Massive Spectroscopy of DES Targets

The Dark Energy Survey and

The Dark Energy Spectrograph

Josh FriemanDES Project Director

Portsmouth, June 2011

Josh Frieman, DESpec meeting Portsmouth, June, 20112

Massive Spectroscopy of DES TargetsWould enable (see today’s talks):

• Clusters: cluster spec. z’s and dynamical masses from velocity dispersions: improve mass-observable calibration

• WL+RSD: DESpec Redshift Space Distortions plus DES WL: powerful probe of DE and test of GR+DE vs Modified Gravity

• LSS: radial BAO: H(z), and improved DA(z)

• SNe, galaxy evolution: host-galaxy z’s and spectroscopic typing (metallicities, stellar masses) to control systematics

• All: calibrate photo-z’s via angular cross-correlation?

• Local dwarf galaxies?: small-scale probes of gravity

• Constraints strongest if imaging and spectroscopy cover same sky, as DES+DESpec will

Weak Lensing and Redshift Space Distortions

Gaztanaga, Eriksen, et al


Bernstein & Cai

Dark Energy Spectrograph: DESpec

• Upgrade of DES currently under study: – multi-object prime focus spectrograph for the Blanco 4m,

interchangeable with DECam, 3.8 sq. deg. FOV– λ=600-1000 nm with resolution R~3300 at 1000 nm – Use DECam infrastructure (cage, barrel, hexapod, most

optics, shutter, 20 spare CCDs,…): substantial cost saving– 10 low-cost spectrographs, 4000 robotically positioned fibers– Redshifts for ~7 million DES galaxies (in ~270 nights), ~20

million from DES+LSST (~800 nights)• Enhance Dark Energy science reach of DES by factor ~several,

especially testing DE vs. modified gravity: Stage IV DETF• Enhance DE science reach of Stage IV LSST• Uniquely synergize with DES and LSST: power of `same sky’


The Dark Energy Camera

• DECam mounted on Telescope Simulator at Fermilab


The Dark Energy Camera

• DECam mounted on Telescope Simulator at Fermilab

• To first approximation, DESpec would interchange the imager with a fiber-positioning system that feeds multiple spectrographs

DESpec Concept Optical Design

C1C2C3

NewADC

C4

NewC6 C5

See talk by Steve Kent

Fiber Positioner Example

• FP with “Cobras”, “twirling posts” with rotating fiber. Two axes of rotation

9

FiberPatrol Radius

“Cobra”

See talk by Richard Ellis

Fiber Positioner Example

• Operating fiber-positioner on Subaru with ~400 fibers.

• Spines pivot from mounts near the bases

• Naturally handles a varying target density because the tips are small

10

FMOS Echidna on the Subaru

See talk by Will Saunders


Rationales for Blanco Spectroscopy

• Uniform, deep imaging catalogs from DES+VHS for targeting:

enable powerful new science beyond what redshifts alone provide (e.g., WL+RSD)• Maximally enhance science reach of DES: improve all the DE methods+enable new methods (RSD, radial BAO)• Hemispheric synergy with LSST: part of a broader eventual strategy for LSST follow-up: extend to ~15,000 sq deg• Excellent site: 0.65” seeing (0.9” Mosaic), high number of useable nights (80%) yield fast (hence cheap) survey• Lower cost & schedule risks by reusing/capitalizing on many DECam components: optics, CCDs, cage, hexapod, shutter


Caveats

• NOAO does not currently have plans for an Announcement of Opportunity for new instrument on the Blanco following DECam.

• Future of NOAO facilities will depend on outcome of NSF Astronomy Division Portfolio Review in 2011-2012.

• Need to further evaluate DESpec in the global context of planned & proposed spectroscopic survey facilities (e.g., BigBOSS, Sumire, 4MOST,…): DESpec should complement rather than compete with other projects where possible.

DESpec and BigBOSS

• Both projects involve 4000-5000-fiber spectrographs on ~identical 4m telescopes, with related science goals.

• DE reach increases with survey area, so ideally would survey both north (BB) and south (DESpec).

• Similar survey power (area/depth per unit time) for the two concepts: BB larger FOV, DESpec higher fiber density.

• DESpec uniquely covers entire survey areas of DES and LSST, maximizing synergistic science (WL+RSD).

• DESpec would reuse much of the DECam infrastructure, resulting in cost savings.

Path Forward• DESpec a natural “upgrade” to the science capability of DES.

Project could structurally follow the path blazed by DES: international collaboration with DOE+NSF support in the US, building on the successful DES collaboration, with opportunities for new partners.

• Material above presented to Fermilab PAC last week, with very favorable response.

• Revise and then release White Paper (1st draft exists) that lays out the science case, reference survey strategy & reference technical design motivated by science requirements.

• Next few months: optimize target selection for key DE probes, confirm with more detailed simulations. Continue to elaborate science case, carry out technical R&D, and build collaboration. Aim for external review in the Fall. Data release policy?

Massive Spectroscopy of DES Targets

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