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Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2021)

Chapter: Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground

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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix K: Report of the Panel on Optical and Infrared Observations from the Ground." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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K Report of the Panel on Optical and Infrared Observations from the Ground EXECUTIVE SUMMARY We face a future filled with extraordinary opportunities. New ground-based Optical-Infra-Red (OIR) observational facilities are central to addressing the most pressing and fundamental questions in astronomy and astrophysics, as assessed by the six Science Panels of the Astro2020 decadal survey.1 The importance of some of these questions transcends the boundaries of science: How did we get here? Are we alone? To exploit these opportunities, the United States—which has led the world for decades in ground-based OIR astronomy—must overcome some enormous challenges. First, to maintain a leadership position in the 2030s and beyond, investments of an unprecedented scale by NSF in ground-based OIR astronomy will be required. The panel has carefully evaluated the proposal to Astro2020 to create and fund a unified U.S. Extremely Large Telescope Program (U.S.- ELTP) that will combine the resources and capabilities of NSF’s NOIRLab, the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT). Combined with Key Science Programs facilitated by NSF’s National Optical-Infrared Research Laboratory (NOIRLab), this U.S.-ELTP will create a system for the broad U.S. community that is fully competitive with, and complementary to, the European Extremely Large Telescope (E-ELT), and one that maximizes synergies with the current U.S. multi-billion-dollar bi-hemispheric system of ground-based astronomical facilities. The programmatic challenges facing the U.S.-ELTP are daunting indeed, and it is not at all clear that there will be adequate financial capacity to complete the construction and fund the operations of this two-telescope system. The panel has reached the consensus that the rewards of a successful outcome are high enough for NSF and the other GMT and TMT partners to be given the opportunity to try to achieve this success. The panel believe a zero-ELT outcome will gravely damage the U.S. astronomy community for decades. The second challenge is posed by the need to exploit the immense investment that has already been made in the past 30 years to create a powerful and flexible U.S. ground-based OIR system. This is particularly pressing because the scientific payout from a U.S.-ELTP is over a decade away. The panel has reviewed fifty thoughtful white papers from the ground-based OIR community, and has identified a set of thematic areas in which relatively modest investments (e.g., at or below the level of the NSF Mid- scale Research Infrastructure-2 (MSRI-2) program) in existing telescopes could reap major returns during the 2020s. The panel also highlights several opportunities in this medium-scale range to build new special-purpose telescopes or telescope arrays. In some cases, these could be interagency projects (e.g., NSF and NASA, or NSF and DOE). In other cases, they could be in the context of an international partnership. Last, the panel emphasizes the importance of modest strategic investments in technology development and software, and in the further development of the systems-level approach to optimizing the performance of the OIR system in an era of time-domain/multi-messenger astrophysics. For this plan to succeed, there needs to be a fundamental change in the way in which the federal, state, and private funding sources for ground-based OIR facilities interact. A broad partnership is 1 See Appendix A for the overall Astro2020 statement of task, for the set of panel descriptions that define the panels’ tasks, and for additional instructions given to the panels by the steering committee. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-1

necessary for the United States to maintain leadership. NSF will need a major boost in the Major Research Equipment and Facilities Construction Funding (MREFC) line, a robust MSRI program, and a new model for how operations of new facilities are paid for. If we can accomplish all of this, we can fully reap the extraordinary scientific harvests for decades to come. K.1 INTRODUCTION K.1.1 Setting the Stage It is axiomatic that major scientific discoveries are driven by new technology, and in no field is this clearer than in astronomy. Galileo Galilei did not invent the telescope, but he was the first to use it to observe the sky and record his discoveries. His book The Starry Messenger (1610) reported on his observations of the Moon, Jupiter, and the Milky Way. These observations revolutionized our understanding of the cosmos and ushered in centuries of discoveries to come based on ever-more- powerful telescopes. The era of astrophysics can be said to have begun roughly a century ago, launched by the construction of large telescopes armed with spectrographs. For many decades, the United States was the unrivaled leader in the construction and utilization of such facilities, from the 100" at Mount Wilson (1917) to the 200" at Mount Palomar (1948). This U.S. leadership was made possible largely through an unmatched level of philanthropy. Such days are over. Starting in the late 1960s the level of funding provided by the U.S. federal government and of other nations produced a suite of telescopes that matched the capabilities of the largest private/state-funded facilities. This situation has continued into the current era of very large telescopes, which started to come into operation in 1990s. The importance of the next generation Extremely Large Telescopes (ELTs) has been recognized for at least 20 years, and indeed an ELT was the top ground- based recommendation of the 2000 decadal survey. Yet, as we survey the landscape today, we see that the scale of investment needed to construct and operate the next generation of Extremely Large Telescopes (ELTs) is severely straining the financial model that has served the U.S. astronomical community so well for over a century (which has largely segregated private/state-, and federally funded telescopes). Initially two competing ELT projects with major U.S. involvement emerged: the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT). Since the previous decadal survey, New Worlds, New Horizons (NWNH), there has been an enormous amount of work done to retire the most challenging technical risks to the construction of GMT and TMT. These two projects have now joined forces with NSF’s NOIRLab to propose a unified U.S.-ELT program seeking substantial federal funding for, and providing access to both ELTs by the U.S. community. In addition, two major new ground-based OIR telescopes have just seen (or soon will see) first light. The 4 meter Daniel K. Inouye Solar Telescope is the largest solar telescope in the world, with a focus on understanding the Sun’s explosive behavior. The Vera C. Rubin Observatory (the highest ranked ground-based project in NWNH) will undertake an astronomical survey, the Legacy Survey of Space and Time (LSST) that will fully open the window of time-domain astronomy. Together, these two facilities cost just over $1 billion in 2020 dollars for construction, and continuing investment will be needed to realize a commensurate return on investment. The past decade has also seen investment in new capabilities for existing U.S. ground-based OIR telescopes. The twin national 4-meter telescopes have been converted to wide-field survey machines with the Dark Energy Camera (Blanco), and the Dark Energy Spectroscopic Instrument (Mayall) made possible by an NSF/DOE partnership. The 3.5-meter WIYN telescope is now equipped with a state-of-art spectrograph (NEID) for precision radial velocity measurements of exoplanets, thanks to an NSF/NASA partnership. The development and deployment of multi-object and integral-field spectrographs, and of advanced Adaptive Optics and High Contrast Imaging systems has made these technologies powerful PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-2

tools on the largest telescopes. Pioneering work on optical/near-IR interferometers has opened up a whole new terrain mapped with high angular resolution. In this report, the panel assesses the investments needed in the next decade to ensure that the United States remains at the forefront in addressing the most important and enduring scientific questions (many of which can only be answered with the next generation of ground-based OIR observatories—see Section K.2 below). In the case of the ELTs (discussed in Section K.3), the cost will be considerable, with the investment needed now. The scientific payoff—while immense—will be more than a decade away. The panel argues in Section K.4 that additional investment is essential so that, in the meantime, we can fully realize the potential of the powerful and flexible OIR system constructed at great cost over the past few decades. K.1.2 Input to the Panel The analyses reported below are based on extensive input to the panel. The panel individually discussed 50 white papers from the community. These were used primarily to define the topical areas described in Section K.4 and guided the panel’s thinking in these areas. The panel had two presentations by NSF: one in which they provided an overall budgetary framework and entreated the panel to think boldly, and a second in which they clarified the nature of the cross-divisional support in NSF for solar astronomy and the role of Astro2020 in providing guidance in this area. The panel received and analyzed the more detailed “Request for Information Version 1” (RFI 1’s) from the GMT, TMT, and NSF’s NOIRLab for the U.S.-ELT Program, CHARA (OIR Interferometer), COSMO (solar observatory), and the Mauna Kea Spectroscopic Explorer (massively multiplexed spectrograph). The panel received and analyzed the less detailed RFI 2’s from the FOBOS and MegaMapper massively multiplexed spectrographs, the NPOI and MROI OIR interferometers, and the Liger and WFAO next-generation AO systems. In the case of the U.S.-ELT program, the panel received full Technical, Risk, and Cost Evaluation (TRACE) analyses. (See Appendix O.) The panel went through three iterations of questions and answers with the U.S.-ELT Program based on the RFI 1’s and initial TRACE results. The panel also had a face-to-face meeting with the U.S.-ELT Program leads and key personnel (from AURA, GMT, NOIRLab, and TMT). The panel did not request a TRACE for the other projects with RFI 1 or 2, because we did not deem them as being high-priority candidates for NSF MREFC-funding. K.2 THE SCIENCE FRONTIER Before considering the different proposed programmatic elements in the future U.S. Ground- Based OIR system, the panel would like to set the stage by highlighting four areas at the science frontier for which these facilities would play the most essential role. These are drawn from the reports of the science panels, representing this panel’s synthesis of their analyses. K.2.1 Exoplanets and Astrobiology Is the Solar System a cosmic rarity or a galactic commonplace? How do Earth-like planets form, and what determines whether they are habitable? Is there life on other worlds? The National Academies Exoplanet Science Strategy and the Panel on Exoplanets, Astrobiology, and the Solar System have identified two ground-based capabilities in optical and infrared astronomy that are essential to realize the great opportunity in exoplanets and astrobiology that is open before us: Firstly, the GMT and TMT will open an unprecedented discovery space in the study of planet formation, mature gas giants, and even terrestrial worlds. The unprecedented contrast and angular PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-3

