Finding: The Euclid mission promises to make important contributions to probing dark energy and to the measurement of cosmological parameters.
The Euclid payload includes a 1.2-m obscured-aperture telescope, an optical array of 36 4k x 4k charge-coupled devices (CCDs) with 0.1 arcsec pixels, and an infrared array with 16 2k x 2k HgCdTe sensors with 0.3 arcsec pixels. These arrays have fields of view of 0.56 square degrees and 0.55 square degrees. A filter wheel holds a grism for spectral resolution R ~ 250 spectroscopy in addition to the nearinfrared Y-, J-, and H-band imaging filters.7 The core science observations of the mission are planned to take 6.25 years. The European Space Agency (ESA) plans to launch Euclid in late 2019.8
Euclid will use both weak gravitational lensing and the large-scale distribution of galaxies (including baryon acoustic oscillations, BAO) to measure the geometry of the universe and a combination of weak gravitational lensing, redshift-space distortions, and galaxy cluster measurements to probe the growth rate of structure. Together, measurements of the growth of structure and the distance-redshift relation will constrain the nature of dark energy and the physical origin of cosmic acceleration. Euclid measurements should represent a very significant advance over current measurements of structure growth and the geometry of the universe. They have the potential to revolutionize our understanding of dark energy.
Euclid plans to carry out optical plus near-infrared (NIR) imaging observations over an area of 15,000 square degrees (NIR limiting magnitudes YJHAB = 24) in order to obtain weak gravitational lensing shear measurements of ~1.5 billion galaxies. The statistical power of this technique to probe dark energy derives from the ability to measure cosmic shear in tomographic redshift slices. To maximize weak gravitational lensing signal-to-noise, Euclid visual imaging will be carried out in a single, broad optical band. Weak lensing measurements require both excellent image quality and very large uniform samples (ideally hundreds of millions) of galaxies. Euclid will be the first mission to attempt these challenging measurements in space for large numbers of galaxies and as such represents a significant advance in techniques for studying cosmic acceleration. Euclid’s combined visible and near-infrared survey will have no precedent and will likely constitute the premier space-based dark energy survey until the Wide-Field Infrared Survey Telescope (WFIRST) is launched.
Finding: The Euclid mission will provide a valuable legacy of survey science data. These data can be expected to be a unique resource for the astronomical community.
Beyond its cosmological studies using BAO and weak-lensing measurements, Euclid will provide the astronomical community with extremely valuable survey data that can be exploited for a wide range of scientific investigations. Two surveys are planned for the Euclid mission:9 a wide survey covering 15,000 square degrees of sky and a deep survey covering nominally 40 square degrees studying the
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2 Findings THE PROPOSED EUCLID MISSION Finding: The Euclid mission promises to make important contributions to probing dark energy and to the measurement of cosmological parameters. The Euclid payload includes a 1.2-m obscured-aperture telescope, an optical array of 36 4k x 4k charge-coupled devices (CCDs) with 0.1 arcsec pixels, and an infrared array with 16 2k x 2k HgCdTe sensors with 0.3 arcsec pixels. These arrays have fields of view of 0.56 square degrees and 0.55 square degrees. A filter wheel holds a grism for spectral resolution R ~ 250 spectroscopy in addition to the near- infrared Y-, J-, and H-band imaging filters.7 The core science observations of the mission are planned to take 6.25 years. The European Space Agency (ESA) plans to launch Euclid in late 2019.8 Euclid will use both weak gravitational lensing and the large-scale distribution of galaxies (including baryon acoustic oscillations, BAO) to measure the geometry of the universe and a combination of weak gravitational lensing, redshift-space distortions, and galaxy cluster measurements to probe the growth rate of structure. Together, measurements of the growth of structure and the distance-redshift relation will constrain the nature of dark energy and the physical origin of cosmic acceleration. Euclid measurements should represent a very significant advance over current measurements of structure growth and the geometry of the universe. They have the potential to revolutionize our understanding of dark energy. Euclid plans to carry out optical plus near-infrared (NIR) imaging observations over an area of 15,000 square degrees (NIR