T he Case for Massive Spectroscopy in the South

1. The Case for Massive Spectroscopy 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 LSST galaxies, capitalizing on those investments. Maximum overlap requires a southern site.

3. 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 counting Distances+growthImagingSpectroscopy • Baryon Acoustic Oscillations Distances and H(z) ImagingSpectroscopy • Redshift Space Distortions Growth Spectroscopy Growth of Density Perturbations Distance vs. Redshift

4. Massive Spectroscopic Surveys in the Southern Hemisphere • 8-million Galaxy Redshift Survey in ~250 nights • Uniformly selected from deep, homogeneous DES+VHS imaging over 5000 sq. deg. (2018+) • 15-million Galaxy Redshift Survey in ~500 nights • Uniformly selected from deep, homogeneous LSST imaging of additional 10,000 sq. deg. (2021+) or from DES extension • Photometric+Spectroscopic Surveys over same sky • Enable powerful new science beyond what either can provide alone • Deep, uniform multiband imaging from DES, LSST • Enable efficient, well-understood selection of spectroscopic targets, control systematic errors

5. 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

6. RSD, BAO determine galaxy surveydensity ~1500 per sqdeg to z~1 (for nP>1) slide from Enrique Gaztanaga Slide from Enrique Gaztanaga

7. Weak Lensing and Redshift Space Distortions • Powerful test of Dark Energy vs. Modified Gravity • 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 MacDonald & Seljak, Bernstein & Cai, Cai & Bernstein, Gaztanaga, etal, Kirk, et al (in prep)

8. Weak Lensing and Redshift Space Distortions: DETF FoM for 15,000 sq. deg. surveys LSST WL MSDESI RSD+BAO 255 113 • Constraints much stronger if imaging and spectroscopy cover same sky: galaxy-shear cross-correlations Combine: Different sky 1126 Same sky 4240 Assuming GR Not Assuming GR M. Eriksen

9. Weak Lensing and Redshift Space Distortions: Jointly Constraining DE and Gravity LSST WL MS-DESI RSD+BAO Combined: Different Sky Same Sky • Constraints much stronger if imaging and spectroscopy cover same sky: galaxy-shear cross-correlations constrain bias Assuming GR

10. Relative Gain in Modified Gravity Figure of Merit from Same Sky Note WL data not available in the north LSST WL DES WL MS DESI RSD Cai & Bernstein

11. 2-parameter Modified Gravity model Different Sky Same Sky Different Sky Same Sky Assuming GR Modified Gravity constraints Kirk etal

12. 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

13. 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

14. DETF FOM gain for clusters Slide from Sarah Hansen

15. 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 full 3.8 deg2DECam Field of View • Fiber positioner rapidly interchangeable with DECam imager • Maintain wide-field imaging capability for the Blanco • Use much of the DECam infrastructure installed on Blanco • Prime focus cage, hexapod, 4 of the 5 optical corrector elements, shutter • DESpec White Paper • arXiv: 1209.2451 (Abdalla, etal)

16. DECam Prime Focus Cage Installed on Blanco Telescope

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

18. Cerro Tololo Blanco Telescope High, dry site: 80% useable nights, 0.75” site seeing Next door to LSST and Gemini. 18

19. Dark Energy Spectroscopic Survey • Redshift Survey optimized for • Baryon Acoustic Oscillations • Redshift Space Distortions • Target DES+VHS Galaxies (from grizYJHK colors, fluxes) • 19million Emission Line Gals (to z~1.5, BAO) 1200/sq deg • 4million 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.

20. DES deep multiband image Excellent spectrosocopic target source and WL shapes Deep targeting images with WL over large area notavailable from Northern hemisphere

21. Target Selection Simulations ELG goal: 1200/deg2 LRG goal: 300/deg2 Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

22. Target Selection Simulations ELG goal: 1200/deg2 LRG goal: 300/deg2 LRG goal: 300/deg2 Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

23. 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 cal) and deep, uniform target selection (Cf. SDSS): Figures of Merit increase by factor 2-3 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 provide world-class imaging plus spectroscopy facility for the astronomy community