Galaxy Clusters
as
Cosmic Probes
Subha Majumdar
Cluster Cosmology - A flavour of decades old results
Pre WMAP - 1999 Science article
Bahcall, Ostriker, Steinhardt
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In this era of CMB, SNe, etc Why do we need clusters?
LSST forecast: Strengths of different approaches within one single
survey
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Cluster surveys / observational programs (incomplete list)
Red Sequence Cluster Survey (RCS)
Spitzer Adaptation of the RCS (SpARCS)
Spitzer Legacy Extremeley Wide Survey (SLEWS)
Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS)
South Pole Telescope (SPT)
APEX-SZ
Atacama Cosmology Telescope (ACT)
Blanco Cosmology Survey
Sunyaev-Zeldovich Array (SZA)
ROSAT
XMM-LSS Serendipitous Survey
XMM-Cluster Survey
Pan-Starrs
Dark Energy Survey (DES)
Hyper SuprimeCam (HSC)
Large Synoptic Survey Telescope (LSST)
eROSITA,
Wide Field Xray Telescope
AMIBA, SuZIE,
Cluster Imaging Experiment, Cluster Cosmology Atacama Telescope
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South Pole Telescope papers are rolling in…
6+1 paper
since June
2009
Results are
full of
surprises!!
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Atacama Cosmology Telescope papers are not behind…
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What are clusters?
How do they form?
What do they contain?
AND
How are they distributed?
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The WMAP Sky
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QuickTime™ and a
decompressor
are needed to see this picture.
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Counting clusters: mass function, growth fn & 8:
Universality when written in terms M
Seth-Tormen: ellipsoidal collapse & Nbody
Jenkins etal, bestfit to simulations
Also Warren etal, Lukic etal latest.
[8 for R=8h-1 Mpc]
Cosmology with Cluster Number Counts
(with apologies to)
1. with Xray luminosity function
2. Xray temperature function
3. Xray gas mass fraction
4. Optical/Xray cluster 3D corr fn
5. Optical/Xray 2D angular correlations
6. SZ+XR method of Hubble Const
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Cosmology affects cluster counts -
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Almost 10 years back…
Weller, Battye & Kneissi 2002
Fixed Mlim --> so NO uncertainty in cluster physics --> unrealistic ,
good as first attempt.
Ncl = 13600
SNAP
Ncl = 5200
Ncl = 1972
Ncl =90
Problem is we have to deal with real clusters!
Large peak in matter density
– Dark matter clump (~80% of
mass)
– Many luminous galaxies (~2%:
10% of baryons)
Chandra Image of Zw38
• BCG and red sequence
• Additional galaxies
• Diffuse light
– Hot gas (~18%: 90% of baryons)
• Emits X-rays
• Causes SZ decrement in
microwave background
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Cluster Physics Interplay 
Example SZ scaling: f x z 4  d L2  AM  E 2 z 1  z 

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Example - Lets talk about scatter…
Scatter can boost signal, ex: Cl’s or n(z)
Reason : shape of the mass function. Widens the range of possible masses
for fixed value of the observable
Include it by convolving the mass fn with distribution of the scatter
scaling
mass fn
Distribution of observable
Probability of assigning obs mass-true mass
Standard assumption: gaussian scatter in loglog plane
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Scatter and cosmology :
Scatter – 8 degeneracy in dn/dz
Pdf of 8 with priors on scatter
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Self-Calibration - A paradigm shift in cluster cosmology
Use complimentary information from the survey itself
e.g. the mass dependence of bias / spatial distribution
1.
2.
3.
Using the cluster power spectrum and P(k) oscillations
Adding information from counts-in-cell
Shape of mass-function in redshift slices
(Majumdar & Mohr, Lima & Hu)
Unbiased parameter estimation with small error bars in the presence
of systematics.
This idea has now propagated to other fields in cosmology Weak Lensing
Galaxy Bias
BAO
Photo-z
Non-gaussianity from LSS etc
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Self Calibration using Cluster Power Spectrum
Power spectrum of dark matter density fluctuations P(k)
– Clusters are biased: 20,000 clusters comparable to ~5x105
galaxies
– Turnover on large scales- “standard rod” calibrated by primary
From redshift surveys, we
CMB fluctuations
will get P(k) for free !
DUET P(k)
Unfortunately, only P(k)
gives almost no constraints
on `w’. Combined with CMB
priors, one can constraint
w ~ 25-30%
Things become interesting
when dn/dz and P(k) are
Combined.
SM & J.Mohr 2003
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Prospects for Self-Calibrating Cluster Survey…
SM & J Mohr, 2003
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Some ideas on what to expect on DE …
Xray - < 5-10% for eROSITA
~ 1% like from WFXT
Similar constraints from optical with similar cluster numbers
Ignore SZE for now!
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`The Proof of the Pudding is in Eating’
or
Real Results from Real Survey
(RCS1 and RCS2)
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We are in a little spot with recent SZE results…
•
•
•
Pre 2009 SPT/ACT surveys forecasted to yield 4-7clusters/deg
(Planck ~ 1000 to few 1000)
•
•
•
Post 2009 SPT/ACT finds 0.05-0.12 clusters/degsq
Also finds less SZ power in SZ Cl’s
Implication - Cosmological constraints are washed out !