limiting magnitudes YJHAB = 24) in order to obtain weak gravitational lensing shear measurements of ~1.5 billion galaxies. The statistical power of this technique to probe dark energy derives from the ability to measure cosmic shear in tomographic redshift slices. To maximize weak gravitational lensing signal-to-noise, Euclid visual imaging will be carried out in a single, broad optical band. Weak lensing measurements require both excellent image quality and very large uniform samples (ideally hundreds of millions) of galaxies. Euclid will be the first mission to attempt these challenging measurements in space for large numbers of galaxies and as such represents a significant advance in techniques for studying cosmic acceleration. Euclid’s combined visible and near-infrared survey will have no precedent and will likely constitute the premier space-based dark energy survey until the Wide-Field Infrared Survey Telescope (WFIRST) is launched. Finding: The Euclid mission will provide a valuable legacy of survey science data. These data can be expected to be a unique resource for the astronomical community. Beyond its cosmological studies using BAO and weak-lensing measurements, Euclid will provide the astronomical community with extremely valuable survey data that can be exploited for a wide range of scientific investigations. Two surveys are planned for the Euclid mission:9 a wide survey covering 15,000 square degrees of sky and a deep survey covering nominally 40 square degrees studying the 4
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redshift 6-8 universe. Both surveys will be carried out in visible and near-infrared bands and also include spectroscopic measurements in the near-infrared. Figure 2.1 indicates the sensitivities for the Euclid wide survey; the deep survey will go approximately 6 times (2 magnitudes) fainter. The Euclid Wide Survey is expected to cover a larger area than will the WFIRST survey. The Euclid surveys will be unprecedented, producing a massive data set of deep images and spectra covering a significant fraction of the sky. The Euclid Wide Survey will provide images of billions of galaxies at redshifts of 1 < z < 3 and near-infrared spectroscopy of tens of millions of galaxies, thereby allowing determination of their redshifts. Its 0.13 arcsecond spatial resolution in the visible will be a significant improvement over ground-based surveys whose resolution is limited because of the atmosphere. The Euclid Deep Survey is designed primarily to detect and study a sample of high-redshift star-forming galaxies at 6 < z < 8, thus providing new insights into galaxy formation and evolution at early epochs.10 The Euclid surveys will be a unique resource for the astronomical community, not unlike the earlier Sloan Digital Sky Survey, and can be expected to impact all areas of astronomical research. In addition to the obvious impact on studies of galaxies at moderate and high redshifts, the Euclid surveys will also provide a vast catalogue of stars in our own Galaxy and in nearby galaxies, expected to make important contributions to studies of both galaxy structure and stellar populations. U.S. participation in Euclid through the Euclid Science Team (EST) and Euclid Consortium (EC) could be very beneficial for the development of the tools and expertise within the U.S. astronomical community needed to effectively exploit the Euclid survey data when it becomes public, nominally 14 months after the start of the survey. Access to and widespread use of the Euclid survey data will help leverage the considerable investment made by the United States in both JWST and large ground-based projects such as Large Synoptic Survey Telescope (LSST) and the Atacama Large Millimeter Array. THE PROPOSED WFIRST MISSION Finding: The Wide Field Infrared Survey Telescope (WFIRST) is a highly capable mission with an exciting and broad scientific program spanning exoplanets to cosmology to infrared surveys. NWNH envisaged WFIRST to be a 1.5-meter obscured-aperture telescope; recent work by the Science Definition Team (SDT) has suggested an alternate 1.3-meter off-axis telescope design, with equivalent detecting area but a more compact point spread function.11 In this new candidate design, the focal plane consists of 36 2k x 2k HgCdTe infrared detectors, of which 8 are dedicated to distinct spectroscopic channels with 0.45 arcseconds per pixel and spectral resolution R ~ 200.12 The imaging scale is approximately 0.18 arcseconds per pixel with a 0.291 degree field of view. Spectroscopy is always enabled in parallel with the imaging arrays and covers 1.1 to 2.0 microns. A filter wheel for the imaging array also includes six filters and an additional R ~ 75 prism. WFIRST can take images and spectra over the 0.6 to 2.0 micron wavelength range. WFIRST enables a broad spectrum of science and the large discovery potential of a guest observer program. The Euclid and WFIRST missions have different objectives, different hardware, and different survey designs. Euclid will make high-resolution optical images that are the key observation for its core program of using weak lensing to study dark energy. WFIRST use its near-infrared observations to address the broad research program outlined in NWNH. COMPARISON OF WFIRST AND EUCLID CAPABILITIES Finding: Both Euclid and WFIRST should make important contributions to the understanding of cosmic acceleration. While Euclid should advance our understanding of 5
