Transcript The Dark Energy Survey J. Frieman, M. Becker, J. Carlstrom, M.

The Dark Energy Survey
J. Frieman, M. Becker, J. Carlstrom, M. Gladders, W. Hu, R. Kessler, B. Koester, A.
Kravtsov, for the DES Collaboration
Overview
The Dark Energy Survey (DES) is a 5-year, 5000 sq. deg. optical imaging survey in
grizY bands that will use a new mosaic CCD camera with 520 megapixels covering a 3
sq. deg. field of view on the Blanco 4m telescope at the Cerro Tololo Inter-American
Observatory (CTIO). DES will observe about 300 million galaxies and probe the nature
of the dark energy using four complementary methods: (i) the abundance and clustering
of galaxy clusters, in concert with the South Pole Telescope Sunyaev-Zel’dovich (SZ)
survey, (ii) weak gravitational lensing tomography, (iii) baryon acoustic oscillations via
galaxy angular correlations, and (iv) Type Ia supernova distances from repeat imaging of
a smaller area. Combining results from the different methods will provide tighter
cosmological constraints; comparing results from the different methods will provide
important cross-checks on systematic errors. The processed DES data will provide a
long-term resource for the astronomy community, and the Dark Energy Camera will be
available for general purpose observing on the Blanco telescope.
Clockwise from top left: 1. N-body
simulation of large-scale structure
covering the DES footprint;
2. Blanco 4m dome with
Magellanic clouds and Milky Way
overhead; 3. Blanco telescope with
prime focus cage at top; 4. layout of
Dark Energy Camera, showing
corrector optics and new prime
focus cage; 5. Multi-CCD test
vessel in U. Chicago machine shop;
6. DECam focal plane layout with
62 2kx4k CCDs; 7. CCD wafer: the
fully depleted, thick CCDs provide
superior quantum efficiency in the
red passbands, important for
observations at z 1.
Probing Dark Energy
Three views of Galaxy Clusters: Left: SDSS optical image of a distant cluster; DES
will detect many tens of thousands of clusters to redshift z~1.3 as concentrations of red
galaxies; Center: Weak lensing reconstructed mass map (contours) superimposed on
optical image of a low-redshift cluster using data from the Blanco 4m (Joffre et al.
2000); DES will provide statistical calibration of cluster masses via weak lensing;
Right: Dark matter distribution in a cosmological simulation of a cluster-size halo;
simulations indicate a robust and tight scaling relation between cluster mass and
integrated SZ flux.
Sensitivity to Dark Energy: Upper left panel: predicted number of detected clusters vs.
redshift in SPT+DES for dark energy equation of state parameter w0 = 1 [solid curves,
with power spectrum normalization 8=0.75 (black) and 0.9 (blue)] and w0 = 0.8
(dotted), with statistical errors shown; Upper right panel: baryon acoustic oscillation
signal in galaxy angular power spectrum in four redshift slices (z = 0.3, 0.7, 1.1, 1.5)
with varying w0; Lower panel: weak lensing cosmic shear angular power spectrum in
four redshift slices, for w0 = 1 (black) and 0.9 (red), with binned statistical errors
shown; only modes with multipole l < 1000 (excluding grey region) are included in the
forecasts shown in next column.
Forecast Constraints
w(z) =w0+wa(1–a)
Forecast DES constraints (griz data only) on dark energy equation of state parameters w0
and wa. Spatial curvature has been marginalized over and a Planck CMB prior assumed.
Photometric redshift systematic error nuisance parameters also marginalized (see below).
Table shows marginalized constraints and the Dark Energy Task Force Figure of Merit,
proportional to the inverse area of the constraint ellipses above. `Stage II’ denotes dark
energy constraints expected from on-going projects.
Photometric Redshifts
Multi-band imaging enables approximate galaxy redshift estimates based on observed
colors: Left panel: early-type galaxy spectra at redshifts z = 0, 0.7, 1.4 overlaid on griz
filter response curves; Right panel: estimated photometric redshift vs. true redshift for
simulated DES galaxy survey, including DES grizY and JHK near-infrared imaging
from the planned Vista Hemisphere Survey at ESO. Photo-z error distributions,
critical for precise dark energy constraints, will be well measured using existing deep
spectroscopic data.
The National Science Foundation