishek Prakash97, Gabriel Pratt98, Chanda Prescod-Weinstein99, Clement Pryke100, Giuseppe Puglisi1,3, David Rapetti101,102, Marco Raveri5,35, Christian L. Reichardt60, Thomas H. Reiprich103, Mathieu Remazeilles104, Jason Rhodes58, Marina Ricci79, Graça Rocha, Benjamin Rose105, Eduardo Rozo32, John Ruhl106, Alberto Sadun107, Benjamin Saliwanchik89, Emmanuel Schaan30,108, Robert Schmidt109, Sébastien Fromenteau77, Neelima Sehgal9, Leonardo Senatore3, Hee-Jong Seo39, Mauro Sereno7, Arman Shafieloo110, Huanyuan Shan111, Sarah Shandera112, Blake D. Sherwin11,38, Sara Simon, Srivatsan Sridhar110, Suzanne Staggs49, Daniel Stern58, Aritoki Suzuki30, Yu-Dai Tsai6, Sara Turriziani113, Caterina Umiltà26, Franco Vazza33, Abigail Vieregg35, Alexey Vikhlinin61, Stephen A. Walker51, Lingyu Wang128,129, Scott Watson114, Reinout J. van Weeren115,61, Jochen Weller28, Norbert Werner116,117,118, Nathan Whitehorn119, Ka Wah Wong120, Adam Wright1,3, W. L. K. Wu5, Zhilei Xu121, Siavash Yasini94, Michael Zemcov122, Yuanyuan Zhang6, Gong-Bo Zhao123,124, Yi Zheng125, Ningfeng Zhu121, Irina Zhuravleva35, Joe Zuntz126

1 Stanford University, Stanford, CA 94305

2 SLAC National Accelerator Laboratory, Menlo Park, CA 94025

3 Kavli Institute for Particle Astrophysics and Cosmology, Stanford 94305

4 Cornell University, Ithaca, NY 14853

5 Kavli Institute for Cosmological Physics, Chicago, IL 60637

6 Fermi National Accelerator Laboratory, Batavia, IL 60510

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

8 University of Michigan, Ann Arbor, MI 48109

9 Stony Brook University, Stony Brook, NY 11794

10 Massachusetts Institute of Technology, Cambridge, MA 02139

11 DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge, UK, CB3 0WA

12 Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA

13 Department of Physics, Lower Mountjoy, South Rd, Durham DH1 3LE, United Kingdom

14 Lawrence Livermore National Laboratory, Livermore, CA, 94550

15 SISSA - International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, Italy

16 IFPU - Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy

17 INFN – National Institute for Nuclear Physics, Via Valerio 2, I-34127 Trieste, Italy

18 CSEE, West Virginia University, Morgantown, WV 26505, USA

19 Center for Gravitational Waves and Cosmology, West Virginia University, Morgantown, WV 26505, USA

20 University of North Dakota, Grand Forks, ND 58202

21 University of Bonn, Bonn, Germany

22 National Center for Nuclear Research, Ul.Pasteura 7,Warsaw, Poland

23 HEP Division, Argonne National Laboratory, Lemont, IL 60439, USA

24 Dipartimento di Fisica, Università La Sapienza, P. le A. Moro 2, Roma, Italy

25 Istituto Nazionale di Fisica Nucleare, Sezione di Roma, 00185 Roma, Italy

26 University of Cincinnati, Cincinnati, OH 45221

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

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

29 Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada

30 Lawrence Berkeley National Laboratory, Berkeley, CA 94720

31 Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France

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

33 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

34 University of California at Santa Cruz, Santa Cruz, CA 95064

35 University of Chicago, Chicago, IL 60637

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

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

38 Kavli Institute for Cosmology, Cambridge, UK, CB3 0HA

39 Department of Physics and Astronomy, Ohio University, Clippinger Labs, Athens, OH 45701, USA

40 Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA

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

42 University of California, Irvine, CA 92697

43 Department of Physics, Harvard University, Cambridge, MA 02138, USA

44 University of New Mexico, Albuquerque, NM 87131

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

46 Laboratoire Astroparticule et Cosmologie (APC), CNRS/IN2P3, Université Paris Diderot, 10, rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France