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dark energy, WFIRST has the more robust and powerful approach. WFIRST should make significant advances in dark energy research beyond Euclid’s own contributions. In the report of the NWNH Science Frontiers Panel on Cosmology and Fundamental Physics, that panel noted that the key requirements for studying cosmic acceleration were measurements over a wide redshift range, rather than an emphasis on a redshift z < 1.13 There have been no observational results since NWNH that argue for a major deviation from that panel’s conclusions. There are three primary, complementary techniques for studying dark energy: weak gravitational lensing, three-dimensional clustering of galaxies, and supernova measurements. Only WFIRST will use all three, including the supernova technique used by U.S. astronomers and their colleagues in their Nobel Prize-winning discovery of cosmic acceleration. Supernova measurements complement the other two techniques and currently provide the strongest dark energy constraints; all three are needed for a robust dark energy program. Both Euclid and WFIRST plan to use measurements of the three-dimensional distribution of galaxies to trace the geometry of the universe. During the first 400,000 years after the Big Bang, sound waves in the cosmic fluid imprint a structure known as baryon acoustic oscillations in the three- dimensional distribution of galaxies. Measurements of these BAO features provide a “standard ruler” for measuring the expansion rate of the universe as a function of time (or equivalently, redshift). According to the current plans of both missions, the Euclid mission would cover a wider area of the sky than the WFIRST mission.14 Because the WFIRST mission delivers a higher number density of galaxies, the effective volume of its survey is larger; thus it will be better able to make the key measurement of the relationship between distance and redshift (z). The proposed WFIRST BAO survey will make significantly more accurate distance measurements than the Euclid BAO survey at redshifts z > 1, when the universe was approximately half its present age, and nearly an order of magnitude better at z = 2.15 With its higher number density of galaxies, WFIRST will be better able to use redshift distortions to measure the growth rate of structure. The combination of growth rate measurements and geometry measurements tests whether cosmic acceleration is due to dark energy or the breakdown of general relativity. Weak gravitational lensing, the distortion of the observed shapes of distant galaxies due to the bending of light rays by the clumpy distribution of matter, is a potentially powerful technique for tracing its large-scale distribution. The measurement of weak gravitational lensing is a primary objective of both the Euclid and the WFIRST missions. For Euclid this involves measuring the shapes of galaxies with optical CCDs. WFIRST measures the shapes of galaxies in the infrared, where galaxies are brighter and smoother. The WFIRST reference mission, as described in the interim report of the WFIRST SDT,16 allocates 1 year of survey time for a wide-area weak gravitational lensing program in comparison to 6.25 years for Euclid. If limited by statistical errors, then the expected weak gravitational lensing dark energy science return will be somewhat greater from Euclid than WFIRST.17 The weak gravitational lensing signal is very subtle, however, and its detection is vulnerable to systematic errors introduced by the instrument,18 by the atmosphere (when observed from the ground), and by astrophysical effects such as intrinsic alignments.19 The WFIRST multiband approach to weak gravitational lensing is more robust than Euclid’s single very broad band, which is potentially vulnerable to galaxy color gradients.20,21 Because WFIRST measures lensing in three passbands, its data can be internally cross-correlated to help mitigate systematic measurement error. Since the WFIRST approach to weak gravitational lensing measurement appears to be more robust, it may produce better constraints on dark energy properties. Euclid’s and WFIRST’s measurements are not duplicative and the combinations will be more powerful than any single measurement. Combining WFIRST with Euclid and with ground-based data sets, such as that expected from LSST,22 should further enable astronomers to address the systematic challenges that previous ground-based weak gravitational lensing measurements have experienced.23 These combined data sets will likely overcome systematic limitations and realize the full potential of this powerful technique. 6
