The Dark Energy Survey - DES

1
The Dark Energy Survey - DES

• Jochen Weller
• University College London
• (Josh Frieman, Ofer Lahav, The DES Collaboration)

2
What would we like to know about Dark Energy in
the Future?

• Can we measure a difference from a cosmological
  constant? (w(a)?)
• Are there traces of early dark energy?
• Can we find signatures of modified gravity?
• Need to measure
• Expansion history of the Universe (geometry)
• growth of structures
• large scale anisotropies and power spectra
  forclumping of dark energy

3
Cosmological Dark Energy Observations

• Geometry
• Type Ia Supernovae (luminosity distance)
• Cosmic Microwave Background Anisotropies
  (angular diameter distance)
• Baryon Acoustic Oscillations (angular diameter
  distance)
• Growth of Structure (usually also geometry)
• Weak lensing tomography
• Galaxy Cluster Counts
• Redshift space distortions
• Dark Energy Clumping
• Large Scale CMB anisotropies (ISW) correlated to
  power spectrum

4
The Dark Energy Survey

• Study Dark Energy using
• 4 complementary techniques
• Cluster counts
• Weak lensing
• Baryon Acoustic Oscillations
• SNe Ia distances (Joe Bernsteins
  talk)
• Two multi-band surveys
• 5000 deg2 g, r, i, Z, Y to 24th mag
• 9 deg2 repeat (SNe)
• Build new 3 deg2 camera
• Survey 2011-2016 (525 nights)

Blanco 4-meter at CTIO
300 million galaxies
5
The DES Collaboration
Fermilab University of Illinois at
Urbana-Champaign University of Chicago Lawrence
Berkeley National Lab NOAO/CTIO DES Spain
Consortium DES United Kingdom Consortium Universit
y of Michigan Ohio State University University of
Pennsylvania DES Brazil Consortium Argonne
National Laboratory
6
The DES Telescope

• NOAO/CTIO 4m Blanco telescope
• 1970 era, equatorial mount
• An existing, working telescope
• On-going studies finite element
• analysis, laser metrology, PSF pattern
  modeling
• Solid primary mirror
• 50cm thick Cervit, 15 tons
• Mechanical mirror support system
• radial purely mechanical
• axial 3 load cell hard points controllable
  support cells
• Primary cage
• DES will replace entire cage
• will have radial (alignment) movement
• Cerro Tololo
• site delivers median 0.65 Sept-Feb
• current Mosaic IItelescope delivers median 0.9
  Sept-Feb

3 Hard Points
Abbott, Walker, Peoples, Bernstein...
24 Radial Supports
33 Pressure Pads
7
The DES Instrument DECam
F8 Mirror
Filters Shutter
3556 mm
CCD Read out
Hexapod
Optical Lenses
1575 mm
8
DES CCDs

• 62 2kx4k fully depleted CCDs
• 520 Megapixels
• Excellent red sensitivity
• Developed by LBNL

9
Photometric Redshifts
Measure relative flux in multiple filters
track the 4000 A break Estimate individual
galaxy redshifts with accuracy ?(z)
0.1 (0.02 for clusters) This is sufficient
for Dark Energy probes Note good detector
response in z band filter needed to
reach zgt1

including sensitivity to Y-band
10
Synergy with VISTA Hemisphere Survey
ESO Public Survey late 2008 JHK imaging over
DES area
11
Galaxy Photo-z Simulations
DES
10? Limiting Magnitudes g 24.6 r 24.1
i 24.0 z 23.9 2 photometric
calibration error added in quadrature
Z 23.8 Y 21.6
Developed improved Photo-z Error Estimates and
robust methods of outlier rejection Cunha, Lima,
Frieman, Lin and Abdalla, Banerji. Lahav
ANNz low depth survey training sets in place
12
DES Area and Depth Synergy with South Pole
Telescope

• South Pole Telescope (10m, bolometer array, 150,
  250, 270 GHz)
• 4000 sq. deg. survey
• Detect 10,000 clusters
• through Sunyaev-Zeldovich
• effect
• Dark Energy Survey
• measure photometric
• redshifts for these clusters
• to z 1-1.3

Galactic Dust Map
13
Cosmology Dependence of Galaxy Cluster Number
Counts

• concordance cosmology ?m 0.3 ?8 0.78
  n1, h0.72 w-1, ?? 4.000 deg2 Mlim
  1.7?1014h-1M?
• ?m 0.4
• ?8 0.85
• w - 0.8
• w - 0.7

Weller, Battye Kneissl 2002
Problem Need to understand mass - observable
relation for SZ surveys or for
optically selected clusters
14
Weak Lensing Calibration of Mass - SZ Observable
Relation with optical surveys