resolution of the GMT and TMT will enable profound advances in imaging and spectroscopy of entire planetary systems, over a wide range of stellar and planetary masses, semimajor axes, and wavelengths, including both reflected light and thermal emission. For low-mass stars, the reduced glare of the central star may permit the GMT and TMT to image temperate rocky worlds. These facilities will also allow the detection of newly formed planets in their natal disks, providing the ground truth for the time scale of planet formation and permitting studies of the dynamical interaction between disks and planets. With the high spatial resolution of the GMT and TMT, researchers will finally be able to search the inner parts of planet-forming systems. The unprecedented light gathering capability of the GMT and TMT, coupled with high-resolution optical and infrared spectrographs will be powerful tools for studying the atmospheres of transiting and nontransiting close-in planets. For the closest and least massive stars, these observatories may detect molecular oxygen in the atmospheres of transiting terrestrial planets orbiting within the habitable zone of their stars, landmark discoveries with implications far beyond astronomy and even science. Secondly, as mass is the most fundamental property of a planet, the astronomical community needs to develop the ability to measure the radial velocities of stars to a precision sufficient to measure the masses of their attendant temperate terrestrial exoplanets. These mass measurements are essential both for determining the planet’s bulk composition (and, by inference, its formation history), and for interpreting studies of the planetary atmosphere, since the atmospheric scale height depends on a combination of surface gravity, temperature, and mean molecular weight. Radial velocity measurements are currently limited by variations in the stellar photosphere, instrumental stability and calibration, and spectral contamination from telluric lines. Hence progress will require a coordinated national initiative in extreme precision radial velocities that includes observers, instrument builders, stellar astrophysicists, heliophysicists, and statisticians. Although the bulk of this effort will involve smaller ground-based telescopes, the photon gathering capability of the GMT and TMT will also play a role, enabling photon- noise-limited Doppler precision of several cm/s on time scales of a few minutes, allowing researchers to disentangle the signals from the stellar photosphere and orbital motion. The grand opportunities now afforded by exoplanets cannot be addressed with a single telescope: The optimal targets for some of the most demanding exoplanet studies will be rare and demand access to the entire celestial sphere. In addition, the technology roadmap to enable the full exoplanet science potential of GMT and TMT will be realized only by leveraging the existing network of U.S. centers and laboratories and current 8- to 10-meter class facilities. K.2.2 The Fundamental Physics of the Universe Over the past century some of the most revolutionary discoveries in fundamental physics have come from OIR astronomy: from the confirmation of Einstein’s prediction of the trajectory of light passing close to the sun and Hubble’s discovery of the expansion of the universe, to the discovery of dark matter and of an accelerating universe (likely owing to dark energy). In this vein, there are a host of exciting paths to follow over the next decade and beyond. Here the panel focuses on cases in which ground-based OIR observations are essential. Tests of General Relativity (GR): The Galactic Center is an ideal laboratory for testing GR in the context of a supermassive Black Hole (BH). In the coming decade, it will be possible to use the next generation of Adaptive Optics to precisely measure the orbits of stars close to Sag A*. Such measurements will test both the Einstein equivalence principle and the form of the Kerr metric around a BH, allowing a search for a massive graviton or additional interactions (like scalar fields). By using ELTs to go 5 magnitudes fainter than the confusion limit today, there will likely be multiple stars with periods as short as 1 to 2 years, providing even more stringent tests of GR. With the improved sample and astrometric and radial velocity precision enabled by ELTs, it will be possible to detect GR effects that have no analog in classical dynamics such as the precession of the periapse and the Lense-Thirring or frame dragging effect, which is owing to the spin of the BH. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-4

The nature of dark matter. Models of dark matter span an enormous range in particle mass, and in the degree to which the dark matter particles self-interact. The combination of light-gathering power and high angular resolution will enable the new ELTs to test these models. By measuring precise radial velocities and proper motions, it will be possible to construct maps of the 3D stellar orbits at dwarf galaxy centers to map the radial distribution of dark matter, to thereby determine whether self-interacting dark matter is required. Similarly, precise maps of gravitationally lensed background sources utilizing the sensitivity and spatial resolution of an ELT will make it possible to determine the dark halo mass function at low masses, which depends on the nature of dark matter. Tests of the expansion history of the universe will not only constrain the nature of dark energy, but can also look for signs of “new physics,” which is hinted at by the so-called “H0 tension” (an apparent difference in the Hubble constant based on local scales versus the cosmic microwave background). Ground-based OIR astronomy will play a crucial role in this. Key programs include improved measurements of the evolution of the large-scale structure of the universe (made using the next generation of massively multiplexed spectrographs), and improved and refined measurement of the distance scale using new standard candles, standard sirens (electromagnetic counterparts to gravitational wave events), and standard clocks (time delay measurements of lensed Type Ia supernovae). These techniques really require the sensitivity and spatial resolution provided by ELTs to achieve their full potential. ELTs are required to reach 1 percent precision H0 measurements with multiple techniques. With ELTs, tip of the red giant branch distances will reach deep into the Hubble flow to ~100 Mpc, enabling large numbers of direct measurements, while geometric eclipsing binary distances will be possible for all Local Group galaxies anchoring these distances. Gravitational lensing systematics, both the mass distribution of quasar hosts and the line-of-sight mass distribution, can be minimized with deep and high-resolution IFU observations. Standard siren host galaxies are likely to be faint; ELTs will be needed for spectroscopic follow-up to obtain redshifts. Understanding the physics of cosmic inflation. Maps of the large-scale structure of the universe made possible with galaxy redshift surveys undertaken with a future generation of massive-multiplexed wide-field OIR spectrographs are crucial here. For example, a clear detection of non-Gaussianity in the primordial fluctuations would rule out the simple “single field” inflation scenario. The detection of departures from a power-law form of the primordial power spectrum would offer the opportunity to explore the history of the primordial seeds of large-scale structure during their production in the era of inflation. These require sampling significantly larger volumes with a much larger number of galaxies than can be done with existing or nearly complete facilities. K.2.3 Galaxy Evolution In the next decade and beyond, ground-based OIR facilities will remain key to unraveling the fundamental processes that drive galaxy evolution. Future capabilities will transform our view of galaxies, their constituent components, and their vital link with the circum- and intergalactic medium (CGM/IGM) via baryon cycling. Specific areas of progress are captured by the themes identified by the Science Panel on Galaxies (1) the evolution of the IGM and the sources of radiation during the first billion years from cosmic dawn throughout the epoch of reionization; (2) the gas, metals, and dust flows into, through, and out of galaxies at all epochs; (3) the formation of supermassive black holes and the coupling of their growth with that of their host galaxies; (4) the impact of the assembly history of galaxies and dark matter (DM) halos on their observable properties; and (5) the discovery and investigation of the first galaxies. The complex interplay between baryons and DM, and the richness of physical processes involved, call for multi-scale multi-wavelength approaches probing the full ecosystems across time. Ground-based OIR systems play a pivotal role by accessing a wealth of key spectral tracers of stars, gas, and metals. Transformative advances rely on progress along two synergistic axes: spatially+spectrally detailed information and multiplexing. Key enabling capabilities are (1) R~5000–50000 spectroscopy; (2) PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-5

resolution in the infrared down to 0.01–0.02” (roughly 100 pc); (3) dense/full spectroscopic coverage up to ~2 arcmin scales (roughly 1 Mpc at z > 1); and (4) 10–100x higher sensitivity than existing facilities. Detailed information is vital to unveil physical processes at the level of individual stars and their surrounding nebulae in the Local Group, of individual 100pc-scale star-forming complexes and globular cluster progenitors at z > 1, and of the first galaxies at z > 10 with compact <<1 kpc sizes. The characterization of their dynamical, chemical, and thermal state requires velocity resolution <10–100 km/s. Achieving the corresponding angular resolution of 10–20 mas and spectral resolution of several 103 to a few 104, with the necessary sensitivity for such faint sources, will only be possible with ground-based ELTs, coupled with high-performance AO systems. With 5–10 times sharper views, ELTs+AO will map z > 1 galaxies in similar detail as we have now for galaxies a mere 100 Mpc away—a drastic advance reaching fundamental sub-galactic components. The ELTs will thus uniquely enable major breakthroughs in galaxy evolution. Multiplexing is essential to build up sample statistics and to cover wide contiguous areas. This is needed for (1) a complete census and detailed characterization of ~108 stars and the low-mass satellites that encode the Milky Way’s chemo-dynamical history and the baryon-DM interplay, (2) to connect directly individual galaxies and their surrounding CGM/IGM, and (3) to map the distribution and properties of the first generations of galaxies holding keys to the history and topology of reionization. Ongoing projects and future concepts for massively multiplexed spectrographs—be they multi-object or integral field instruments—will deliver the necessary 10-100 times larger samples. Coupled with high sensitivity and angular resolution, they will reach unexplored low mass/luminosity regimes that are very sensitive to the physics of star and galaxy formation, overcome crowding in nearby studies, and greatly boost observational efficiency for distant sources. K.2.4 A Stellar Renaissance Over the past 15 years, the solar and stellar astrophysics community has ignited a scientific renaissance through a remarkable investment in facilities for global synoptic and high-resolution solar observations, extreme-precision stellar radial velocity, and milliarcsecond imaging of nearby stars, as well as ultra-widefield surveys focused on high-precision position measurements, high-cadence time-domain measurements, medium-resolution optical-infrared spectroscopy, and multi-band precision photometry. Ground-based OIR facilities have played, and will continue to play, the central role. A crowning achievement for ground-based solar astronomy is the recent completion of NSF’s 4 m Daniel K. Inouye Solar Telescope (DKIST), whose unprecedented spatial resolution and spectropolarimetric sensitivity will open a new window on the magnetohydrodynamic phenomena that affect convective motions and drive the storage and release of magnetic energy (reconnection). These in turn result in sunspots, flares and eruptive events, solar energetic particles, coronal heating, and perhaps most fundamentally the solar cycle, all of which have impact on our home planet through space weather. Solar measurements made from the ground (DKIST, GONG, SOLIS, GST, and other observatories, as well as the planned COSMO, ngGONG, and DKIST-II) and from space (SDO, SOHO, Solar Orbiter (with ESA), Parker Solar Probe, and the in-development Solar C/EUVST (with JAXA)), the coming decade will provide significant new insight into physical processes that apply to all stars. High time- cadence observations by NASA’s Kepler mission have opened an analogous window on the interiors of distant stars via asteroseismology and photometry, while high spatial-resolution observations with ground-based optical interferometers such as CHARA and VLTI are producing astonishing insights into the surface phenomena and asymmetries of nearby stars. Meanwhile, the combination of ground- and space-based ultra-wide-field imaging, spectroscopic, and high-precision astrometric surveys (e.g., SDSS-IV Apogee, ESA’s Gaia, DOE/NSF Dark Energy Survey) are enabling a giant leap forward in researchers’ ability to disentangle galactic structure and more precisely constrain the dynamical and chemical evolution of the Milky Way over its entire lifetime, as well as providing new, more precise measurements of fundamental stellar properties and lifecycle stages PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-6