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Finding: Euclid and WFIRST both will use a combination of ground-based optical measurements and space-based near-infrared measurements to determine the distances of galaxies observed in their weak gravitational lensing surveys. An essential part of any weak gravitational lensing observation is an approximate determination of the distance (more precisely the redshift) to the lensed galaxy. Both Euclid and WFIRST must augment their space-based near-infrared galaxy observations with ground-based optical data in order to obtain approximate estimates of distance known as photometric redshifts (photo-z’s). The Euclid mission plans to obtain the needed multiband optical data over its survey area from a combination of ground-based observatories. A number of facilities are carrying out, planning, or considering wide-area, multiband, optical surveys to a depth useful for Euclid photo-z’s, including Pan- STARRS,24 Dark Energy Survey (DES),25 LSST,26 KIDS,27 HyperSubprimeCam,28 and the William Herschel Telescope.29 The first three of these are U.S.-led projects, and each has had independent discussions with the Euclid mission about the possibility of collaboration. In the same timeframe as Euclid and WFIRST, LSST should also carry out its deeper optical survey. There are clear, but notably asymmetric, potential synergies between these projects and Euclid: multiband optical data from the ground are necessary for Euclid to meet its weak gravitational lensing science requirements; on the other hand, while Euclid data would enhance the science reach of the ground-based surveys, those data are not seen as necessary to achieving the primary science goals of those surveys. These same considerations apply to WFIRST and the ground-based surveys. Finding: Critical elements of the WFIRST mission are outside the scope of Euclid’s core science mission. WFIRST’s guest observer program will enable a broad range of astronomical science; its gravitational microlensing survey for exoplanets will provide an essential complement to Kepler30 in detecting Earth-mass planets at a wide range of separations, from the habitable zone to infinity; and its deep infrared surveys are well matched to the LSST optical survey. Among the other unique aspects of the WFIRST mission are its supernovae survey and its survey of the galactic plane. Surveys of large areas of the sky enable a wide range of astronomical studies. During the past decade, the Sloan Digital Sky Survey (SDSS), a ground-based optical survey, was by some metrics the most productive astronomical instrument in the world.31 U.S. astronomers are leading several deeper optical surveys (Pan-STARRS and DES) that will map the sky much more sensitively than SDSS. The NWNH’s top priority for large ground-based astronomy projects is the LSST, which will undertake a very deep optical survey of the sky (as well as time domain studies). While optical surveys are essential for many areas of science, much of today’s forefront astronomical science requires NIR data to further understanding. Earth’s atmosphere is quite bright in the NIR (in addition to blurring the images), which significantly hampers ground-based NIR observations. While Euclid’s infrared imaging data will enable an important wide survey, its 0.3 arcsecond pixels do not fully sample its point spread function and effectively smear out galaxy images. WFIRST takes full advantage of the space environment by surveying the sky in three NIR filters at high resolution with a well-sampled point spread function. As shown in Figure 2.1, WFIRST’s NIR surveys are substantially deeper than Euclid’s (with higher spatial resolution) and, importantly, are well matched in depth to the optical surveys planned by the LSST, although they cover less area. WFIRST should, for instance, revolutionize studies of galaxy formation and evolution during the epoch when galaxies such as the Milky Way were most vigorously forming stars.32 WFIRST will map the structure of the Galaxy using red giant clump stars as tracers and will probe the epoch of reionization by detecting bright quasars and the most massive galaxies shining less than half a billion years after the Big Bang. The versatility and sensitivity of WFIRST’s wide-field NIR camera and spectrometers are crucial for enabling a robust Guest Observer program, with at least 10 percent of the mission lifetime available to the community through peer- reviewed, open competition. 7