47 Brown University, Providence, RI 02912

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

49 Princeton University, Princeton, NJ 08544

50 Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France

51 Goddard Space Flight Center, Greenbelt, MD 20771 USA

52 Astronomy Centre, School of Mathematical and Physical Sciences, University of Sussex, Brighton BN1 9QH, United Kingdom

53 University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

54 Department of Physics, McWilliams Center for Cosmology, Carnegie Mellon University

55 Swinburne University of Technology, Hawthorn, Victoria 3122, Australia

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

57 University of Florida, Gainesville, FL 32611

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

59 Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, AlbaNova, Stockholm SE-106 91, Sweden

60 School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia

61 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138

62 The Ohio State University, Columbus, OH 43212

63 Dunlap Institute for Astronomy and Astrophysics, University of Toronto, ON, M5S3H4

64 Department of Astronomy and Astrophysics, University of Toronto, ON, M5S3H4

65 University of California at Santa Cruz, Santa Cruz, CA 95064

66 Laboratoire Univers et Particules de Montpellier, Univ. Montpellier and CNRS, 34090 Montpellier, France

67 Columbia University, New York, NY 10027

68 Institute for Basic Science (IBS), Daejeon 34051, Korea

69 Johns Hopkins University, Baltimore, MD 21218

70 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005 India

71 Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

72 University of California at Davis, Davis, CA 95616

73 Department of Physics, Ben-Gurion University, Be’er Sheva 84105, Israel

74 Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, 40122, Italy

75 University of Miami, Coral Gables, FL 33124

76 Dipartimento di Fisica e Astronomia ‘‘G. Galilei’’,Università degli Studi di Padova, via Marzolo 8, I-35131, Padova, Italy

77 IFUNAM - Instituto de Física, Universidad Nacional Autónoma de Mético, 04510 CDMX, México

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

79 Laboratoire Lagrange, UMR 7293, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France

80 Department of Physics and Astronomy, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada

81 Department of Physics and Astronomy, York University, Toronto, Ontario M3J 1P3, Canada

82 Perimeter Institute, Waterloo, Ontario N2L 2Y5, Canada

83 Southern Methodist University, Dallas, TX 75275

84 International Centre for Theoretical Physics, Strada Costiera, 11, I-34151 Trieste, Italy

85 The Inter-University Centre for Astronomy and Astrophysics, Pune, 411007, India

86 Siena College, 515 Loudon Road, Loudonville, NY 12211, USA

87 European Southern Observatory, Garching, Germany

88 Institut d’Astrophysique de Paris (IAP), CNRS & Sorbonne University, Paris, France

89 Department of Physics, Yale University, New Haven, CT 06520

90 Brookhaven National Laboratory, Upton, NY 11973

91 Harvard Data Science Initiative, Harvard University, Cambridge, MA 02138

92 Department of Physics, Nara Women’s University, Kitauoyanishi-machi, Nara, Nara 630-8506, Japan

93 Instituto de Física, Universidade de Brasília, 70919-970, Brasília, DF, Brazil

94 University of Southern California, CA 90089

95 Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

96 IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France

97 California Institute of Technology, Pasadena, CA 91125

98 IRFU, CEA, Université Paris-Saclay, 91191, Gif-Sur-Yvette, France

99 University of New Hampshire, Durham, NH 03824

100 University of Minnesota, Minneapolis, MN 55455

101 University of Colorado, Boulder, CO 80309, USA

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

103 Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

104 Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester, M13 9PL, UK

105 Space Telescope Science Institute, Baltimore, MD 21218

106 Case Western Reserve University, Cleveland, OH 44106

107 University of Colorado, Denver, CO 80204, USA

108 Department of Physics, University of California Berkeley, Berkeley, CA 94720, USA

109 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, D-69120 Heidelberg, Germany

