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)

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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.

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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 deg2 grizY to 24th mag 30 deg2 time-domain griz (SNe)• New 3 deg2 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

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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

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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

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• 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 DensityPerturbations

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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

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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

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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

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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)

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sky: galaxy-shear cross-correlations

Weak Lensing and Redshift Space Distortions: Jointly Constraining DE and Gravity

LSST WL 1272MS-DESI RSD/BAO 508

Combined: Different Sky 1718 Same Sky 4197

DETF FoM

15,000 sq deg

incl Planck+SNe

Same SkyFoMfactor ~2.4 DETF~1.4 γ

Eriksen, Gaztanaga, etal

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Cai & Bernstein

Relative Gain in Modified Gravity Figure of Merit (growth rate) from Same Skyfactor ~1.4 in γ

Note WL data not available in the north

LSST WL

MS

DE

SI R

SD

DES WL

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More General Modified Gravity model

DE Assuming GR Modified Gravity constraints

Kirk etal

Different SkySame Sky

Different SkySame Sky

Same sky FoM factor ~3

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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

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ClustersNumber 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

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Slide from Sarah Hansen

DETFFOMgain for clusters

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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 deg2 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)

DECam Prime FocusCage Installed on Blanco Telescope

DECam Prime FocusCage Installed on Blanco Telescope

+DESpec

Saunders, etal

High, dry site: 80% useable nights, 0.75” site seeingNext door to LSST and Gemini.

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Blanco Telescope

Cerro Tololo

Els, etal

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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.

DES deepmultiband image

Excellent spectrosocopictarget sourceand WL shapes

Deep targetingimages with WLover large area not availablefrom Northernhemisphere

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Target Selection Simulations

Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

ELG goal:1200/deg2

LRG goal:300/deg2

Using COSMOS Mock Catalog

SELECTION METHOD

Photometry (Total) Selection

Successful ELG z’s [1< z< 1.7]

PTF + WISE color

1747 552

iDES < 23.5 & ANNz sel

1502 1430

iDES < 23 & ANNz sel

585 600

ELG goal:1200/deg2

Exp time = 20 minWavelength 0.6 – 1 micron Jouvel, etal

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Target Selection Simulations

Deep, homogeneous parent catalogs from DES, VHS, LSST enable efficient selection and sculpting of redshift distributions

ELG goal:1200/deg2

LRG goal:300/deg2

Using COSMOS Mock Catalog

SELECTION METHOD

Photometry (Total) Selection

Successful LRGs [0.5< z <1.2]

PTF + WISE color

247 143

DES S/N > 3 + ANNz

433 433

DES+VHS color

373 264

LRG goal:300/deg2

Exp time = 20 minWavelength 0.6 – 1 micron

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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