<<

Surveys DOI: 10.18727/0722-6691/5127

4MOST Consortium Survey 8: Survey (CRS)

Johan Richard 1 sion-line and quasars, totalling with spectroscopic redshift information Jean-Paul Kneib2 about 8 million objects over the redshift in order to construct the most precise Chris Blake 3 range z = 0.15 to 3.5, will allow definitive available observational tests of gravity, Anand Raichoor 2 tests of gravitational . Many key and mitigate the most significant system- Johan Comparat 4 questions will be addressed by atic effects that limit the efficacy of these Tom Shanks 5 combining CRS spectra of these targets tests, such as the calibration of photo- Jenny Sorce 1, 6 with data from current or future facilities metric and bias. Spec- Martin Sahlén 7 such as the Large Synoptic Survey troscopy of the southern hemisphere is Cullan Howlett 8 ­Telescope, the Square Kilometre Array vital to enable these advances; there is Elmo Tempel 9, 6 and the mission. currently no existing large-scale southern Richard McMahon10 hemisphere redshift survey beyond the Maciej Bilicki 11 local . DESI will survey the Boudewijn Roukema 12, 1 Scientific context ­northern sky, and the future Taipan Gal- Jon Loveday 13 axy Survey 3 and Euclid satellite will map Dan Pryer 13 A wide variety of cosmological observa- structure in the redshift ranges z < 0.2 Thomas Buchert 1 tions suggest that, in the standard inter- and 1 < z < 2, respectively, missing Cheng Zhao 2 pretation, the Universe has entered out the 0.2 < z < 1 interval which is key and the CRS team a phase of accelerating expansion pro- for tracing the physical effects of dark pelled by some form of . energy. Moreover, current deep imaging The physical nature of dark energy is not from the (DES) 4 1 Centre de Recherche Astrophysique de yet understood, and may reflect the and Kilo-Degree Survey (KiDS) 5, future Lyon, France ­general-relativistic nature of structure for- imaging by the Large Synoptic Survey 2 Laboratoire d’astrophysique, École mation, new contributions in the matter- Telescope (LSST) 6, CMB Stage 4 experi- ­Polytechnique Fédérale de Lausanne, energy sector, or new fundamental the- ments, and future radio surveys by the Switzerland ory, such as modifications to gravitational Square Kilometre Array (SKA) and its 3 Centre for Astrophysics and Super- physics on cosmic scales. Past studies ­precursors, MeerKAT and the Australian computing, Swinburne University of of the effects of dark energy have par- Square Kilometre Array Pathfinder Technology, Hawthorn, Australia ticularly focused on mapping the expan- (ASKAP), will all concern the southern 4 Max--Institut für extraterrestrische sion history of the Universe, for example, hemisphere. Southern-hemisphere spec- Physik, Garching, using baryon acoustic oscillations (BAO) troscopic follow-up with 4MOST is critical 5 Department of Physics, Durham as a standard ruler or Type Ia Superno- for successfully completing the multiple ­University, UK vae as standard candles. These probes science cases for these facilities. 6 Leibniz-Institut für Astrophysik ­Potsdam have yielded important constraints on (AIP), Germany the homogeneous expanding Universe, The 4MOST CRS will make a fundamen- 7 Department of Physics and , including ~ 1% distance measurements tal contribution to tests of gravitational Uppsala universitet, Sweden and a ~ 5% determination of the equation physics by constructing a unique red- 8 International Centre for Radio Astron- of state of dark energy. Future surveys, shift- map of the large-scale struc- omy Research/University of Western for example by the Dark Energy Spectro- ture for ~ 8 million galaxies and quasars Australia, Perth, Australia scopic Instrument (DESI) 1 or Euclid 2 in the southern hemisphere out to red- 9 Tartu Observatory, University of Tartu, will improve these distance constraints to shift z = 3.5. This map will be cross-cor- Estonia sub-percent measurements in narrow related with complementary current and 10 Institute of Astronomy, University of redshift bins. future datasets to carry out key cosmo- Cambridge, UK logical tests. The area of overlap between 11 Sterrewacht Leiden, Universiteit Leiden,­ However, in order to distinguish between CRS and lensing-quality the Netherlands the different possible manifestations of deep imaging is about three times that 12 Torun Centre for Astronomy (TCfA), dark energy, these measurements of currently planned for DESI, thus enabling Nicolaus Copernicus University, expansion must be supplemented by compelling and competitive science. 13 University of Sussex, Brighton, UK accurate observations of the gravitational growth of the inhomogeneous clumpy Universe. There are several important sig- Specific scientific goals The 4MOST Cosmology Redshift Sur- natures of gravitational physics which vey (CRS) will perform stringent cosmo- may be used for this purpose, including Testing gravitational physics with logical tests via spectroscopic cluster- the peculiar motions of galaxies or clus- ­overlapping lensing and spectroscopy ing measurements that will complement ters and the patterns of weak lensing Weak gravitational lensing and galaxy the best lensing, cosmic microwave imprinted by the deflections of light rays peculiar velocities imprinted in redshift- background and other surveys in the from either distant galaxies or the cosmic space distortions are complementary southern hemisphere. The combination microwave background (CMB). These observables for testing the cosmological of carefully selected samples of bright probes rely, to a significant degree, on model because they probe different galaxies, luminous red galaxies, emis- the cross correlation of imaging datasets ­combinations of the metric potentials.