across the HR diagram. Indeed, such large surveys have been critical in detecting examples of statistically rare, short-lived stellar types. The renaissance in solar and stellar astrophysics will continue and accelerate in the decade ahead thanks to OIR ground-based facilities that have just started operation or will begin soon such as NSF/DKIST, DOE/NSF/VRO, DOE/NSF/DESI-Stellar, NASA/NSF/NEID and ESO/4MOST. High-time cadence information will be added for large stellar samples by NASA/TESS and ESA/PLATO. Many of the brightest stars in those samples will be observable by ground-based optical interferometers and high- spectral resolution spectrometers and spectropolarimeters to open a new window on the connection between stellar surface and interior phenomena. Looking further into the future, the spatial resolution of ELTs will isolate single stars below the main-sequence turnoff, map the orbits of tight binaries, and enable proper motion selection in distant clusters in currently inaccessible environments throughout the Local Volume. The ELTs will also be useful to follow up the detections of candidates for Pop III stars in the halo of the Milky Way and in other Local Group galaxies. Affiliating and effectively using these highly multi-dimensional peta-scale data sets will require new approaches to managing and analyzing positional, photometric and spectroscopic information, likely incorporating database and machine-learning techniques developed outside astronomy. Astronomers have always worked closely with mechanical, optical, electrical, and computer engineers. In the decades ahead, astronomers will also embrace working with data scientists in ways not broadly appreciated even 10 years ago. K.3 THE U.S. EXTREMELY LARGE TELESCOPE PROGRAM K.3.1 Introduction We stand at a watershed moment. A new generation of Extremely Large Telescopes is essential to answering the most important questions in astronomy and astrophysics in the 2030’s and the decades that follow. It seems clear that without a major federal investment, the two ELT programs with U.S. stakeholders, the Giant Magellan Telescope (GMT) and the Thirty-Meter Telescope (TMT), will both fail, severely damaging U.S. astronomy for decades. At this pivotal time, the two ELT projects have joined forces with NSF’s NOIRLab to propose a radically new concept to Astro2020: a unified U.S.-ELT Program that will provide the broad U.S. astronomy community with access to both GMT and TMT in exchange for a substantial federal investment in the ELTs, and with an interface to this broad community provided by NOIRLab. The panel then considered two stark choices: Endorse an unprecedented level of NSF investment and a revolutionary new “business model” to capitalize on this investment, or cede U.S. leadership in the frontier science enabled by ELTs for decades to come. The panel argues below in favor of the first choice on the basis of the review of a proposed U.S.-ELT Program uniting GMT, TMT, and NOIRLab. K.3.2 The Science Case In Section K.2, the panel described four broad areas at the scientific frontier where the next generation of ground-based OIR facilities are most essential and explicitly identified there the need for ELTs, from the search for life on distant planets, to the births and lives of galaxies, to the fundamental physics of the cosmos. Here the panel reports on its analysis of the reports of the Science Panels to identify which of the specific Questions and Discovery Areas (DAs) will need ELTs. For each of these, the panel has evaluated whether the ELTs are essential (18 cases), useful (8 cases), or not needed (4 cases) to answer the questions posed. The essential items are listed in Table K.1, which makes it clear that the science case for PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-7

ELTs is extraordinarily broad and deep (and likely unmatched by any single other new ground- or space- based observatory currently under consideration). TABLE K.1 List of Essential Needs for ELTs in Science Frontier Panel Reports Panel on Compact Objects and Energetic Phenomena Q2: What powers the diversity of explosive phenomena across the electromagnetic spectrum? Q4: What seeds supermassive black holes and how do they grow? DA: Transforming the view of the Universe by combining new information from light, particles, and gravitational waves. Panel on Cosmology Q2: What are the properties of dark matter and the dark sector? Q3: What physics drives the cosmic expansion and large-scale evolution of the universe? Panel on Galaxies Q1. How did the intergalactic medium and the first sources of radiation evolve from cosmic dawn through the epoch of reionization? Q2: How do gas, metals, and dust flow into, through, and out of galaxies? Q3: How do supermassive black holes form, and how is their growth coupled to the evolution of their host. Q4: How do the histories of galaxies and their dark matter halos shape their observable properties? DA Mapping the circum-galactic medium and the inter-galactic medium in emission. Panel on Exoplanets, Astrobiology, and the Solar System Q1. What is the range of planetary system architectures? Q2. What are the properties of individual planets, and what processes lead to planetary diversity? Q3: How do habitable environments arise and evolve within the context of their planetary systems? Q4. How can signs of life be identified and interpreted within the context of their planetary environments? DA: The search for life on exoplanets. Panel on the Interstellar Medium and Star and Planet Formation Q3. How does gas flow from pc-scales down to proto-stars and their disks? Q4. Is planet formation fast or slow? DA: Detecting and characterizing forming planets. Panel on Stars, the Sun, and Stellar Populations The ELTs were deemed useful but not essential to the four questions and discovery area identified by the Panel on Stars, the Sun, and Stellar Populations. K.3.3 The Components of the U.S.-ELT Program The U.S.-ELT Program (U.S.-ELTP) as proposed to Astro2020 is made up of three elements: The GMT, the TMT, and NOIRLab. The primary mirror of the GMT consists of seven 8.4 m segments, for a total diameter of 24.5 meters. It is a two-mirror Aplanatic Gregorian telescope yielding a 25 arcmin field-of-view (FOV) at f/8 with Ground-Layer Adaptive Optics. Alignment and phasing is achieved with Adaptive Secondary Mirrors. Laser Tomography Adaptive Optics utilizes six sodium laser beacons and edge-sensors. With this, diffraction-limited images of 10 mas at 1 μm can be achieved over a 20–30 arcsec FOV. The first generation of instruments includes the GMT Multi-object Astronomy and Cosmology Spectrograph (GMACS), the GMT Integral Field Spectrograph (GMTIFS), the GMT Consortium Large Earth Finder (G-CLEF), and the GMT Near-IR Spectrograph (GMTNIRS). The GMT will be located at the Las Campanas Observatory in Chile. The majority of the GMT partners are U.S. institutions, with PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-8

international partners in Australia, Brazil, and Korea. The site has been excavated, the site water and electrical distribution upgrades have been completed, and the sixth mirror has been cast. The primary mirror of the TMT consists of 492 1.44 meter mirror segments, for a total diameter of 30 meters. It is a three-mirror Ritchey-Chretien telescope delivering a 20 arcmin FOV. Ground Layer AO can improve image quality over a 2 arcmin FOV. Six sodium lasers and 3 tip/tilt natural guide stars will be employed. With this Narrow Field InfraRed Adaptive Optics System (NFIRAOS), diffraction- limited images of 8 mas will be achieved over a 15–30 arcsec FOV. Instruments will be deployed at the Nasmyth platforms. The first-generation instruments include the Infrared Imaging Spectrometer (IRIS), the Wide-Field Optical Spectrometer (WFOS), and the Multi-Object Diffraction-limited High-Resolution Infrared Spectrograph (MODHIS). The TMT will either be sited at the Mauna Kea Observatory in Hawaii (MKO), or at Roque de los Muchachos Observatory in the Canary Islands (ORM). The majority of the TMT partners are international, with the participation of institutions in the United States, Canada, China, India, and Japan. NSF’s NOIRLab is the U.S. national center for ground-based, nighttime optical and infrared astronomy. It operates five programs—Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), Gemini Observatory, Kitt Peak National Observatory (KPNO) and the Vera C. Rubin Observatory (VRO). Its roles in the U.S.-ELTP will be to (1) provide access to a bi- hemispheric ELT system; (2) enable and support large-scale, systematic, collaborative research (Key Science Programs—see Section K.3.10 below); (3) provide user support, broaden participation in TMT/GMT science, and foster research inclusivity; and (4) engage and represent the whole U.S. community in GMT and TMT governance, scientific planning, and instrumentation development. K.3.4 Technical Risks Since New Worlds, New Horizons, both the GMT and TMT projects have devoted significant resources to identify and retire the major technical risks. It is the panel’s assessment that—for the most part—these are now both technically mature projects which are now mastering the implementation of new technology. However, there are still issues to be solved. The TRACE analysis by the Aerospace Corporation gave both projects a medium rating in technical risk. The largest concerns for both projects were the active and adaptive optics systems. For GMT the most important issue is the complexity of 7-segment adaptive secondary mirrors, since phasing and alignment to desired specifications across 4 modes is required. The on-axis guide star and laser guide star modes were found to be the most challenging. Quantitatively, the adaptive seven- segment secondary mirror has 675 actuators per segment. Challenges with this system could lead to schedule delays and may limit desired scientific performance across select observing modes. The Laser Tomography Adaptive Optics mode depends on six sodium lasers and edge sensing to < 15 nm RMS. Its design requires significant development. For TMT, the biggest concern is that the NFIRAOS AO instrument is complex, with multiple sodium lasers, wave-front sensors, deformable mirrors and downstream imagers and spectrographs. It is a single point failure whose performance directly impacts two of three first-generation science instruments. It still requires significant development across multiple partners. The primary mirror integration and wave-front control system for the 492 segments involves 1476 actuators and 2772 edge-sensors, and will also be challenging. Problems with this system could lead to delays. K.3.5 Construction Costs and Funding Model In the panel’s view, the most serious risk to the successful construction for both the GMT and the TMT projects is financial rather than technical. The cost estimates provided by the projects are considerably higher than those previously understood by the astronomy community and will severely PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-9