So, will doing cosmology with clusters become secondary
science for these SZ surveys ?
Need to do something More with these surveys!
Our Options 1. Come up with new ideas for cluster surveys
2a) Make progress with understanding of cluster physics
(for cases wherecluster modeling cannot be avoided)
2b) Make atleast a useful ‘working’ model of cluster
3) Fall back on some old ideas of combining diff SZE
outputs
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Quick Thinking 1 - Change Survey Design
tobs
2000 deg2
Example ACT - 2000 deg2 survey
with tobs~107 s
ftime = fractional time
spent on the smaller
patch.
1800 deg2
200d
eg2
tS = ftimetobs
tL = (1-ftime)tobs
Magic of a ‘tiered’ survey (in numbers) …
(from Satej’s talk)
Single area
survey
(2000)
deg2.
A, α, γ unkown
γ unkown
with follow-up
Ωm
0.676
0.019
0.032
w
1.343
0.400
0.147
σ8
2.849
0.121
0.028
factor of 4
BIG improvement!
improvement!
Wedding
Cake
survey
(200+1800)
deg2.
better than
mass follow-up
Ωm
0.105
0.009
0.030
w
0.115
0.095
0.088
σ8
0.116
0.019
0.028
Bottomline - A simple change of survey plan can do wonders
Overlapping areas for SZE+Xray surveysAn ensemble of dual detected clusters for free
XRay clusters
Temp
Sizes
Common
SZ clusters
mock catalog of 430 clusters created
from ACT/SPT+eROSITA for Nbeam=2
with minimum 20% errors in dA(z).
dA from clusters ‘CAN BE LIKE’ dL from Sne !!
100 mass follow-up:
30-100 % error
ACT/SPT
dN/dz + dA(430)
ACT/SPT
dN/dz only
ACT/SPT dN/dz +
SNe(307)
Use flux counts with SZ Cl to remove cluster uncertainties …
Diego & SM
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Higher - z and Larger Surveys -
1. Completed Optical
2. Proposed (and failed) Optical
3. Upcoming and funded Xray
4. Proposed XRay
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PI - G Wilson (USA)
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…more
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What about masses? -
For the cluster in SpARCS SOUTH
Vdisp = 1050 +/- 230 km/s
M = 9.4 +/- 6.2 x 1014 Msun
5.7 x 1014 Msun (from scaling)
z = 1.34
This cluster just should not exist !!
Or maybe a sign of non-gaussianity
We are working on it now.
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A comparison of survey volumes …
SDSS: 10000 deg, z<0.5,
Vol ~ 7 Gpc3
SPT/ACT : 2000- 4000deg, z=0.1 -1, Vol ~ 7-14 Gpc3
RCS2: 900 deg, z=0.2-0.9,
Vol ~ 2.4 Gpc3 &
SpARCS ~ 0.5 Gpc3
350 deg, z=1-2
Vol ~ 3.6 Gpc3
high-z survey having smaller area has same leverage due to
Its higher volume!
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Becoming more ambitious - More with Spitzer
Proposed, for ~3000 hours (i. e., 4 months of Spitzer time) making it a legacy
class survey:
The Spitzer Legacy Extremely Wide Survey (SLEWS)
PI: Jonathan Gardner (NASA) & Gillian Wilson (UCRiverside)
(Multi country, multi supportive PI from other surveys, huge team
Total members : 84 (with different backgrounds and expertise)
Roughly 59 (USA), 24 (Europe + Japan), 1 (India)
Basic coverage: 350 deg2 , 1< z < 2
Basic aim: a) To do clusters at high z
b) To do quasars at z>6.5 when reionization is about to occur and
there are probable stromgen spheres
Passed initial stages but finally lost to a ExoPlanet Survey :-(
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Into the ‘not too distant future’ - Wide Field Xray Telescope
(Mission pushed ahead by R. Giacconi , 2010 Decadal Survey)
If funded, this will be the mother of all cluster surveys
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Forecast for WFXT - using N(z) + P(k)
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Cluster constraints on ‘generalised’ modified gravity models
with
 = 0.55 for standard model (GR+LCDM)
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Can India have a world class optical survey ???
A comparison Telescope
RCS on 4m CFHT
2m in Hanle
Seeing
0.7 arc sec
<1 arc sec
Resolution
0.187 arc sec
0.45 arc sec
FOV
1 deg sq
0.7 arc-min sq
30 arc-min sq
RCS1 (100 degsq) Equivalent time
~ 27 - 40 hrs
~ 1350-2000 hrs
or 54 - 80 hrs
I think its possible with a little effort !!
Technology additions are easy and cheap.
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Thanks
Thanks also to Ravi Subrahmanyan, Biman Nath, Joe Mohr
Martin White, Dick Bond, Wayne Hu, Nabila Aghanim,
Joe Silk, Christoph Pfrommer, Nick Battaglia, Jon Sievers,
Sudeep Das, Jose Diego,Bhuvnesh Jain, Howard Yee, Mike
Gladders, Henk Hoekstra, Gillian Wilson, Adam Muzzin,
Yen Ting Lin , Alexey Vikhlinin, Salman Habib, David Gilbank
Zoltan Haiman
+ Satej Khedekar & Anya Chaudhuri.
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