Massive Spectroscopy for Dark Energy in the South

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. 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. 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 deg2grizYto 24th mag 30 deg2 time-domain griz (SNe) • New 3 deg2FOV, 570 Megapixel cameraon the Blanco 4m Survey 2013-2018 (525 nights) Premiere facility instrument for astronomy community DECam on the Blanco 4m at CTIO

4. DES Science Summary Forecast Constraints on DE Equation of State DES 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 SNeIa to z ~1 • Sensitive to geometry Planck prior assumed Factor 3-5 improvement over Stage II DETF Figure of Merit

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

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

10. Weak Lensing and Redshift Space Distortions • RSD from MS-DESI • Measures degenerate combination of growth fand 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. Weak Lensing and Redshift Space Distortions: Jointly Constraining DE and Gravity DETF FoM LSST WL 1272 MS-DESI RSD/BAO 508 Combined: Different Sky 1718 Same Sky 4197 • Constraints stronger if imaging and spectroscopy cover same sky: galaxy-shear cross-correlations Same Sky FoM factor ~2.4 DETF ~1.4 γ 15,000 sqdeg inclPlanck+SNe Eriksen, Gaztanaga, etal

12. 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 DES WL MS DESI RSD Cai & Bernstein

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

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

15. Clusters Number of clusters above mass threshold • 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 Dark Energy equation of state Mohr

16. DETF FOM gain for clusters Slide from Sarah Hansen

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 deg2DECam 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. Cerro Tololo Blanco Telescope High, dry site: 80% useable nights, 0.75” site seeing Next door to LSST and Gemini. Els, etal 20

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. Target Selection Simulations Using COSMOS Mock Catalog ELG goal: 1200/deg2 ELG goal: 1200/deg2 LRG goal: 300/deg2 Exp time = 20 min Wavelength 0.6 – 1 micron Jouvel, etal Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

24. Target Selection Simulations Using COSMOS Mock Catalog LRG goal: 300/deg2 ELG goal: 1200/deg2 LRG goal: 300/deg2 Exp time = 20 min Wavelength 0.6 – 1 micron Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

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 deeptarget 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