strain the financial capacity of even a full partnership between the projects and the National Science Foundation. The GMT project now estimates a total construction cost of $2 billion in real year (RY) dollars. Of this amount, 20 percent has been spent to-date. An additional 10 percent has been committed by current partners. The project plans for the remaining 70 percent to come from NSF (40 percent), additional (uncommitted) funds from existing partners (15 percent), and funds from unidentified new partners (15 percent). To state it differently, even with all the funds expended and committed, and with an $800 million commitment from NSF, there is still a shortfall of roughly $600 million. The TMT project now estimates a total construction cost of $2.65 billion in RY dollars. Of this amount, 18 percent has been spent-to-date. An additional 41 percent has been committed by the current partners. The project plans for the remaining 41 percent to come from NSF (30 percent) and additional (uncommitted) funds from current partners (11 percent). In this case, with all the committed funds and full funding from NSF, there is still a shortfall of $310 million. The construction costs estimated in the TRACE reports are both about 20 percent higher than the project estimates ($2.4 billion for GMT and $3.1 billion for TMT). They are largely the result of conservative assumptions made by the TRACE analysis as to risk. All these numbers are summarized in Table K.2. TABLE K.2 Summary of Construction Costs and Funds, in Million Dollars and Real Years GMT TMT Funds spent $400 (20%) $475 (18%) Additional funds committed $197 (10%) $1063 (41%) NSF ask $800 (40%) $800 (30%) Missing funds $603 (30%) $310 (11%) TOTAL $2000 $2650 __________ __________ __________ TRACE delta $400 (20%) $450 (17%) TRACE TOTAL $2400 $3100 TRACE missing funds $1003 $760 K.3.6 Programmatic Risks The largest single programmatic risk to each project derives directly from the funding issues summarized above. Both projects need significant additional new funding beyond the planned request from NSF. Both projects believe that the combination of the imprimatur of a top ranking in the decadal survey, followed by the full financial involvement of the U.S. federal government would make it possible to secure additional resources from existing partners and possibly from new partners. Before discussing the two projects, it is important to emphasize that they are based on very different funding models, each with different potential risk factors. The majority of the GMT partners are U.S. universities (plus the Carnegie Institution for Science). The model for the contribution and allocation of financial resources is strongly cash-based. In contrast, the majority of the TMT partners are international entities funded by their respective national governments. The TMT funding model is largely based on significant in-kind contributions from these partners in terms of completion of assigned technical work packages. The GMT project’s estimate of the additional funds needed in addition to the funds currently committed and the funds requested from NSF is $600 million. If the construction cost for the TRACE is adopted, this increases to $1000 million. A further risk factor for GMT is the relative immaturity of estimates for cost-to-go. Only 16 percent of these are based on signed contracts or detailed bids, with the PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-10

other 84 percent being based on “Rough-Order-of-Magnitude” or project estimates. This leads to additional uncertainty in cost and schedule. The TMT project’s corresponding estimate of these additional new funds is $310 million. If the TRACE cost estimate is adopted this becomes $760 million. A further potential risk factor is TMT’s reliance on international partners. This is based on a model of low-cost in-kind critical components and sub-systems. While TMT has agreements in place that a given international partner would shoulder any additional costs associated with the delivery of their work packages, this model has not been tested under extreme circumstances. If this funding model is successful, the TRACE delta could be significantly decreased. An additional potential programmatic risk for TMT is posed by the uncertainty in its choice of site. Based on the documents presented by TMT, which were analyzed by the panel and in the TRACE report, a timely decision to build TMT on ORM would not lead to an increase in cost or a delayed schedule compared to MKO. Moreover, the panel has reviewed the relevant metrics on site quality and finds that—while MKO is the superior site—the ORM site is acceptable. The largest impact would be in the thermal infrared and in the ultraviolet near the atmospheric cut-off. Despite this assessment, the choice of a site still poses a significant programmatic risk since it could adversely affect the partnership. The TRACE analysis—based primarily on the considerations above—gave both projects a medium-high programmatic risk. The TRACE evaluation flagged the schedules as being too aggressive. GMT plans for 12 years, including LTAO commissioning, while the TRACE estimate was 13 years. TMT plans for 10 years, while the TRACE estimate was 13 years. The panel notes that there is better agreement with risk-adjusted schedules from the projects: 13.7 years for GMT and 11.2 years for TMT. Both stated their costing included the risk-adjusted schedules. K.3.7 Life Cycle Costs As is discussed further in Sections K.3.8 and K.3.9, the panel regards it as essential that a plan is developed to ensure that adequate funding is in place through a combination of federal and project funds to operate the U.S.-ELTs with high efficiency, and to continue to provide them with state-of-the art instruments throughout their scientifically productive lifetimes. The two projects have developed bottom-up estimates for operations of these facilities and have budgeted for partial funding of future instruments. The panel is concerned that the estimated operations and instrumentation budgets are too lean, for both GMT and TMT. GMT has budgeted $30.6 million per year for operations and $10 million per year for new instruments. The total annual amount of $40.6 million represents about 2.1 percent of construction costs (all numbers being in $2020). TMT has an annual budget of $33.3 million for operations and $13.7 million for new instruments. The total annual budget of $47 million represents 2 percent of construction costs. In comparison, the corresponding fractions are 4 percent for the E-ELT and 5 percent for the VRO and ALMA. It is the panel’s assessment that the currently proposed operations budgets are too low to support the type of highly efficient, flexible mode that will be needed to capitalize fully on the financial investment. Likewise, the budget for future instrumentation will not be adequate to ensure a continuing line of state-of-the-art new instruments. K.3.8 Consideration of the U.S.-ELT Program In this section, the panel considers the case for the proposal to Astro2020 by the U.S.-ELTP for federal investment that would unite the GMT, TMT, and NOIRLab. The panel first discusses the question of whether this should be done, and then asks whether it can be done, from a financial point-of-view. The panel emphasizes here that a single proposal was received from the U.S.-ELTP (not separate GMT and TMT proposals). The panel has therefore considered this proposal for investment in a two-ELT PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-11

system. In Section K.3.9, the panel addresses what would happen if NSF and the two ELT projects cannot firmly commit to the total financial resources required. K.3.8.1 The Strategic Case for the U.S.-ELTP The panel believes that the scientific case for a U.S.-ELT program is strong and compelling. The technical risks, while challenging, are manageable. The greatest threat to these projects is financial in nature, and could potentially be retired through an NSF investment, if that is part of a full and robust financial plan with the other partners. Given the status of the GMT and TMT projects, and the on-going construction of the E-ELT, the time to invest is now. Here, the panel summarizes the strategic considerations that informed its thinking. These considerations form the core of the argument to Congress for federal support at such a substantial level. Ensuring U.S. Leadership: The construction of the E-ELT is well under way. Absent a federal investment, it seems clear that the U.S. community will cede leadership in what has been the backbone area of observational astrophysics for over a century. In this context, a federal investment only makes sense if the program it enables is at a level that effectively competes with and complements the E-ELT capabilities over a long lifetime of discovery. The E-ELT (39-meter aperture) is larger than either the GMT (24.5 meters) or TMT (30 meters). A U.S. federal investment in either GMT or TMT on its own, will not achieve parity. In particular, the sizes of the professional astronomical communities in the United States and Europe are similar, so that a fraction of a single ELT would underserve the U.S. community. There are a number of ways in which the sum of the GMT and TMT can offer important advantages over the E-ELT, some of which is described below. Here the panel emphasizes two points. The first is that the United States has a powerful legacy of astronomical discoveries using a bi- hemispheric OIR system of telescope. Unlike Europe, this bi-hemispheric approach has enabled the U.S. community to undertake major surveys or follow-up on high-stakes targets in any hemisphere required. The second is that GMT and TMT have respective unvignetted fields-of-view that are six and four times larger than the E-ELT in terms of solid-angle, making them more efficient for undertaking surveys. The panel notes that the U.S. community has extensive experience in large astronomical surveys. This wide- field, bi-hemispheric capability makes the U.S.-ELTP complementary to the larger-aperture E-ELT. Capitalizing on Investments and Assets: The strongest argument in favor of a bi-hemispheric U.S.- ELTP is the importance of being able to fully exploit the synergy made possible by the investments already made in bi-hemispheric facilities. No matter how powerful the ELTs may be, they are part of a system of ground-based facilities that have been constructed at great cost (over $3 billion). These facilities are needed to unlock the scientific power of the ELTs, and vice versa. These facilities are (by hemisphere, in alphabetical order):  North: ARC 3.5, Gemini North, HAWC, HET, Keck I and II, LBTO, Mayall (DESI), MMT, Palomar 5 m, PTF, Pan-STARRS I and II, Sloan Telescope (SDSS N), Subaru (HSC and PFS), SMA, JVLA, WIYN (NEID)  South: ALMA, Auger Observatory, Blanco (DECam), DuPont (SDSS-S), Gemini S, Magellan I and II, SALT, SOAR, VRO, SPT (CMB/SZ), IceCube In addition to these ground-based facilities, space-based facilities are inherently all-sky, and require bi-hemispheric ground-based facilities to fully exploit their power. Last, there are fields on the sky that are only accessible to either a northern- or southern-hemisphere ELT, and represent either an enormous existing investment in space-based and/or ground-based observations, or are astronomically unique, including:  North: EGS, Euclid NEP, GOODS-N, Kepler, M 31, SDSS (main), TESS-N PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-12