110 Korea Astronomy and Space Science Institute, Daejeon 34055, Korea

111 Shanghai Astronomical Observatory (SHAO), Nandan Road 80, Shanghai 200030, China

112 The Pennsylvania State University, University Park, PA 16802

113 Computational Astrophysics Laboratory -- RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

114 Syracuse University, Syracuse, NY 13244

115 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

116 MTA-Eötvös University Lendület Hot Universe Research Group, Pázmány Péter sétány 1/A, Budapest, 1117, Hungary

117 Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno, 611 37, Czech Republic

118 School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan

119 University of California at Los Angeles, Los Angeles, CA 90095

120 University of Virginia, Charlottesville, VA 22903

121 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

122 Rochester Institute of Technology

123 National Astronomical Observatories, Chinese Academy of Sciences, PR China

124 Institute of Cosmology & Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK

125 School of Physics, Korea Institute for Advanced Study, 85 Hoegiro, Dongdaemun-gu, Seoul 130-722, Korea

126 University of Edinburgh, EH8 9YL Edinburgh, United Kingdom

127 Dartmouth College, Hanover, NH 03755

127 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany

127 Space Research Institute (IKI), Profsoyuznaya 84/32, Moscow 117997,Russia

128 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands

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

130 IIT Hyderabad, Kandi, Telangana 502285, India

1 Introduction

Observations of clusters of galaxies provide a powerful probe of cosmology and astrophysics (Voit 2005, Allen, Evrard & Mantz 2011, Borgani & Kravtsov 2011). Statistical measurements of the evolution of the cluster population over time constrain both the growth of cosmic structure and the expansion history of the Universe. Such observations have played a key role in establishing the current ‘‘concordance’’ model of cosmology, in which the mass-energy budget of the Universe is dominated by dark matter and dark energy, with the latter being consistent with a cosmological constant (e.g. White et al. 1993, Allen et al. 2004, Vikhlinin et al. 2009, Mantz et al. 2010). Clusters are also remarkable astrophysical laboratories, providing unique insights into, e.g., the physics of galaxy evolution (von der Linden et al. 2010) and structure formation (Simionescu et al. 2019, Walker et al. 2019), the role of feedback processes (Fabian 2012, McNamara & Nulsen 2012), the history of metal enrichment (Mernier et al. 2018), the nature of dark matter (Clowe et al. 2006), and the physics of hot, diffuse, magnetized plasmas (Markevitch & Vikhlinin 2007, Brunetti & Jones 2014, van Weeren et al. 2019). Clusters also serve as natural gravitational telescopes with which to observe the most distant reaches of the Universe (Treu et al. 2015).

The key observations enabling robust population studies of galaxy clusters are: a sky survey on which cluster finding can be systematically performed with a clean selection function (below), accurate redshift estimates, robust absolute mass calibration (typically provided by weak lensing measurements), and targeted follow-up observations (especially at X-ray wavelengths) to provide precise centers and relative masses for the clusters, and measurements of their dynamical states.

2 Exploiting multi-wavelength synergies in cluster searches

Galaxy clusters produce observable signals across the electromagnetic spectrum. At X-ray wavelengths, spatially extended bremsstrahlung emission from the hot intracluster medium (ICM) can be clearly identified. In optical and IR data, we can search for overdensities of galaxies, as well as the red colors typical of cluster members. At mm wavelengths, the spectral distortion of the cosmic microwave background (CMB) due to inverse-Compton scattering with the ICM (the Sunyaev-Zel’dovich or SZ effect) provides a nearly redshift-independent way to find clusters.