50 The Messenger 175 – March 2019 The overlapping datasets created by the Auxiliary science Science requirements 4MOST CRS are particularly beneficial for these tests (Kirk et al., 2015) because: Large-scale structure mapping – The minimum survey area needed is (1) they allow for the additional measure- CRS will offer a spectroscopic view of 6000 square degrees. The minimum ment of galaxy-galaxy lensing, which is both large and small scales of the cosmic survey area for subject to a lower level of systematics web. In particular, its high galaxy number calibration is 1000 square degrees. than cosmic shear; (2) measurements of density will allow structural studies of – The minimum required target densities quasar magnification bias can be com- voids down to relatively small scales over for each target category (Bright Galax- pared with these other lensing measure- a wide redshift range. At larger scales, ies — BG, Luminous Red Galaxies — ments and redshift-space distortion CRS will further enable cosmological dis- LRG, Emission-Line Galaxies — ELG, ­analyses in the same volume; (3) imaging tance and effective expansion rate meas- Quasars — QSO) are defined such can mitigate key redshift-space distortion urements accurate to 1–5% to be made that clustering measurements are not systematics by constraining galaxy bias in bins of dz = 0.1 up to z = 3.5 using gal- limited by Poisson noise. models; and (4) the same density fluctua- axies and quasars, complementing DESI – We require a spectroscopic success tions generate both the lensing and clus- BAO measurements in the northern hem- rate (SSR) > 95% for BGs, > 75% for tering signatures, thus potentially reduc- isphere. The CRS Lya survey will exploit LRGs, > 80% for ELGs and > 50% for ing statistical uncertainties. the higher spectral resolution compared QSOs. to DESI (by a factor of almost 2) to meas- Source redshift distributions via ure structure in the Lya forest down to The survey area is required to be as wide cross-correlations sub-Mpc scales, allowing new limits to be as possible, a requirement driven by Weak gravitational lensing is one of the placed on warm models as ­carrying out the best measurements and most powerful and rapidly developing well as high-redshift BAO measurements covering all of the existing high-quality probes of the cosmological model, being (for example, Bautista et al., 2017). Com- imaging in the southern hemisphere from particularly advanced in the southern bined with chronometric measures of the DES and KiDS. The minimum area of sky thanks to imaging surveys such as effective expansion rate, these BAO dis- 6000 square degrees (current baseline at KiDS, DES, and LSST. A principal source tances will also provide tests of average 7500 square degrees) is based on ensur- of systematic error for cosmic shear curvature and effective expansion rate ing a much wider area (and therefore a tomography is the calibration of the consistency (for example, Clarkson et al., strong impact) for CRS compared to the source redshift distribution which enters 2008), to test the standard hypothesis planned overlap area of 3000 square the cosmological model. Different meth- that comoving space is rigid (Roukema et degrees for DESI at z < 0.7, where targets ods of calibrating this distribution exist al., 2015). have the strongest galaxy-galaxy lensing and usually they require spectroscopic signal and are best placed to lens overlap, which should, however, be deep Synergies with other surveys sources in DES and KiDS. The latter two enough. The planned 4MOST galaxy and Cross-correlation of large-scale H I inten- requirements are there to ensure sample- quasar redshift surveys will allow for sity maps across the southern sky with variance-limited measurements on large this calibration to be accomplished up to optical spectroscopy will allow the evolu- scales and for the efficiency of the sur- high redshifts for all overlapping imaging tion of the neutral hydrogen content of vey; these are based on previous experi- surveys in the southern sky. galaxies to be mapped in detail, paving ments with similar target types (for exam- the way to surveys with the SKA (Wolz ple, eBOSS). Synergies with CMB experiments et al., 2017). CRS cross-correlations with As the only planned large southern spec- overlapping optical and eROSITA X-ray troscopic survey at intermediate red- imaging will allow us respectively to Target selection and survey area shifts, 4MOST is uniquely positioned for measure the effect of quasar feedback synergies with CMB Stage 4 experiments on the local clustering environment and Cross-correlation with deep lensing and mapping the CMB across the southern to investigate novel routes to cosmological CMB surveys motivates the use of LRG hemisphere with unprecedented resolu- parameters (for example, Risaliti & Lusso, as the most efficient tracers of large- tion and accuracy. The CMB contains 2018). CRS, in conjunction with the scale structure with the maximal lensing a wealth of information about the late- 4MOST TIDES Survey (Survey 10; Swann imprint. These targets should span a time cosmic evolution through its interac- et al., p. 58), can map the host-galaxy range of redshifts where the lensing tions with the large-scale structure. redshifts of a significant population of geometry is most efficient and cosmolog- Of particular importance are the Sunyaev-­ SNe discovered by LSST, allowing pre- ical physics is dark-energy dominated, Zel’dovich and integrated Sachs-Wolfe cise gravitational tests using peculiar i.e., z < 0.7. Photometric redshift calibra- effects, and weak gravitational lensing of velocities (Howlett et al., 2017). CRS will tion by cross-correlation requires full red- the CMB. CRS will provide growth-rate also be a valuable tool to follow up the shift coverage to the limit of the source measurements by allowing cross-correla- numerous galaxy-galaxy strong lensing sample, where each target class covers tion with spectroscopically confirmed events found by Euclid and LSST (Collett, at least 1000 square degrees (Newman et targets. 2015), which can be used as probes al., 2015). for the dark matter distribution at galactic scales.