 South: GOODS-S and Euclid DF-S, Euclid DF-Fornax, LMC, SMC, TESS-S, Roman ST HLS, SDSS (S) There are also very rare and important sources to be discovered that may only be visible from one hemisphere, and there are monitoring programs where the longitudinal coverage afforded by two ELTs would be valuable. Risk Mitigation: There are serious programmatic risks for each of the ELTs, leading to multiple ways in which either could fail. By pursuing the as-proposed U.S.-ELTP, the risk of a zero-ELT outcome is reduced. A Platform for Innovation: A two-telescope system provides a broader platform for innovation. Firstly, two telescopes provide more nights of observations to the community, making a broader scientific program possible. Secondly, while the first generation of instruments on GMT and TMT have overlapping capabilities, this need not be the case in the future. The U.S.-ELTP could decide on a strategic plan through which future generations of instruments are complementary and access is shared across the partners. This would add to the advantage that the smaller plate scales for GMT and TMT mean that instruments will be less expensive compared to those on the E-ELT, making it possible to achieve greater diversity in capabilities for a fixed investment. The NOIRLab as a “Force-Multiplier”: A crucial strategic consideration in the panel’s view is the use of the NOIRLab as a “force-multiplier” in terms of return to the community. The panel agree with their plan (described in Section K.3.10 below) to develop a set of large strategic community-driven Key Science Programs on a scale that would greatly boost the impact of the U.S.-ELTP. In fact, the panel believes that the true power of this approach really requires that the non-U.S.-federal partners also contribute observing time to these programs and are fully engaged with the broader U.S. community in terms of selecting, designing, and executing these programs. A Trans-Pacific Partnership and a Worldwide ELT System: The successful formation of the U.S.- ELTP would join together the United States in a scientific partnership spanning the Pacific Ocean, from North America (Canada and the United States) and South America (Chile), to Australia, East Asia (China, Japan, Korea), and South Asia (India). This partnership would be able to forge collaborations with the European ELT from a position of strength and in so doing form a powerful worldwide ELT system that fully exploits the complementary strengths of these facilities for decades to come. K.3.8.2 Can This Be Done? As discussed in Section K.3.5, the intention of the U.S.-ELTP is to request a total of $1.6 billion RY dollars funded by NSF’s MREFC line. These funds would be split evenly between GMT and TMT. Can this be accommodated within NSF’s budget? NSF presented an aspirational MREFC budget for the coming decade that represents a very significant increase over the mean annual budget over the past decade. The U.S.-ELT program has also presented a baseline for when the funds from NSF would be needed. The comparison between this request and the notional NSF MREFC budget is shown in Figure K.1 below. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-13

FIGURE K.1 Comparison between the U.S.-ELT request and the total notional NSF MREFC budget. The green line (marked by an arrow) represents NSF’s guidance to Astro2020. From this, two conclusions are obvious. Yes, the U.S.-ELTP can fit within the NSF guidance. However, the MREFC line pertains to the entire NSF program (not just astronomy), and over the 5 fiscal years starting in 2023, the U.S.-ELTP by itself would require 70 percent of the total MREFC budget. In other words, NSF might have the capacity to provide the requested funds, but only if the U.S.-ELTP was the number one major NSF construction project until late in the decade. The panel has expressed its concerns in Section K.3.7 that the proposed funding level for operations and new instruments may be insufficient to realize a commensurate return on the enormous investment in construction. Even the NSF share of these budgets as proposed by the projects is problematic, since the current NSF model is one in which operations costs of observatories built using MREFC funds are borne by the Division for Astronomical Sciences from its existing budget. This cannot work in this context. The panel was briefed by NSF on this issue and told that changes are being considered. Nevertheless, this remains a major concern of the panel. K.3.9 Conditions for an NSF Investment The panel believes that maximum flexibility on the part of NSF, in the terms of the final arrangements it negotiates with the U.S.-ELTP, will lead to the best outcome. However, in order for NSF to agree to provide the requested funds to the U.S.-ELTP, the panel believes that there are critical conditions to be met. These are of two kinds. The first are programmatic in nature, and the panel assumes that they would be met through the due diligence of NSF. The second concerns the critical role played by the NOIRLab on behalf of the U.S. community. K.3.9.1 Programmatic Conditions The following programmatic considerations would be preconditions for NSF investment in the U.S.-ELTP: First, the partners in GMT and TMT present a full financial plan that has all the resources to complete construction, including adequate contingency. Second, NSF caps their investment at the requested amounts of $800 million (RY) for each project. Third, the share of observing time secured by NSF is in direct proportion to their investment in fixed-year dollars (and hence larger than the notional 25 percent if $1.6 billion is allocated). Last, the current U.S.-ELT partners and NSF need a firm and realistic plan for life-cycle costs for at least the first decade (covering both operations and new instruments). This plan would be rigorously reviewed by NSF to determine whether the proposed budgets represent the best PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-14

trade-off between cost and scientific return. The NSF portion of the operations budget would explicitly include a program of competitively selected grants to enable research using data obtained as part of the Key Science Programs (Section K.3.10) undertaken with the ELTs. If it is not possible for all parties to commit to the full financial requirements, NSF could then examine whether it is possible to achieve a full financial commitment for one telescope, and decide whether to proceed with that plan to realize a U.S.-ELTP involving NOIRLab. K.3.9.2 The Role of the NOIRLab The second set of conditions deals with the role that the NOIRLab would play in making the U.S. community a full scientific partner in the U.S.-ELT program and ensuring that the return on the federal investment to this community is maximized. The U.S. federal government, through the NSF investment, will be far-and-away the largest single stakeholder in each of these telescopes. Thus, a strong U.S. leadership position would be enabled by empowering NOIRLab to act on behalf of the full U.S. community. The federal investment in the U.S.-ELTP needs to accomplish more than simply providing much of the missing financial resources needed to construct and operate these observatories. This investment can be leveraged as part of the negotiations to create the right framework: the NSF resources can be used in part to provide the “glue” to form a true partnership. NSF can ensure this by empowering the NOIRLab to play a central role in the partnership. More specifically, the panel envisages that the NOIRLab would be charged to: 1. Provide access to a bi-hemispheric U.S.-ELT system. 2. Engage and represent the whole U.S. community in GMT and TMT governance and scientific planning. 3. Provide user support, broaden participation in GMT and TMT science, and foster research inclusivity. 4. Assist the community in developing Key Science Programs (see Section K.3.10 below), and solicit the active involvement of the entire GMT/TMT U.S. and international partnership in these, including contribution of their observing time. 5. Develop the capability for queue scheduling applied to all GMT/TMT observations to optimize the way in which data are taken and maximize their immediate and legacy scientific value. 6. Facilitate plans for new instruments (including a process of open competition for building these instruments by members of the U.S. community when federal funds are involved). 7. Ensure that well-calibrated and well-characterized data and resulting data products are in an easily used archive containing all TMT/GMT data. K.3.10 The Key Science Programs and the Need for U.S.-ELT Science Centers The panel believes that the Key Science Programs are a critical component of the U.S.-ELTP. These are community-driven ELT programs that are being facilitated with the help of the NOIRLab. They are based on the principle of creating a powerful scientific legacy through systematic investment in large- scale, transformative research. These are projects on scales difficult to realize within shares of any of the current GMT/TMT partners, and are envisaged as utilizing at least half of the U.S.-ELTP observing time. As emphasized above, the panel believes the true value of the programs will only be realized if all the ELT partners participate in them, with not only scientific contributions, but also observing time. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-15

These are planned to have broad, inclusive scientist participation via open collaboration models, to harness resources of a diverse research community, spread scientific benefits widely through the community, and produce data products with high archival reuse value. While the panel finds these to be an exciting concept, it worries that there is no current funding mechanism in the United States that can support them, nor are they part of the current U.S.-ELTP operating budget as presented to the panel. Nonetheless they are critical to supporting the development of the most effective science program from the U.S.-ELTP. Funds for Science Centers would enable collaborations of people and institutions to carry out leadership science and create data, data products, and knowledge infrastructure that would enable multiple generations of scientific usage from large coherent data sets. The creation of a funding mechanism to create Science Centers may potentially be a way for NSF to partner with private foundations, a partnership of increasing interest at NSF.2 The national benefit of such Science Centers associated with the U.S.-ELTP Key Programs would be many-fold. They would provide the data and knowledge infrastructure necessary to carry out these large, long-term, multi-partner programs. They would produce myriad scientific discoveries derived from these highly leveraged, large coherent data sets, drive the hardware development and produce software tools for enabling science, and create a team of people capable of exciting and effective public engagement and stewardship of the major funding investments made within the U.S.-ELT partnership. The panel believes that without these Science Centers, the revolutionary potential of a U.S.-ELTP will not be fully realized. K.3.11 Summary The coming generation of Extremely Large Telescopes (ELTs) will be scientifically essential for decades to come. They will be necessary to address the majority of the questions posed, and discovery areas identified, by the Science Frontier Panels. The European Southern Observatory is well-along in the construction of the European ELT (E-ELT) with an aperture of 39 meters. It follows then that without a U.S. response, the United States will be ceding leadership in astronomy (and not just ground-based OIR astronomy) for at least a generation. It is also important to emphasize that—historically—large ground- based OIR telescopes remain highly productive for 50 to 70 years. The cost of the U.S.-ELTP is high, but it can be “amortized” over many decades. There are currently two ELTs with significant U.S. private and state participation, the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT). It seems clear that neither project can be successfully completed without an unprecedented level of federal support from NSF. The GMT and TMT, in concert with NSF’s NOIRLab have therefore proposed to Astro2020 to create an integrated U.S.- ELT Program (U.S.-ELTP) that will provide access to both GMT and TMT in exchange for federal investment of $800 million in each of these two telescopes and also in the necessary NOIRLab infrastructure. In the panel’s view, in order to be fully competitive with the E-ELT, to ensure full synergy with the extensive system of U.S. bi-hemispheric ground-based astronomical facilities, and to strengthen the case for this level of financial support (which will take the enthusiastic support of Congress), NSF will need to work with all three U.S.-ELTP elements to create the proposed system of two telescopes, and implement a bold and visionary new model that forms a true partnership with U.S. community engagement (enabled by the NOIRLab). NSF will be by far the largest single partner in this program, and can therefore play an appropriate leadership role. The path ahead is very difficult. There are serious risks associated with both ELTs, and several ways each could fail. The panel believes the engagement of NSF with both projects helps mitigate the risk of no U.S.-ELTP. 2 See, for example, https://www.msri.org/workshops/785. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-16