The primary observation enabling galaxy cluster science is a sky survey on which cluster finding can be systematically performed, ideally over a large sky area and wide range in redshift. While the construction of cluster catalogs in any single waveband can quickly become a frustrating endeavour hampered by systematic limitations, the complementary nature of X-ray, optical and mm-wavelength data provides direct, observational solutions to most issues. X-ray observations, for example, can provide clean, complete catalogs of clusters, as well as multiple low-scatter mass proxies: quantities that are relatively immune to projection effects, correlating tightly with the true, three-dimensional halo mass. The primary disadvantages of X-ray measurements are the need to make them from space (which brings associated cost and risk), the impact of surface brightness dimming (though this is mild at $z>1$; Churazov et al. 2015), and the inability to provide precise absolute mass calibration directly. SZ surveys provide a more uniform selection in redshift, with only their sensitivity determining the mass down to which clusters can in principle be detected. This technique provides our best route for finding clusters at high redshifts, although care is needed to understand the impact on selection of radio- and infrared-emissive cluster galaxies, especially at higher redshifts. Future SZ surveys will also have the ability to provide absolute cluster mass calibration through CMB-cluster gravitational lensing (e.g. Hu et al. 2007). Like X-ray surveys, optical and near-infrared (OIR) surveys are most effective at low-to-intermediate redshifts, but have the benefit of finding larger numbers of clusters down to lower masses. The primary challenges for optical cluster selection are projection effects (which can lead to overestimated richnesses for some clusters) and the relatively complex nature of the intrinsic mass-–observable scaling relations. Nonetheless, optical surveys provide an essential complement to X-ray and SZ data in cluster identification, and uniquely provide essential redshift information (from precise multiband photometry or spectroscopy) and precise absolute mass calibration (through galaxy-cluster lensing). Supporting these observational cornerstones, numerical simulations have emerged as a powerful, complementary tool, providing informative priors on the expected correlations between the measured signals (Stanek et al. 2010, Farahi et al. 2018, Truong et al. 2018).

Figure 1 illustrates the mass-redshift coverage for two of the leading, current cluster surveys, which have been used extensively for both cosmology and astrophysics studies, and the expected reach of a number of projects, most of which are approved and funded (for more detail see Section 3). The figure demonstrates how the forthcoming surveys will vastly increase the size and redshift reach of cluster catalogs, extending out to the epoch when massive clusters first formed and when star formation and AGN activity within them peaked.

Uncovering this distant cluster population is non-trivial. At X-ray wavelengths, it requires an imaging facility with a large collecting area (especially at soft X-ray energies, $<1$ keV) and sufficient spatial resolution to distinguish truly extended emission from the intracluster medium (ICM) from associations of point-like AGN sources. SZ surveys likewise require a combination of sensitivity and spatial resolution to detect clusters, as well as sufficient frequency coverage to spectrally distinguish measurements of the SZ effect from emissive radio and infrared sources (which contaminate the SZ signal at lower and higher frequencies, respectively). To provide both good redshift estimates and accurate shape measurements for a robust weak lensing mass calibration, optical surveys require exquisite photometric calibration and image quality. To extend the reach of optical measurements significantly beyond z\ {\raise-3.22916pt\hbox{\buildrel>\over{\sim}}}\ 1, space-based near IR measurements are needed, with sufficient resolution and depth to appropriately complement the optical data.

3 The Landscape of Approved Projects

A number of facilities that are approved and in construction will contribute substantially to the future of cluster science. Of special note are the new, dedicated survey instruments: eROSITA in X-rays, LSST and Euclid at OIR wavelengths, and several ‘‘Stage 3’’ ground-based mm-wavelength observatories. Also of note is the Athena observatory, which will devote a significant fraction of its observing time to performing a deep X-ray survey of several hundred square degrees.

The German-Russian SRG mission, bearing the eROSITA X-ray survey instrument (Merloni et al. 2012), will launch later in 2019. eROSITA will have 30--50 times the sensitivity of the previous all-sky X-ray survey by ROSAT. Figure 1 shows that the all-sky eROSITA survey is expected to identify essentially all groups out to $z\sim 0.3$, all intermediate mass clusters to $\sim 0.6$, and the most massive clusters at z\ {\raise-3.22916pt\hbox{\buildrel<\over{\sim}}}\ 2. The FoV-averaged spatial resolution of $26^{\prime\prime}$, while an improvement over ROSAT, will be limiting at high redshifts, where the angular extent of clusters is small. Distinguishing ICM and AGN contributions to the emission from faint, modestly extended sources will require follow-up measurements with higher-spatial-resolution X-ray observatories.