The Messenger 175 – March 2019 51 Surveys Richard J. et al., 4MOST Consortium Survey 8

Table 1. Properties of each target category in CRS.

Name z Selected (AB) R-band Sky area Density Colour Redshift Number of targets magnitude range (magnitude [AB]) (deg2) (deg2) selection completeness (106) BG 0.15–0.4 16 < J < 18 20.2 ± 0.4 7500 250 J–Ks, J–W1 95% 1.88 LRG 0.4–0.7 18.0 < J < 19.5 21.8 ± 0.7 7500 400 J–Ks, J–W1 75% 3.00 ELG 0.6–1.1 21.0 < g < 23.2 23.9 ± 0.3 1000 1200 g–r, r–i 80% 1.20 QSO 0.9–2.2 g < 22.5 22.2 ± 0.7 7500 190 g–i,i–W1,W1–W2 65% 1.43 QSO-Lya 2.2–3.5 r < 22.7 22.2 ± 0.7 7500 50 g–i,i–W1,W1–W2 90% 0.38

10

0 1000

–10 –2

–20 ee e) re

100 degr eg –30 r pe

n (d io –40 ounts c

Declin at –50

4MOST/CRS 10 Object DESI –60 Euclid DES –70 ATLAS not DES KiDS –80 1 18 h 12h 6h 0h 24h Right ascension (degree)

Figure 1 (above). Footprints of the discussed imaging BG (250/deg2) surveys and target densities from mock catalogues. The CRS area (7500 square degrees), demarcated LRG (400/deg2) by a thick cyan line, consists of DES and VST-ATLAS ELG (1200/deg2) excluding DESI and of the two main KiDS regions. QSO+Lyα (240/deg2) The 1000 square degrees covering ELGs is shaded in yellow, with a higher target density. 102 2 g de N/

101

Figure 2 (left). The expected redshift distributions for the different tracers. These are obtained by 10 0 applying our target selection on real data, using the HSC photometric­ redshifts for the BG/LRG/ELG, 0.0 0.51.0 1.52.0 2.53.0 and using SDSS DR14 spectroscopic redshifts for Redshift the QSO.

52 The Messenger 175 – March 2019 BG (16.0 < VHSJ < 18.0) LRG (18.0 < VHSJ < 19.5) ELG (22.0 < DESg < 23.2) 1.5 1.5 2.0 z = 0 (star) z = 0 (star) z = 0 (star) phot phot phot 0 < z < 0.4 0 < z < 0.7 1.0 0.005 < z < 0.4 1.0 phot phot phot 0.4 < z < 0.8 0.7 < z < 1.1 0.4 < z phot 1.5 phot phot 0.8 < z 1.1 < z 0.5 0.5 phot phot % i W1 W1 S E E 1.0 70 0.0 0.0 – DE r – WIS – WIS J J –0.5 –0.5 S S S 30% 0.5 70% DE