• Simple estimate 10 background galaxies/sq.
  arcmin
• Distribution dn/dz exp(-z/zc) zc0.5

Dodelson Weller (see also Natasha
Hurley- Walkers talk)
15
Constraint on Dark Energy - South Pole Telescope
and Dark Energy Survey

• significant improvement of constraint by using WL
  prior
• Not used shape of mass function
• Power spectrum of clusters (be careful might be
  correlated to other probes)

Weller, Tang Dodelson
16
Weak Lensing Tomography

• Cosmic Shear
• Angular Power
• Spectrum in 4
• Photo-z Slices
• Shapes of 300 million galaxies
• ?z?0.7
• Primary Systematics
• photo-zs, PSF anisotropy,
• shear calibration

Statistical errors shown
17
BAO in DES Galaxy Angular Power Spectrum
Wiggles due to BAO
Systematics photo-zs, correlated photometric
errors, non-linearity, scale-dependent bias
Fosalba Gaztanaga
Blake Bridle
18
DES Forecasts Power of Multiple Techniques

w(z) w0wa(1a) 68 CL
Assumptions Clusters ?80.75, zmax1.5, WL
mass calibration BAO lmax300 WL
lmax1000 (no bispectrum) Statisticalphoto-z
systematic errors only Spatial curvature,
galaxy bias marginalized, Planck CMB prior

DETF Figure of Merit inverse area of ellipse

• geometric
• growth

Stage II not included here
geometric

19
Forecast Constraints
DETF FoM

• DESStage II combined Factor 4.6 improvement
  over Stage II combined
• Consistent with DETF range for Stage III DES-like
  project

20
Conclusion

• DES is a multiprobe instrument to take the next
  step in probing the Dark Energy, aiming toward
  greater understanding of the physics of cosmic
  acceleration.
• DES has ability to calibrate mass - observable
  relation for SZ clusters
• Using multiple techniques to probe Dark Energy is
  important to control systematic errors and
  achieve robust constraints.

21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
Spectroscopic Redshift Training Sets for DES
Redshift Survey Number of Redshifts Overlapping DES
Sloan Digital Sky Survey 70,000, r lt 20
2dF Galaxy Redshift Survey 90,000, bJlt19.45
VIMOS VLT Deep Survey 60,000, IABlt24
DEEP2 Redshift Survey Magellan PRIMUS Survey 30,000, RABlt24.1 100,000, Rlt23
25
Weak Lensing Systematics Anisotropic PSF
Focus too low
Focus (roughly) correct
Focus too high

• Whisker plots for three BTC camera exposures
  10 ellipticity
• Left and right are most extreme variations,
  middle is more typical.
• Correlated variation in the different exposures
  PCA analysis --gt
• can use stars in all the images much better
  PSF interpolation

Jarvis and Jain
26
PCA Analysis Improved Systematics Reduction
Focus (roughly) correct
Focus too high
Focus too low

• Remaining ellipticities are essentially
  uncorrelated.
• Measurement error is the cause of the residual
  shapes.
• 1st improvement higher order polynomial means
  PSF accurate to smaller scales
• 2nd Much lower correlated residuals on all
  scales!

Jarvis and Jain
27
Understanding Improving Blanco Image Quality

• Ray tracing model of Mosaic correctorprimary
• Reproduces PSF patterns qualitatively explains
  PCA components
• Focus errors coupled to primary astigmatism
• MisalignmentADC induces coma
• Guiding errors
• Trefoil distortions of primary due to mirror
  supports
• Improvements for DES
• New corrector with small smoothly
• varying PSF
• active focus sensors on focal plane
• wavefront (curvature) sensors
• active control of prime focus cage tilt
• active control of primary (low order)
• fix radial support system
• improve thermal environment
• (reduce local power dissipation)

Mosaic II an observed PSF pattern
Mosaic II optical model ADC - neutral position
axis to right Primary misaligned
by 0.2 mm x -0.7 mm y results in Coma
Offsets of this amount have been measured
due in part to broken radial supports
28
Parameterizations of Dark Energy

• Background evolution
• w w0
• w w0w1z
• w w0? ln(a) (Efstathiou 1999)
• w w0wa(1-a) (Chevalier 2001, Linder 2003)
• binned w(z) (parameter free)
• Perturbations cs2,?, ...

29
Figure of Merit

• Figure of merit inverse area of 95 contour in
  w0-wa space
• This is some indication but be careful
  parameterization imposes certain redshift
  sensitivity, which does not necessarily reflect
  particular survey