LSST will survey the entire southern sky in ugrizy over a 10 year period, beginning in 2022. It will identify clusters down to the group scale, constrain their redshifts photometrically, and provide precise, stacked weak lensing mass measurements out to a redshift of $\sim 1.2$ (LSST Dark Energy Science Collaboration 2012). Note that the redshift limit reflects the redshift at which the 4000 Å break moves out of the reddest band. Combining LSST data with near-IR data from Euclid, an ESA M-class mission scheduled for launch in 2021, will extend the range further. Conversely, while Euclid will identify overdensities of IR-luminous galaxies out to high redshifts (Laureijs et al. 2011), its ability to characterize the cluster population will be enhanced greatly through combination with precise LSST photometry (as well as complementary X-ray and mm observations).

The ‘‘Stage 3’’ CMB (i.e. mm-wavelength) surveys most relevant to cluster science are those by SPT-3G, AdvancedACT (both ongoing), and the planned Simons Observatory and CCAT-prime. Taking advantage of the SZ effect, these surveys will break new ground in providing the first large, robustly selected catalogs of clusters at $z>1.5$, as well as the first informative absolute mass calibration from CMB-cluster lensing. They will find $>3000$ clusters at $z>1$ and $\sim 50$ at $z>2$ (Benson et al. 2014, De Bernardis et al. 2016, The Simons Observatory Collaboration 2018, Stacey et al. 2018). However, few detections are expected above $z\sim 2.3$ (Fig. 1).

Athena, an ESA mission with NASA involvement, will be the next flagship-class X-ray facility (Nandra et al. 2013). Scheduled for launch in 2031, Athena will combine an order of magnitude increase in effective area compared to XMM-Newton, with a smaller $5^{\prime\prime}$ (HPD) PSF on axis, degrading only to $\sim 10^{\prime\prime}$ at $30^{\prime}$ radius. Athena’s grasp significantly exceeds that of any previous X-ray instrument, including eROSITA. Athena will also carry the first large, high-spectral-resolution IFU X-ray calorimeter. With all these advances, we expect to find (Zhang et al. 2019, in prep) and study (Ettori et al. 2013, Pointecouteau et al. 2013) very distant galaxy groups and clusters at z\ {\raise-3.22916pt\hbox{\buildrel>\over{\sim}}}\ 2 over a modest fraction of the sky with Athena, revolutionizing studies of cluster evolution, dynamics, thermodynamics and metal enrichment. However, due to the small size of these objects (typically \ {\raise-3.22916pt\hbox{\buildrel<\over{\sim}}}\ 50^{\prime\prime} in diameter), these studies will rely on spectral modeling to distinguish emission from AGN and the ICM, rather than directly resolving AGN and small-scale structure within clusters.

4 New Opportunities

While the projects described above will undoubtedly transform cluster studies, they are limited in their ability to probe the highest redshifts of interest ($z>2$; due to limited sensitivity and/or sky coverage) and, especially, in their ability to study the astrophysical processes within and around these systems. To do so will require new multi-wavelength facilities with improved sensitivity and enhanced spatial and spectral resolution (Figure 2).

At X-ray wavelengths, the primary requirement is for an observatory with comparable throughput and spectral capabilities to Athena, but an order of magnitude higher spatial resolution ($\sim 0.5^{\prime\prime}$). This would open the door to groundbreaking astrophysical measurements, especially (though not exclusively) in the high-$z$ regime (Figure 1). Recent advances in lightweight, high-resolution, high-throughput X-ray optics have made this goal achievable, as is discussed by the Lynx and AXIS teams (Zhang et al. 2018, The Lynx Team 2018). The ability to spatially resolve and separate AGN within clusters, and to cross-match these sources with ground- and space-based observations in other wavebands, will transform our ability to study how the triggering and quenching of star formation and AGN activity correlates with the evolution of galaxies and their surrounding large scale structure. Resolving the thermodynamic structure and turbulent gas motions within halos, and the distribution of metals within the diffuse cluster gas, will reveal the interwoven stories of galaxy evolution and structure formation, and the roles of feedback from AGN and stars (e.g. Gaspari et al. 2012, McDonald et al. 2018), spanning the epochs when the massive virialized structures first formed and AGN and star formation activity within them peaked.