For NSF to proceed with this plan, it would need to ensure that each of the other GMT and TMT current and future partners commit the financial and technical resources to fully complete construction and provide the funds needed for operations and a line of new instrumentation. The panel believes that the two-telescope U.S.-ELTP is needed to maintain U.S. leadership, and that it would be disastrous if no U.S.-ELTP is realized. K.4 A BALANCED PROGRAM FOR THE NEXT DECADE AND BEYOND K.4.1 Introduction As exciting and extraordinarily important as the U.S.-ELTP would be, the scientific payoff to the community is over a decade away. Moreover, while the multi-billion dollar U.S.-ELTP price tag may seem daunting, it is crucial to recognize the comparable investment made in existing OIR ground-based observatories, starting with the two Keck telescopes in the 1990s. For telescopes with U.S. stakeholders, this amount is roughly $2.4 billion in 2020 dollars (just in initial construction costs). The Daniel K. Inouye Solar Telescope) (which has just seen first-light), and the Vera Rubin Observatory (operations starting in 2023) together account for about $1.0 billion of this total. At great cost, we have built a powerful and flexible system of ground-based OIR facilities. How do we exploit this investment during the next decade and beyond? In this section of report, the panel describes what it believes to be the most essential components of this system. The primary goal is to highlight how relatively modest investment either in, or in support of, the existing (or soon-to-exist) ground-based OIR telescopes will pay major dividends during the decade to come. In some cases, the panel also describes possible new telescopes or telescope arrays that could either plausibly be fit into the NSF Mid-Scale Research-2 (MSRI-2) program or funded by NASA. The panel does not consider any of these facilities as rising to the level requiring the MREFC. These are described in alphabetical order. K.4.2 Adaptive Optics/High-Contrast Imaging Adaptive optics (AO) is one of the transformational technologies that have driven breakthroughs in many areas of astronomy and astrophysics in the past two decades, by enabling ground-based telescopes to image at their diffraction-limit. AO development has led, notably, to the discovery of the supermassive black hole at the Galactic Center, the first images and spectra of exoplanets, and direct evidence for the existence of dark subhalos as predicted by Cold Dark Matter models. AO is a key component for the success of DKIST. Many current and new instruments at existing OIR facilities are equipped with AO capabilities. AO is intrinsically embedded in the design and operations of ELTs. Thus, the central role of AO instrumentation and the importance of further development are rapidly growing, with novel concepts pushing toward wider areas, higher performance, and extended wavelength coverage.  Wide-field AO (GLAO, MCAO, MOAO, LTAO) delivers uniform wavefront correction over large areas, achieved by sensing the atmospheric turbulence profile with multiple laser beams assisted with natural guide stars, and serving a very wide range of areas from Galactic to extragalactic science. Examples include censuses of stellar populations in the Milky Way and Local Group where AO significantly reduces the impact of crowding, and surveys of resolved morphologies and kinematics of distant galaxies whose apparent sizes are of order of the seeing disk and smaller. Wide-field AO has matured in the 2010’s, with first systems now in science operations (e.g., GeMS on Gemini-South, ARGOS at the LBT).  Extreme AO (ExAO) lies at the opposite end of the AO parameter space and aims to provide exceptionally high performance (Strehl ratios in excess of 90 percent) over a narrow field. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-17

Technical implementation of the ExAO concept is highly challenging. Coupled with high- contrast observations (>105), the primary goal is the high-priority exoplanet science case. The challenges and scientific rewards have motivated massive efforts worldwide. Pathfinders are in operation (e.g., GPI on Gemini South, SPHERE at the VLT) but significant development notably in deformable mirror technology, speed of AO systems, and wavefront control is needed in the 2020’s to achieve the scientifically driven technical requirements (contrast of 106 or better).  Visible AO has high potential scientific return by opening up an entire wavelength regime to high angular resolution studies. The goal is to exploit the smaller diffraction limit (∝λ/D) of telescopes in the optical, yet both the coherence length and time decrease at shorter wavelengths (∝λ6/5) requiring wavefront sensing at high spatial and temporal frequencies that are currently technologically challenging. This is an important developing area for the 2020s– 2030s. The panel received and reviewed RFI 2’s for GNAO (at Gemini-North) and LIGER (at Keck). GNAO development was initiated as part of the NSF-funded program GEMMA (Gemini in the Era of Multi-Messenger Astronomy) for a queue-operated MCAO facility that will deliver diffraction-limited imaging over a ~1.5 arcmin field at Gemini North. The ultimate goal of transforming Gemini-North into a full AO telescope will require deployment of an Adaptive Secondary Mirror (ASM) and integration of multi-laser guide star wave-front sensors (WFS) into the Acquisition and Guiding (A&G) unit. LIGER is an innovative next-generation AO-fed integral field spectrograph and imaging camera, fully funded through the Final Design Phase (until December 2020), and will take advantage of the NSF-funded KAPA (Keck All-Precision AO) system. With investments at the level of ~$15 million each for each project (equipment and labor), over timelines of 5–10 years to completion (2025 for LIGER, 2029 completion of ASM + A&G camera for GNAO), these programs are NSF MSRI-2 candidates. The review of the Programs (and Science) White Papers by the U.S. community, along with the RFI 2 documents, led the panel to conclude that the case for continued support of development over the next decade is strong: 1. AO/High-Contrast Imaging are key enabling technologies for high science priorities identified by the Exoplanets, Stars, and Galaxies science frontiers panels; 2. They play an essential role in boosting the scientific return and efficiency of existing facilities (e.g., Gemini, Keck, Magellan, DKIST) with modest-scale investment throughout the 2020s; 3. NSF Mid-scale program opportunities have been identified (e.g., GNAO, GmagAO-X, LIGER) to nurture existing 6–10 m class telescopes. 4. Such investments in AO systems development is a key risk mitigation strategy for ELTs, whose full resolution and sensitivity potential can only be realized with AO, and which is recognized as the most important technical risk for both GMT and TMT. K.4.3 Extreme-Precision Radial Velocities Mass is the most fundamental property of a planet, and knowledge of a planet’s mass is essential for two reasons. First, a measurement of a planet’s mass is necessary to constrain its bulk density, and in turn infer something of its composition and ultimately its formation. Second, a planet’s mass is a key input for interpreting spectroscopic features in its atmosphere, since the atmospheric scale height depends on the planetary surface gravity, in addition to the mean molecular weight and temperature. There is keen interest in studying terrestrial planets, including those orbiting at Earth-like insolations around Sun-like stars, motivating a push for mass measurements to the sensitivity required for such worlds. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-18

The National Academies Exoplanet Science Strategy (ESS) had two key findings related to precise radial velocities, both echoed by the Panel on Exoplanets, Astrobiology, and the Solar System:3 ESS Finding: The radial velocity method will continue to provide essential mass, orbit, and census information to support both transiting and directly imaged exoplanet science for the foreseeable future. ESS Finding: Radial velocity measurements are currently limited by variations in the stellar photosphere, instrumental stability and calibration, and spectral contamination from telluric lines. Progress will require new instruments installed on large telescopes, substantial allocations of observing time, advanced statistical methods for data analysis informed by theoretical modeling, and collaboration between observers, instrument builders, stellar astrophysicists, heliophysicists, and statisticians. The panel advocates that together NASA and NSF address the grand challenge of achieving the precision required to measure the masses of terrestrial planets orbiting Sun-like stars, which implies a single measurement precision of 10 cm/s and control of systematics at the level of 1 cm/s. While such measurements will be done from the ground, they are inextricably linked to the scientific success of numerous current and proposed missions, namely the legacy Kepler/K2 data set, the ongoing TESS Mission, and a future direct imaging mission. For a direct imaging mission, such precise radial velocities will identify terrestrial exoplanets orbiting nearby stars, determine when they are situated at quadrature, and remove degeneracies from the interpretation of atmospheric spectra features. NASA has tackled formidable technology challenges in the past in pursuit of its scientific goals, and the panel advocates here that this same coordinated effort be brought to the EPRV challenge. The panel concurs with the recommendation from the NAS Exoplanet Science Strategy,4 namely, ESS Recommendation: NASA and NSF should establish a strategic initiative in extremely precise radial velocities (EPRV) to develop methods and facilities for measuring the masses of temperate terrestrial planets orbiting Sun-like stars. Following this recommendation, NASA and NSF jointly commissioned a community-based Extreme Precision Radial Velocity Working Group, which recently presented the blueprint for a strategic EPRV initiative.5 K.4.4 OIR Interferometers The renaissance in stellar astrophysics in the 2010s was driven by new data and capabilities that enabled enormous advances in precision measurements of fundamental physical properties of stars: masses, radii, and luminosities. The Panel on Stars, the Sun, and Stellar Populations’s first science priority is measuring fundamental stellar properties across the H-R diagram, with an emphasis on precision stellar masses and radii. Ground-based OIR interferometry is the key enabling technology, with increased polarimetric sensitivity providing significant opportunity for new discovery. The 2010s saw significant advances in OIR interferometry, with CHARA and NPOI becoming fully operational and scientifically productive. During this same time there was steady progress in construction of the ambitious 3 National Academies of Sciences, Engineering, and Medicine, 2018, Exoplanet Science Strategy, The National Academies Press, Washington, DC. 4 National Academies of Sciences, Engineering, and Medicine, 2018, Exoplanet Science Strategy, The National Academies Press, Washington, DC. 5 See https://exoplanets.nasa.gov/exep/NNExplore/EPRV/. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-19