VH VH % 30% % –1.0 –1.0 % 90 90 70 0.0 –1.5 –1.5

–2.0 –2.0 –0.5 –1.0 –0.5 0.0 0.51.0 1.5 –1.0 –0.5 0.00.5 1.01.5 –0.5 0.00.5 1.01.5 2.0

VHSJ – VHSK VHSJ – VHSK DESg – DESr

Figure 3. Colour selection for the BG (left), LRG metric selection. In addition, colour area, which is the main criterion for high (middle), and ELG (right), using real data (VHS, DES, selections are applied to each target accuracy in clustering, as well as the high and the CFHT Legacy Survey photometric redshifts). For each tracer, we display the typical loci of the based on empirical regions in the colour- total number of targets N. Each part is objects passing the magnitude cut reported in the colour diagrams.­ The colour selections equally accounted linearly in the figure of title: grey contours are for stars, blue/green/red con- are based on the availability of the rele- merit calculation. tours are for objects with redshifts lower/within/ vant filters in the imaging data contained higher than the aimed redshift range; our selections are shown using black semi-transparent dots. in each region (combining DES as well as the VISTA Hemisphere Survey [VHS] Acknowledgements and WISE). The selections foreseen are We acknowledge support from the French Pro- tuned to obtain the desired target den- gramme National Cosmologie Galaxies (PNCG), Targets from the CRS are therefore sity, ­maximising the fraction of targets in the ERC starting Grant 336736-CALENDS and the divided into the following subcategories: the desired redshift range and favouring ERC advanced Grant 740021-ARTHUS. BG, LRG, ELG, QSO, including quasars a certain type of objects (red for BG and probed through their Lyman-forest at LRG, blue for ELG, see Figure 3). References z > 2.2 (QSO-Lya). This allows the survey to cover targets at all redshifts from z = 0 Bautista, J. E. et al. 2017, A&A, 603, 12 to z = 3.5 (Figure 2). Table 1 summarises Spectral success criteria and figure of Clarkson, C. et al. 2008, Physical Review Letters, 101, 011301 the main properties of the magnitude and merit Collett, T. E. 2015, ApJ, 811, 20 colour selections. Comparat, J. et al. 2016, A&A, 592, 121 We use the following spectral success Howlett, C. et al. 2017, ApJ, 847, 128 There are two main survey regions: criteria to estimate the usefulness of a Kirk, D. et al. 2015, MNRAS, 451, 4424 Newman, J. A. et al. 2015, Astroparticle Physics, one larger area of 7500 square degrees given target to achieve our science goals: 63, 81 for BG, LRG, QSO and QSO-Lya targets, – BG and LRG: median signal-to-noise Risaliti, G. & Lusso, E. 2018, Nature Astronomy, and a smaller region of 1000 square S/N > 1 per Å in continuum region arXiv:1811.02590 degrees for ELGs (included in the larger 4000–8000 Å. Roukema, B. F. et al. 2015, MNRAS, 448, 1660 Wolz, L. et al. 2017, MNRAS, 470, 3220 one). The baseline sky area (7500 square – ELG: S/N > 0.5 per Å in continuum degrees) for CRS is constructed by com- region near 6700 Å or 9000 Å. bining the DES, KiDS and VST-ATLAS – QSO low-z: S/N > 1 per Å in continuum Links area which are not covered­ by DESI (Fig- region near 6700 Å. 1 Dark Energy Spectroscopic Instrument (DESI): ure 1). The 1000-square-degrees area – QSO Lyman-alpha: S/N > 0.1 per Å in www.desi.lbl.gov for ELG targets is chosen within the best Lyman-alpha forest. 2 Euclid: https://www.euclid-ec.org quality imaging region (KiDS-S and DES, 3 Taipan Galaxy Survey: Figure 1). There is almost no overlap in These spectral success criteria are very https://www.taipan-survey.org 4 Dark Energy Survey (DES): ELG targets with the 4MOST WAVES Sur- similar to the ones used for the eBOSS https://www.darkenergysurvey.org vey (Driver et al., p. 46), which targets survey (for example, Comparat et al., 5 Kilo-Degree Survey (KiDS): lower redshift sources. 2016) and correspond to our goal of http://kids.strw.leidenuniv.nl 6 reaching a certain redshift completeness Large Synoptic Survey Telescope (LSST): https://www.lsst.org To achieve the 4MOST CRS science at the faintest magnitudes (Table 1). goals, it is important to reach a suffi- ciently large target density in each target The figure of merit accounts for the category. This density directly translates achieved surface density of successful into a magnitude range in the photo­ targets and its homogeneity over a large

The Messenger 175 – March 2019 53