Cosmology with Large Optical Cluster Surveys
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Transcript of Cosmology with Large Optical Cluster Surveys
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Cosmology with Large Optical Cluster Surveys
Eduardo RozoEinstein Fellow
University of Chicago
Rencontres de MoriondMarch 14, 2010.
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The Take Home Message• Galaxy clusters probe the physics behind our accelerating universe in a way that is fundamentally different from geometrical probes (e.g. BAO or SN).
• Optical clusters already provide cosmological parameters constraints of comparable accuracy to those from other methods, and in particular X-rays.
• Different methods for studying clusters are highly complementary to each other, and can also break degeneracies inherent to other LSS probes.
• Realizing the statistical power of upcoming experiments is difficult: detailed simulations of upcoming experiments are necessary to test our understanding of systematics.
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Two Questions
• Why clusters?
• Why optical?
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Why Clusters?The universe is accelerating. Two possible culprits:• dark energy exists• GR breaks down at large scales.But- how to distinguish between the two?
GR is a highly predictive theory:Once the geometry of the universe is measured (e.g. from BAO+SN), GR+CMB predict the growth of LSS.
(See Mortonson’s talk on Tuesday).Cluster test gravity on cosmological scales in a way
that is fundamentally different from geometrical probes.
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0 1 2 3 z
0.05
0.00
-0.05
ΔG/G
GR Predictions for Growth Relative to CMB from Current
Data
Mortonson, Hu, and Huterer 2009
Our job is to put data points on this plot.
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Why Optical?• Galaxy clusters are easy to find with optical data.
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Why Optical?• Galaxy clusters are easy to find with optical data.
• There is lots of data available, and even more to come.
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Dark Energy Survey
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Why Optical?• Galaxy clusters are easy to find with optical data.
• There is lots of data available, and even more to come:
Optical cluster samples already contain of order a few tens of thousands of clusters.
Future surveys will produce many tens of thousands of clusters.
Incredible raw statistical power.
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w0
w a
Dark Energy Survey Forecasts
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Cosmology with Galaxy Clusters
Cluster Counting
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Cosmology with Galaxy Clusters: What do Clusters
Measure?No. of halos in a survey = (halo density)x(survey
volume)
It is the cosmological dependence of the halo density that makes galaxy clusters special.
GeometrySensitive to Growth
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Cosmology with Galaxy Clusters: What do Clusters
Measure?Halo (cluster) density depends principally on two things:• The power spectrum amplitude s8(z).• The matter density Wm.
To measure s8(z), we need to know three things:1. How many clusters are there?2. What are their redshifts?
3. What are their masses?
€
dndM
= stuff ×Ωm
M ⎛ ⎝ ⎜
⎞ ⎠ ⎟× exp −
stuff (M /Ωm )σ 8
2(z)
⎡ ⎣ ⎢
⎤ ⎦ ⎥
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€
dndM
= stuff ×Ωm
M ⎛ ⎝ ⎜
⎞ ⎠ ⎟× exp −
stuff (M /Ωm )σ 8
2(z)
⎡ ⎣ ⎢
⎤ ⎦ ⎥
Suppose you measure abundance at one mass (i.e. the amplitude of the halo mass
function).You constrain a combination of s8(z) and
Wm:s8Wm
0.4 = constantBut halo mass M and Wm are degenerate!
The Single Biggest Problem for All Cluster Cosmology
Errors in M translate directly translate into errors in Wm, which result in errors in s8.
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The Mass Function from the maxBCG Cluster Catalog
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€
dndM
= stuff ×Ωm
M ⎛ ⎝ ⎜
⎞ ⎠ ⎟× exp −
stuff (M /Ωm )σ 8
2(z)
⎡ ⎣ ⎢
⎤ ⎦ ⎥
Suppose you measure abundance at one mass (i.e. the amplitude of the halo mass
function).You constrain a combination of s8(z) and
Wm:s8Wm
0.4 = constantBut halo mass M and Wm are degenerate!
The Single Biggest Problem for All Cluster Cosmology
Errors in M translate directly translate into errors in Wm, which result in errors in s8.
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Cosmology With Optical Surveys
- The Three Questions -To measure A(z), we need to know three things:
1. How many clusters are there?i.e. How do we find clusters?
EASY2. What are their redshifts?
EASY3. What are their masses?
HARD
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Optical Clusters: How do We Find them?
Finding Clusters is easy.
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Finding ClustersLook for tight groups of uniformly red galaxies.
Use of red-sequence nearly eliminates projection effects.
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Optical Clusters: What are Their Redshifts?
The location of the red-sequence allows for robust redshift estimates.
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Ben Koester, RCS2 data.
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Optical Clusters: What are Their Masses?
Optical clusters don’t come tagged with mass.They come tagged with richness: a measure of the galaxy content of the clusters.
To do cosmology we must know the statistical relation between a cluster’s mass and its richness (mass
calibration).Many possibilities:• Weak lensing• X-rays (e.g. temperature, brightness, YX, Mgas) • Galaxy velocity dispersion• SZ decrement• Cluster clustering
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So Does it All Work?