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FIGURE 2.1 The point source sensitivity of various planned weak gravitational lensing surveys: LSST (red), WFIRST deep (blue) and Euclid wide (green). Magnitudes are defined so that fainter objects have larger magnitudes. LSST, Euclid’s broad band optical images, and WFIRST deep survey will be able to see fainter galaxies than Euclid’s wide survey. The WFIRST deep survey goes deeper, but the Euclid survey covers a much larger area. SOURCE: Courtesy of Chris Hirata, California Institute of Technology. WFIRST with its three infrared colors and optimized survey capability will be an excellent tool for a variety of modest area (a few square degrees) Guest Observer projects such as mapping galaxy clusters, mapping nearby galaxies much more deeply and with much better spatial resolution than a variety of current Milky Way observational projects. As is usual for new capabilities such as WFIRST will offer, the Guest Observer program will undoubtedly lead to discoveries in unanticipated areas. WFIRST should also significantly advance the census of exoplanets through gravitational microlensing searches. The field of exoplanets has enjoyed rapid and exciting progress during the past decade, with NASA’s recently launched Kepler mission providing the observations that are currently driving the field. Kepler is sensitive to planets in orbits whose semi-major axis are comparable to or smaller than 1 AU, Earth’s distance from the Sun. WFIRST’s gravitational microlensing search will detect planets in orbits not accessible to Kepler. Specifically, the gravitational microlensing technique is sensitive to planets in orbits with semi-major axes larger than ~1 AU, where Kepler’s transit technique is less effective.33 The combination of Kepler and WFIRST survey data will allow a comprehensive census of the prevalence of planets in orbits ranging from very close to their parent stars out to and beyond the “snow line,” where ice formed in the proto-planetary disk.34 WFIRST will also determine the space density of free-floating planets (i.e., planets not bound to individual stars). Euclid does not currently have plans to conduct a search for exoplanets, and, in any case, the Euclid telescope’s small field of regard (i.e., viewable sky angle) would make gravitational microlensing searches with Euclid inefficient. 8
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PREVIOUS NASA-ESA COLLABORATIONS Finding: Previous NASA and ESA collaborations on complementary missions have been successful and mutually beneficial. Previous ESA and NASA astrophysics missions have often featured an ESA mission followed by a NASA mission, and vice versa. ISO (the Infrared Space Observatory) was followed by SIRTF (later the Spitzer Space Telescope);35 XMM (later XMM-Newton) and the AXAF (later the Chandra X-ray Observatory) launched the same year;36 and ESA’s Planck mission is now following NASA’s WMAP. In the case of XMM-Newton, the small investment of NASA hardware in the ESA mission contributed to the success of that mission and led directly to opportunities for U.S. scientists to participate in the science from that mission. Likewise, U.S. scientist participation in the ISO instrument teams and NASA’s contribution of DSN (Deep Space Network) time led to significant opportunities for U.S. scientists using ISO data. Currently, U.S. scientists are contributing about 20-25% of the Planck analysis.37 In each instance, the later missions have had unique and complementary science capabilities that have led to major new discoveries. In many cases, the involvement of U.S. scientists in ESA missions enhanced the scientific productivity of the NASA missions. There is a risk of cost growth associated with any collaboration. NASA’s initial involvement in Planck was at the level now proposed for Euclid. It has grown by an order of magnitude and is now comparable to NASA’s investment in WMAP.38 The Implementation Panel rejected a U.S. involvement in Euclid at this higher level. DESCRIPTION OF A PROPOSED PLAN FOR A U.S. CONTRIBUTION TO THE ESA EUCLID MISSION Finding: NASA could make modest but important hardware contributions to the Euclid mission. The discussions between NASA and ESA have identified three specific options for the U.S. hardware contribution to the Euclid mission: near-infrared detector arrays, reaction wheels, or the filter wheel.39 Corresponding preliminary rough cost