1. Massive Spectroscopy for Dark Energy in the South Josh Frieman MS-DESI Meeting, LBNL, March 2013 Some details in DESpec White Paper arXiv: 1209.2451 (Abdalla, etal)

2. 2 Motivation What is the physical cause of cosmic acceleration? –Dark Energy or modification of General Relativity? If Dark Energy, is it Λ (the vacuum) or something else? –What is the DE equation of state parameter w? The DE program would be substantially enhanced by a massive redshift survey that optimally synergizes (overlaps) with the DES imaging survey and by a larger redshift survey of galaxies with LSST imaging, capitalizing on those investments. Maximum overlap requires a southern site.

3. 3 The Dark Energy Survey Survey project using 4 complementary techniques: I. Cluster Counts II. Weak Lensing III. Large-scale Structure IV. Supernovae Two multiband imaging surveys: 5000 deg 2 grizY to 24th mag 30 deg 2 time-domain griz (SNe) New 3 deg 2 FOV, 570 Megapixel camera on the Blanco 4m Survey 2013-2018 (525 nights) Premiere facility instrument for astronomy community DECam on the Blanco 4m at CTIO

4. 4 DES Science Summary Four Probes of Dark Energy Galaxy Clusters ~100,000 clusters to z>1 Synergy with SPT, VHS Sensitive to growth of structure and geometry Weak Lensing Shape measurements of 200 million galaxies Sensitive to growth of structure and geometry Baryon Acoustic Oscillations 300 million galaxies to z = 1 and beyond Sensitive to geometry Supernovae 30 sq deg time-domain survey ~4000 well-sampled SNe Ia to z ~1 Sensitive to geometry Forecast Constraints on DE Equation of State Factor 3-5 improvement over Stage II DETF Figure of Merit Planck prior assumed DES

5. 5 Benefits of “Same Sky” Deep Photometry and MS-DESI Spectroscopy Maximize Redshift-Space Distortion + Weak Lensing Probe of Dark Energy vs. Modified Gravity Reduce Photo-z and other systematic errors of DES and LSST, enhancing their DE reach –angular cross-correlation method Deep, uniform imaging (DES/LSST) improves MS- DESI target selection efficiency & control –sculpt redshift distributions for maximum DE reach

6. 6 Probe dark energy through the history of the expansion rate: and the growth of large-scale structure: Weak Lensing cosmic shear Distances+growth Imaging Supernovae Distances Imaging Cluster counts Distances+growth Imaging Spectroscopy Baryon Acoustic Oscillations Distances and H(z) Imaging Spectroscopy Redshift Space Distortions Growth Spectroscopy What can we probe? Distance vs. Redshift Growth of Density Perturbations

7. 7 Massive Spectroscopy of DES and LSST Targets Enables New and Improved DE Probes Weak Lensing and Redshift-Space Distortions –Powerful new test of Dark Energy vs Modified Gravity Galaxy Clustering –Radial BAO for H(z) and improved D A (z) Photometric Redshift Calibration –Determine DES and LSST N(z) from angular correlation, improve DE constraints from all methods in these imaging surveys Galaxy clusters –Dynamical masses for DES/LSST clusters from velocity dispersions, reduce the major cluster DE systematic Weak Lensing –Reduce systematics from intrinsic alignments for DES/LSST Supernovae –Reduce systematics from host-galaxy typing for DES/LSST

8. 8 Massive Spectroscopic Surveys in the Southern Hemisphere with MS-DESI 8-million Galaxy Redshift Survey in ~250 nights –Uniformly selected from deep, homogeneous DES+VHS imaging over 5000 sq. deg. (2018+) 15-million GRS in additional ~500 nights –Uniformly selected from deep, homogeneous LSST imaging of additional 10,000 sq. deg. (2021+) or from DES extension

9. 9 Slide from Enrique Gaztanaga RSD, BAO requirements determine MS-DESI galaxy surface density ~1500 per sq deg to z~1 (for nP>1) slide from Enrique Gaztanaga

10. 10 Weak Lensing and Redshift Space Distortions RSD from MS-DESI –Measures degenerate combination of growth f and bias b Weak Lensing from DES and LSST –Helps break degeneracy RSD and WL over same sky –RSD, shear-shear, plus galaxy-shear correlations lead to larger DE and Modified Gravity Figures of Merit –Same-sky FoM benefit factor ~1.4-3 in literature, depending on FoM, but your mileage may differ MacDonald & Seljak, Bernstein & Cai, Cai & Bernstein, Gaztanaga, etal, Kirk, et al (in prep), McDonald (private comm)