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State of the Art: the SDSS maxBCG Cluster Catalog
maxBCG at a glance: (Koester et al. 2007)
• Red-sequence cluster finder.• ~8,000 deg2 SDSS imaging• ~13,000 clusters• Redshift range 0.1<z<0.3.• Photometric redshift accuracy ≈ 0.01.
Three independent approaches to mass calibration:• weak lensing (Sheldon et al. 2009)• X-ray luminosities (ROSAT) (Rykoff et al. 2008)• velocity dispersions (Becker et al. 2007)
All three of these approaches rely on cluster stacking.
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Cosmological Constraints
8(M/0.25)0.41 = 0.832 0.033
Joint constraints: 8 = 0.8070.020 M = 0.2650.016
Rozo et al. 2010
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0 1 2 3 z
0.05
0.00
-0.05
ΔG/G
GR Predictions for Growth from Current Data
Mortonson, Hu, and Huterer 2009
Current constraints
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Prospects for Further Improvement• Improve the fidelity of cluster richness as a
mass estimator.
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Improving Richness Estimates
Richness
Clus
ter M
ass
Scatter ≈ 15%
Preliminary!
Detailed analysis using a cleanly selected sample is ongoing.
RXJ1504
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Prospects for Further Improvement• Improve the fidelity of cluster richness as a
mass estimator.
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Prospects for Further Improvement
• Improve the fidelity of cluster richness as a mass estimator.• Improved mass calibration.• requires X-ray follow up of optically selected
clusters over a broad richness range.• data useful for future optical cluster surveys
as well.
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Relation to X-rays and SZ Clusters
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Optical vs. X-ray: How Do Optical Clusters Measure Up?
maxBCG
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Optical vs. X-ray: How Do Optical Clusters Measure Up?
maxBCG
This agreement is a testament to the robustness of galaxy clusters as cosmological probes, and
demonstrates that cluster abundance systematics are well understood.
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Different Methods are Highly Complementary
Optical: • Very low mass threshold• Least precise mass estimates
X-rays:• Molderate/high mass threshold• Harder to reach high z (though see A. Finoguenov’s
talk)• Most precise mass estimates
SZ (see Joe Mohr’s talk Wednesday):• Detection efficiency is nearly redshift
independent.• Optical redshifts required• Moderate/high mass threshold• Precise mass estimates
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Optical + X-ray
X-ray Follow-up of small optical sub-samples can significantly improve the utility of optical surveys.
Wu, Rozo, and Wechsler 2010.
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Optical + SZ
Cunha 2009
DES
SPT
DES+SPT
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But wait! Order now, and you will also receive…
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Cluster Clustering is a Sensitive Probe of Non-Gaussianity
Plot from Cunha et al. 2010, but see also Sartoris’s talk next.
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Clusters and Galaxy Correlation Functions
r 1 10
xgg displays strong degeneracies between cosmology and HOD.
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Clusters and Galaxy Correlation Functions
r 1 10
Galaxy clusters provide an independent probe of the galaxy content of dark matter halos, breaking the
cosmology-HOD degeneracy.
xgg displays strong degeneracies between cosmology and HOD.
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Clusters and Galaxy Correlation Functions
Tinker et al., in preparation.
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Parting Thought/Advertisement
This all sounds great, but…
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Realizing the Statistical Power of Optical Cluster Samples is Hard
• Accurate photo-z’s.• Low-scatter and well understood mass estimators.• Well understood centroiding properties.• Well understood purity and completeness.• Precise mass estimation.• …
Requires:
How can we be sure all systematic uncertainties are understood to the accuracy required by the
data?
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Testing Cluster Cosmology in DES
- The Blind Cosmology Challenge -
Perform a top-to-bottom analysis of simulated data to test robustness of cosmological
constraints.1. Run N-body simulations to produce 5,000 deg2
lightcone.2. Populate light cone with galaxies. Include:• 5 band magnitudes.• Galaxy sizes and shapes (required for lensing)
3. Lens all galaxies by ray tracing through full lightcone.
4. Run cluster finders to produce cluster catalogs.5. Analyze mock catalogs as you would the real data.The goal:
recover cosmology used to construct the mock catalogs!
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Meeting the Challenge in the DES
- The Blind Cosmology Challenge -Goals:
• Test precision and biases of cosmological parameter estimation pipelines.• Identify principal sources of systematic
uncertainty so that they may be addressed.• Be 100% ready to run analysis on real data
as soon as data taking commences (Fall 2011).
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The Take Home Message• Galaxy clusters probe the physics behind our accelerating universe in a way that is fundamentally different from geometrical probes (e.g. BAO or SN).
• Optical clusters already provide cosmological parameters constraints of comparable accuracy to those from other methods, and in particular X-rays.
• Different methods for studying clusters are highly complementary to each other, and can also break degeneracies inherent to other LSS probes.
• Realizing the statistical power of upcoming experiments is difficult: detailed simulations of upcoming experiments are necessary to test our understanding of systematics.
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The Power of Numbers
RichnessL X
Stack RASS fields along cluster centers to measure the mean X-ray luminosity as a function of richness.
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Sample Selection
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RXJ 1504