MROI configurable imaging interferometer. The OIR interferometry community is growing, and has presented a unified vision of how to advance science with these three complementary facilities in the coming decade. The panel received and analyzed an RFI-1 from CHARA and RFI-2s from each of NPOI and MROI. These provided further information on the technical and performance goals for the projects, and presented detailed budgets and schedules to achieve those goals during the 2020s. These formed an important component of the panel’s discussions to assess the scale of the required investments. The panel concludes that continued U.S. scientific and technical leadership in ground-based optical interferometry requires strategic investment during the 2020s. Based on the RFI responses, modest ($1 million to $2 million per year) investments would promote continued growth of the interferometry user community and fund new technology development efforts. Mid-scale investments in CHARA and NPOI at the high end of MSRI-2 would enable implementation of larger telescopes, longer baselines, and advanced beam combination technologies needed to deliver the greater angular resolution and photometric sensitivity required to achieve the Stars panel’s science goals. A final phase of the envisioned CHARA upgrade (replacement of all existing 1 m inner-array telescopes with new AO-equipped 2 m telescopes) would require an additional mid-scale investment during the 2030s. Achieving the potential of MROI would require an MREFC-level investment to bring the full 10-telescope array into science operation by the end of the 2020s. The full triad of U.S. OIR interferometers is deeply complementary: no one system can accomplish the science of all three by themselves. However, NSF is unlikely to have sufficient funds to support all three. Thus, the U.S. interferometry community and funding agencies would be best served by formulating a plan to realize this goal collectively instead of through internecine competition. This will strengthen U.S. leadership and make it an important component of a balanced U.S. ground-based OIR portfolio. K.4.5 Massively Multiplexed OIR Spectrographs There is very strong support for massively multiplexed spectroscopy across many sectors of the science community. The survey received five white papers related to the topic (SDSS-V, MegaMapper, SpecTel, DESI, and FOBOS), and this capability was highlighted in multiple science questions by the Cosmology, Compact Objects, Galaxies, ISM, and Stars panels. The survey also received an RFI 1 from Maunakea Spectroscopic Explorer (MSE) and RFI 2’s from MegaMapper and FOBOS. Furthermore, numerous past reports have heavily emphasized this capability. The support stems from the strong feeling that large-format spectroscopy is required to maximize return on investment for the large-area imagers coming online in the 2020s. Many science panels emphasized this capability. The Cosmology panel emphasized large volume large-scale structure studies and the need for photometric redshift calibration. The stars panel discussed “industrial scale spectroscopy” in the context of galactic archeology studies of dwarf galaxies, stellar streams, and the galactic halo, as well as spectroscopically exploring stars with unprecedented photometric data (e.g., from Gaia). The Panel on Galaxies considered the importance of galactic archeology as well as statistical galaxy evolution studies and IGM tomography, and the Panel on Compact Objects and Energetic Phenomena discussed transient follow-up and dynamical searches for compact objects. Massively multiplexed spectroscopy is required to fully realize the primary science goals of the VRO, the Roman Space Telescope, Gaia, and other surveys. Happily, investment in existing 2–10 m capabilities would achieve a large fraction of the main science goals through the continuation of projects like SDSS V, DESI, U.S. access to the Prime Focus Spectrograph on Subaru beyond their nominal missions, or investment in a next generation instrument for an existing telescope, like FOBOS on Keck. A dedicated facility would of course provide advantages over relying solely on existing infrastructure. Most glaring is the lack of high spectral resolution (R~20,000) multi-object spectrographs. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-20

Both the Panel on Galaxies and the Panel on Stars, the Sun, and Spectral Populations emphasized the power of this modality for galactic archeology work. MSE and SpecTel presented plans to the panel for such a mode. MegaMapper offers the widest field of view, as required for any cosmological application. Both SpecTel and MSE have larger apertures, which would be powerful for galaxy evolution and cosmological applications. Last, a Southern telescope (SpecTel or MegaMapper) will have obvious benefits for follow-up in the era of the VRO. In all cases, the United States could envision playing a significant role in these projects through a MSRI-2-level investment, which could provide up to about 20 percent of the cost of a project like MSE, SpecTel, or up to about 50 percent of MegaMapper, perhaps split with DOE. However, the time scales are such that the panel strongly encourages investment in some combination of next-generation SDSS V, DESI, and PFS through the 2020s, which could then be followed by investment in a larger dedicated facility. The panel does not believe an MREFC-level of funding is warranted in this decade. K.4.6 Solar Physics The Sun is the only star observable with high spatial resolution and the only star for which global synoptic observations are made continuously. First light of the 4 m DKIST telescope in early 2020 promises significant progress in the coming decade for understanding detailed physical processes in the photosphere and the low corona related to the causes of flux emergence, the dynamo that drives stellar activity cycles, the mechanisms of coronal heating and solar wind acceleration, the fundamental process of magnetic reconnection, including the triggers for sudden release of stored magnetic energy in the star’s atmosphere, and the effects of stellar activity on the habitability of exoplanets around stars more or less like the Sun. DKIST first-light instruments have break-through capabilities. Plans for the next generation can begin after initial observations indicate the most important capabilities for future instrumentation. To fully address the survey’s science goals for solar and stellar astrophysics, very high-resolution observations in the restricted field-of-view of DKIST would need to be supplemented by global synoptic measurements. Two critical measurement gaps exist that require investment in the coming decade: Measurements of the magnetic field in the global corona and greatly improved synoptic observations of the solar photosphere. Such observations of the Sun are also critical to the national priority to better understand and predict space weather and its effects at Earth and elsewhere. Present coronal observations are mostly limited to intensity/density. Magnetic measurements are required to develop better physics of the corona and for understanding energy storage and release related to explosive events. The Coronal Solar Magnetism Observatory (COSMO), a proposed upper mid-scale project, will measure the global corona using a 1.5 m refractive coronagraph. The project passed a preliminary design review in 2018 and can provide synoptic observations of the coronal magnetic field both above the limbs and for certain structures on the solar disk. The panel received and reviewed an RFI 1 from COSMO. The current ground-based solar synoptic network has two aging components: the Global Oscillation Network Group (GONG) and Synoptic Optical Long-term Investigations of the Sun (SOLIS). The six-site GONG network has limited capability owing to modest spatial and spectral resolution, and the more capable SOLIS instrument suite has only one site that is not currently operating. Space-based observations currently fill this gap, but have finite life expectancy and measure quantitatively the magnetic field at only one height in the atmosphere. Continuous measurements of velocity, magnetic field, and intensity at multiple heights in the photosphere and above are required. Currently a conceptual design, ngGONG would be a global network (six) of highly capable solar observatories, including coronagraphs at three sites. Program Balance: The COSMO, ngGONG, and DKIST 2nd Generation projects are expected to compete against other AST-wide projects in a more robust NSF mid-scale project line. DKIST was an MREFC project that addresses important science goals that could not be achieved in any other way. Funding DKIST operations is a burden for the facilities budget of NSF/AST, but funding for operations of PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-21