11. 11 Constraints stronger if imaging and spectroscopy cover same sky: galaxy-shear cross-correlations Weak Lensing and Redshift Space Distortions: Jointly Constraining DE and Gravity LSST WL 1272 MS-DESI RSD/BAO 508 Combined: Different Sky 1718 Same Sky 4197 DETF FoM 15,000 sq deg incl Planck+SNe Same Sky FoM factor ~2.4 DETF ~1.4 γ Eriksen, Gaztanaga, etal

12. 12 Cai & Bernstein Relative Gain in Modified Gravity Figure of Merit (growth rate) from Same Sky factor ~1.4 in γ Note WL data not available in the north LSST WL MS DESI RSD DES WL

13. 13 More General Modified Gravity model DE Assuming GR Modified Gravity constraints Kirk etal Different Sky Same Sky Different Sky Same Sky Same sky FoM factor ~3

14. 14 DES and LSST Photo-z Calibration DES-BigBOSS Joint Working Group Report Angular Cross- Correlation of Imaging with Redshift Survey Requires same sky coverage of imaging and spectroscopy, improves with overlap area Photo-z systematics could otherwise limit DES, LSST Dark Energy reach

15. 15 Clusters Number of clusters above mass threshold Dark Energy equation of state Spectroscopy of DES/LSST Clusters Determine Cluster velocity dispersion (dynamical mass) using 10’s of redshifts per cluster calibrate mass-richness relation: complement WL, SZ, and X-ray estimates Mohr

16. 16 Slide from Sarah Hansen DETF FOM gain for clusters

17. 17 Dark Energy Spectrograph Concept for MS-DESI 4000-fiber optical spectrograph system for the Blanco 4m Mohawk robotic fiber positioner –Based on Echidna system, has demonstrated requisite pitch High-throughput spectrographs Fibers tile 3.1 deg 2 DECam Field of View Fiber positioner interchangeable with DECam imager –Maintain wide-field imaging capability for the Blanco –No substantial changes to the telescope Use much of the DECam infrastructure installed on Blanco –Prime focus cage, hexapod, 4 of the 5 optical corrector elements DESpec White Paper –arXiv: 1209.2451 (Abdalla, etal)

18. DECam Prime Focus Cage Installed on Blanco Telescope

19. DECam Prime Focus Cage Installed on Blanco Telescope +DESpec Saunders, etal

20. High, dry site: 80% useable nights, 0.75” site seeing Next door to LSST and Gemini. 20 Blanco Telescope Cerro Tololo Els, etal

21. 21 Dark Energy Spectroscopic Survey Redshift Survey optimized for –Baryon Acoustic Oscillations –Redshift Space Distortions Target DES+VHS Galaxies (from grizYJHK colors, fluxes) − 19 million Emission Line Gals (to z~1.5, BAO) 1200/sq deg − 4 million Luminous Red Gals (to z~1.3, RSD) 300/sq deg Strawman Survey Design − 2 exposures each field to reach target density and high completeness (1500 successful redshifts per sq. deg.) − 20-min cumulative exposure times to reach requisite depth (40 min for QSOs) − ~750 total nights to cover 15,000 sq. deg.

22. DES deep multiband image Excellent spectrosocopic target source and WL shapes Deep targeting images with WL over large area not available from Northern hemisphere

23. 23 Target Selection Simulations Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions ELG goal: 1200/deg 2 LRG goal: 300/deg 2 Using COSMOS Mock Catalog ELG goal: 1200/deg 2 Exp time = 20 min Wavelength 0.6 – 1 micron Jouvel, etal

24. 24 Target Selection Simulations Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions ELG goal: 1200/deg 2 LRG goal: 300/deg 2 Using COSMOS Mock Catalog LRG goal: 300/deg 2 Exp time = 20 min Wavelength 0.6 – 1 micron

25. 25 Conclusions Two hemispheres better than one Southern hemisphere has critical science advantages: –DES and LSST photometric surveys for DE synergy (WL+RSD, clusters, photo-z, other systematics) and deep target selection (Cf. SDSS): Figures of Merit increase by factor 1.4-3 with same sky –Synergy with other southern facilities as well (SPT, SKA, …) MS-DESI on the Blanco would capitalize on existing, installed, tested DECam infrastructure –Reduce cost and technical and schedule risks –Fiber system interchangeable with DECam maintains Blanco imaging capability into the LSST era and provides world-class imaging plus spectroscopy facility for the astronomy community