estimates provided by NASA to the committee are, in order, $20 million to $25 million, $10 million to $15 million, and $15 million to $20 million (in 2012 U.S. dollars).40 These are pure hardware costs, without any testing or integration. U.S. industries are the world leaders in the development and manufacture of NIR detectors. While the expertise in the U.S. academic and laboratory community in testing and integrating NIR detectors for astrophysics missions could be valuable to employ in this context, it would approximately double the cost to NASA.41 A U.S. manufactured filter wheel, based on technologies developed for the Hubble Space Telescope, would enable the Euclid telescope to continue its optical observations while the infrared filters are being switched. Reaction wheels would provide enhanced guiding and stabilization for the Euclid spacecraft, compared with the current mission specification of using cold gas thrusters. This would increase the survey efficiency by reducing slew and settling time. A preliminary estimate indicated an overall increase in survey efficiency of approximately 10 percent.42 Finding: ESA has offered the United States an opportunity to participate as a partner in Euclid, including membership on the Euclid Science Team. This opportunity would enhance the involvement of the U.S. community in Euclid and improve the ability of the U.S. community to utilize the Euclid data when they become publicly available. 9
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The Euclid science effort is organized into two main bodies: the EST, which currently has 12 members, and the Euclid Consortium, a much larger group comprising more than 900 members from over a dozen countries, with an EC Board as its governing body. The EST includes the Euclid Project Scientist, nine members of the EC Board, and two legacy scientist positions; one of these legacy scientist positions is currently unassigned. The EST is responsible for defining science requirements and is the Euclid scientific advisory body to ESA. The EC is responsible for the visible and infrared instruments on Euclid, data processing, science products, science exploitation, external ancillary data, and simulations.43 Given the EC’s responsibilities, a NASA-appointed position to both the EST and the EC Board (9 of the current 12 EST members have appointments in both44) could be important to facilitate the U.S. science return from Euclid. Previous experience shows that U.S. involvement in science teams has enhanced the U.S. science return from European-led missions.45 Euclid will release all of its data after its proprietary period—quick- release data will be released 14 months after the start of the survey, and products from all levels will be released 26 months after the start of the survey.46 If NASA is a partner, it will support making these data available to the U.S. community47per NASA policy. With this partnership, U.S. members of the EST and Euclid Consortium members should be able to obtain a deeper understanding of the instrument. This should enhance their ability to carry out analyses of the Euclid data and to combine these data with other surveys.48 PROGRAMMATIC CONSIDERATIONS Finding: NASA has stated that participation in Euclid at the proposed modest level would not delay WFIRST. NASA presented a plan to the committee for a near-term (fiscal year (FY) 2013-2014) hardware contribution to Euclid with a value of up to $20 million to $30 million (in U.S. dollars). In his presentation to the committee, Astrophysics Division Acting Director Paul Hertz stated that this amount would constitute (and be reallocated from) about 20 percent of the funds planned within the run-out of the president’s FY2012 budget request for allocation by the division for NWNH priorities in the same timeframe and would thus have a modest albeit non-negligible impact on those programs. Dr. Hertz also stated that expenditure of these funds on Euclid hardware would not impact the launch schedule for WFIRST, since according to current NASA budget projections significant expenditure on the WFIRST mission would only commence in the 2017-2018 timeframe, when JWST construction spending rolls off. As part of its participation in Euclid, NASA would also support a science team at the $1.5- to $3-million- per-year level expected in the 2016-2025 timeframe.49 Finding: Despite its priority in NWNH, WFIRST has only very low-level support for pre-phase-A studies. While WFIRST does not present major technical challenges, thorough mission studies would enable the mission to move forward quickly once funding is available. WFIRST lacks any grants program that supports mission studies (such as scientific mission simulations) outside the U.S. NASA centers.50 10