major new facilities needs to be addressed more broadly at NSF. The existing suite of U.S. solar GB-OIR observatories includes the 1.6m Goode Solar Telescope (GST, a DKIST precursor), the Mauna Loa Solar Observatory (MLSO, a COSMO precursor) and a few smaller facilities based at universities (see the National Academies 2020 Solar and Space Physics Midterm Assessment). Continued investment in these facilities provides critical measurements enabling new science, continuity of unique long-term synoptic observations, and capabilities for instrument development. Improving programmatic balance, including adequate/increased support for science analysis was a major theme of the 2013 Solar and Space Physics decadal survey. Coordination with in situ and remote-sensing space-based instrumentation including Parker Solar Probe, Solar Orbiter, and Solar Dynamics Observatory is essential to address survey goals. K.4.7 Technology Development: Astrophotonics Well established technologies developed in the past decades for fiber-optics telecommunications and other commercial applications are now emerging as potentially revolutionary strategies for a new generation of astronomical instruments. With micro-electro-mechanical-systems already adopted, for example, by the JWST, the new frontier is represented by astrophotonics, which uses fibers and optical waveguides built in solid state devices to manipulate light collected by a telescope. The current portfolio spans a wide range of devices, including multi-core fibers, photonics lanterns, photonics spectrographs, complex Bragg gratings, on-chip beam combiners, pupil remappers, and laser frequency combs. The GRAVITY instrument at VLTI and the MIRC instrument at CHARA have successfully adopted on-chip beam combiners to combine the light collected by their 4x8 m and 6x1 m telescopes, respectively. The possibility of obtaining extremely high-precision radial velocities, of the order of a 10 cm/s or better, as well as direct imaging of exoplanets—two of the main science cases for the U.S.-ELT system—may largely rely on the maturity of single-mode fibers and on-chip nulling interferometers. Strengthening the coordination between the most active astrophotonics research groups in the United States would optimize resources and facilitate the passage from laboratory research to industrial partnership. This could be done through the creation of a distributed, multi-disciplinary Institute of Astrophotonics to coordinate the teams working in this field. The more coordinated approach adopted by Europe (Germany in particular) and Australia has led to success and leadership in this field. A few tens- of-millions of dollars of funding over the next decade would be needed to significantly advance this technology and reestablish U.S. leadership in astrophotonics. Cost-savings for translational programs in astronomical applications can derive from the availability of past fundamental research, industrial prototypes, and available technological infrastructure for hardware production. The versatility, compactness and lightweight of photonics devices make them ideal for space science applications. NASA, therefore, may want to invest and support these developments. Increasing the coordination between NSF and NASA, as well as other stakeholders like DOE, NIST, and so on would create a coherent and synergistic program that optimizes resources and avoids duplications, taking into account the different strategic goals of the various agencies. K.4.8 Time-Domain Astronomy and Multi-Messenger Astrophysics In the past decade, we witnessed the birth of multi-messenger astrophysics (MMA), and the important role that ground-based OIR telescopes played in identifying and characterizing the electromagnetic counterpart to gravitational waves from a binary neutron star merger. The healthy ecosystem of optical telescopes, including NOAO-access facilities (2 m LCOGT/Faulkes, 4 m Blanco, 4.1 m SOAR, and 8m Gemini-S) enabled a prompt localization and detailed characterization of the optical/infrared kilonova light curve and its spectrum, showing the telltale signatures of the products of r- process nucleosynthesis and their subsequent radioactive decay. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-22

Heading into the next decade, we expect a rich landscape of discoveries in time domain astronomy (TDA). These range from wide-field surveys such as the NSF/DOE VRO in the optical and new wide-field capabilities in the radio, to gravitational wave observatory networks like Advanced LIGO and KAGRA, and ground-based all-sky very high-energy neutrino and gamma-ray and detectors like IceCube and HAWC, as well as space-based gamma-ray burst detectors. Opening up these new windows of discovery will place heavy demands on the ground-based OIR ecosystem, for localization, classification, and the characterization of transients. In many cases, a tiered approach to follow-up OIR observations of MMA and TDA discoveries will be required, engaging the full range of the OIR telescope ecosystem: from wide-field and/or rapid slewing 1–4 m aperture telescopes for candidate discovery and filtering, to 6.5–10 m aperture telescopes for spectroscopic classification, to an extension of VRO LSST operations for deep, wide-field target-of- opportunity gravitational-wave counterpart searches, and ultimately to ELTs for detailed characterization from spectroscopy and late-time light curve evolution. Similarly, the new landscape of TDA surveys, most notably VRO LSST, will yield a population of persistent, variable sources that will demand flexible telescope scheduling for time-sensitive observations. One of the biggest challenges in this exciting era of MMA and TDA will be the efficient, selective, and prompt allocation of follow-up observing resources. Specific needs will include (1) upgrades to enable robotic telescope operations (e.g., LCOGT); (2) coordinated, dynamic telescope scheduling software (e.g., TOMS); (3) real-time communication and data analysis infrastructure (e.g., AEON); (4) automated data reduction and calibration (e.g., using Astropy); (5) machine-learning enabled classification (e.g., Antares); and (6) systems for rapid incorporation and reprioritization from initial follow-up observations taken (e.g., TreasureMap). With investment in these cyberinfrastructure tools and robotic telescope operation and scheduling capabilities, we can fully exploit the existing ecosystem of OIR telescopes to handle this new treasure trove of transients and multi-messenger discoveries in the next decade. Furthermore, by enabling the participation of private telescopes in an NSF OIR-lab follow-up network, we can further expand U.S. OIR access and coordination for effective follow-up in the MMA and TDA era. K.4.9 The NOIRLab and the OIR System The U.S. ground-based OIR research community supports and benefits from a remarkable array of general purpose and specialized facilities of all aperture sizes in the 2 m to 10 m range. Financial support comes from a heterogeneous collection of federal, nonfederal, private, and international sources. The scientific passion and technical ingenuity that led to these capabilities enabled U.S. global leadership for more than 100 years. Since the late 1950s, this so-called OIR System has benefited greatly from NSF financial support for major observatories, telescopes, instruments, and data systems. The formation of NSF’s Optical-Infrared Astronomy Research Laboratory (NOIRLab) is a welcome, long-awaited outcome recommended in whole or part by several national reports including the 2000 and 2010 Decadal Survey reports. The newly constituted center provides (1) critical end-to-end (photons-to-science) infrastructure for wide-field optical imaging (Blanco/DECam, VRO) and optical spectroscopy (Mayall/DESI); (2) time-domain research (public brokers, SOAR, Gemini North, Gemini South, data pipelines, etc.); (3) access, visualization, and analysis of massive object catalogs with millions to billions of objects; and (4) wide-field, high-spatial resolution imaging at the 8 m scale (Gemini North and South). On behalf of NSF, NOIRLab operates and maintains mountain-based research parks in Arizona and Chile for facilities built, deployed, and operated by university-based science collaborations. It also facilitates the long-term development and retention of human capital. Indeed, many of the key managerial and technical leaders for the DKIST, VRO, GMT and TMT projects were initially trained at one of the NOIRLab constituent parts. Last, NOIRLab is the focus for strategic community discussion and planning at NSF’s behest, see Table K.3. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-23

Beyond NOIRLab, there are several facilities that are de facto national assets, with the W.M Keck Observatory being an example. Sustaining a university-based instrumentation and technology development program in support of the OIR System is critically important to retaining and supporting a technical workforce, as well as training students for careers in all branches of science, technology, and engineering. Equally important is continued support to the community for development and maintenance of data analysis and management infrastructure, from Astropy to cloud-based systems needed to work with petascale multi-dimensional object catalogs to science-case driven software systems needed for time- domain, multi-messenger research. TABLE K.3 Critical Leadership Roles NOIRLab Will Play in the Decade Ahead Coordination/leadership of the U.S.-ELT Project on behalf of the NSF investment. Efficient and effective execution of the VRO Legacy Survey of Space and Time (LSST), including provision of world-class, open access data services for the LSST data products. Efficient coordination NOIRLab facilities and data systems with other OIR System facilities to support time- domain observation research, especially in regard to rapid follow-up of short-duration phenomena such hypernovae, massive compact binary objects mergers, SMBH tidal disruption events, etc. Operation of unique capabilities within the OIR System, including wide-field optical imaging (DECam), wide- field multi-object spectroscopy (DESI), wide-field multi-conjugate and ground-layer AO imagers (GNAO/GNAOI, GEMS/GSAOI), and extreme precision radial velocity spectrometer (NEID). Execution of regular community-based strategic planning exercises in coordination with its OIR System partners for the benefit of the entire U.S. community. Contributing to the maintenance of a healthy program of technology R&D in the United States. More explicitly, NOIRLab can provide the coordination needed to support and optimally exploit the system of groups represent the U.S. excellence in design and construction of new instrumentation. Last, just as spectrum management at radio frequencies has been a key NSF mission for many years, the time may have arrived for NSF to take a more active role in managing the use of the optical window. For decades, all major OIR observatories have faced challenges from background light generated by ground-based lights of various kinds. In the past 12 months, OIR astronomy is suddenly facing an almost existential challenge from mega-constellations of satellites in Low Earth Orbit (LEO). Federal action and support are needed immediately, or an entire scientific field may be crippled in an unrecoverable way. K.5 CONCLUSIONS The exciting cases presented by the Science Panels show that even in this era of panchromatic/multi-messenger astrophysics, observations with ground-based OIR telescopes will continue to be essential for addressing many of the most important scientific questions we will face in the decades to come. These telescopes are not only invaluable in their own right, they are also essential in fully unlocking the potential for discovery and understanding provided by the other windows on the universe. The U.S. community will need to successfully deal with some extraordinary challenges if it is to maintain a position of leadership in ground-based OIR astronomy. In fact, ground-based OIR astronomy is so intimately and intricately woven into the fabric of the discipline that researchers face challenges in maintaining leadership in astronomy as a whole. These challenges are twofold: to create a U.S.-ELT Program that is fully competitive and leverages the existing bi-hemispheric investment in astronomical facilities, while at the same time, providing the resources needed to exploit the powerful suite of existing facilities in the current decade and beyond. Meeting these challenges requires a sea-change in the way the U.S. ground-based OIR community and its federal/state/private funding sources work together. Without a vigorous partnership PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-24

between the various components of this system, the United States will not be able to remain a leader. We need to learn from the past and face the future with boldness and vision. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION K-25

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We live in a time of extraordinary discovery and progress in astronomy and astrophysics. The next decade will transform our understanding of the universe and humanity's place in it. Every decade the U.S. agencies that provide primary federal funding for astronomy and astrophysics request a survey to assess the status of, and opportunities for the Nation's efforts to forward our understanding of the cosmos. Pathways to Discovery in Astronomy and Astrophysics for the 2020s identifies the most compelling science goals and presents an ambitious program of ground- and space-based activities for future investment in the next decade and beyond. The decadal survey identifies three important science themes for the next decade aimed at investigating Earth-like extrasolar planets, the most energetic processes in the universe, and the evolution of galaxies. The Astro2020 report also recommends critical near-term actions to support the foundations of the profession as well as the technologies and tools needed to carry out the science.

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