For surveys at mm wavelengths, the primary requirements are for greater sensitivity and improved spectral coverage. At high redshifts, even the largest clusters formed have modest spatial extent, making sensitivity and sufficient ($\sim 1^{\prime}$) spatial resolution the keys to identifying them through the SZ effect, and to providing precise mass calibration from CMB cluster lensing. Adequate spectral coverage is also crucial to separate the SZ effect from emission due to star formation and AGN activity in cluster member galaxies, which are expected to become increasingly important at high redshifts. Configurations such as those being studied for CMB-S4, using multiple, large-aperture telescopes and large, multichroic detector arrays, appear highly promising (CMB-S4 Collaboration 2016, Madhavacheril et al. 2017). These measurements would also provide precise (percent-level) absolute mass calibration and similarly precise measurements of the mean pressure and density profiles of the hot gas around clusters (out to many virial radii), from the stacked thermal- and kinetic-SZ signals. Follow-up SZ measurements with even higher spatial resolution (\ {\raise-3.22916pt\hbox{\buildrel<\over{\sim}}}\ 10^{\prime\prime}) and/or greater spectral coverage (extending above the SZ null) will be possible with ALMA interferometry or single-dish observatories (using successors to the MUSTANG-2 and NIKA-2 instruments and/or new proposed facilities such as CCAT-prime or AtLAST; Stacey et al. 2018, Mroczkowski et al. 2019). From space, a new survey such as the proposed PICO mission could build on the legacy of WMAP and Planck, providing all-sky coverage from 20–800 GHz (albeit with lower spatial resolution than ground-based telescopes), and producing its own catalog of clusters and protoclusters (Hanany et al. 2019). All these measurements could be complemented by high-spectral resolution X-ray grating spectroscopy of background AGN. Together, these new X-ray and SZ facilities would provide an unprecedented view of the hot, high-redshift Universe.

At OIR wavelengths, WFIRST will provide exquisite data for measuring redshifts and weak lensing of high-$z$ clusters (e.g. Akeson et al. 2019). These capabilities, along with those of LSST and Euclid, should be complemented by high-throughput spectrographs with high-multiplexing capabilities on scales of $\sim 10^{\prime}$. Such instruments would enable detailed studies of the star-formation and AGN properties of cluster galaxies, spanning the period when they transition from being dominated by star-forming systems to being red-sequence-dominated. Comprehensive multi-object spectroscopy will also provide a valuable complement to X-ray measurements for dynamical studies of clusters, and will be vital for calibration of photometric redshifts in cluster fields.

Powerful synergies will also be found at radio wavelengths, where SKA and its precursors (e.g. JVLA, LOFAR, MWA, HERA), working in concert with X-ray facilities, will extend studies of AGN feedback out to the highest redshifts. The detection of radio halos and relics, and the correlation of these signals with the dynamic and thermodynamic structure observed at X-ray, optical and mm wavelengths, will reveal the acceleration of particles during subcluster merger events and provide further insight into the virialization process.

ALMA follow-up will open the door to measurements of molecular gas in high-redshift clusters. At the highest redshifts ($z>4$), observations of dusty, star forming galaxies detected by mm surveys will extend studies of dense environments into the pre-virialized, protocluster regime (e.g. Miller et al. 2018). Finally, combining the most powerful facilities across all wavelengths, we will continue to use clusters as gravitational cosmic telescopes, to probe the earliest phases of galaxy evolution, and the roles of young stars and AGN in the reionization of the Universe.

Extracting science from more sensitive measurements requires concurrent advances in modeling, including simulations designed to map physical models directly to the space of observable features. Empowering the interpretation of new observational capabilities over the coming decade will require large simulated ensembles of massive halos from cosmological volumes, as well as improvements in resolution and new physical treatments.