Cosmology with Cluster Surveys Subha Majumdar Canadian Institute of Theoretical Astrophysics Toronto along with Joe Mohr, Martin White & Jose Diego International Conference on Gravitation and Cosmology, Kochi, 5th January 2004

Jan 2004 ICGC – 04, Kochi Overview Introduction to Galaxy Clusters Introduction to the Surveys Studying Dark Energy with Galaxy Cluster Surveys, the stress in on the eqn. of state “w” of dark energy (This was the parameter that was not constrained by WMAP+SDSS! But knowledge of “w” is fundamental to our understanding of dark energy) Something new: Self Calibration in cluster surveys and precision cosmology

Jan 2004 ICGC – 04, Kochi Galaxy clusters are the most massive, collapsed structures in the universe. They contain galaxies, hot, ionized gas ( K) and dark matter. In typical structure formation scenarios, low mass clusters emerge in significant numbers at z~2-3 Clusters are good probes, because they are massive and “easy” to detect through their: What Are Galaxy Clusters? X-ray emission Sunyaev-Zel’dovich Effect Gravitational lensing Chandra Image of Zw3158 SZE X-ray HST

Jan 2004 ICGC – 04, Kochi The Basics of SZ Effect

Jan 2004 ICGC – 04, Kochi Whats nice about SZE? 1) Ofcourse, the distinct spectral signature 2) Measures the total thermal content of the cluster 3) More or less redshift independent 4) Less susceptible to messy cluster substructure, core physics (prop to density and not density squared as in XRays)

Jan 2004 ICGC – 04, Kochi Example: Local Abundance/Mass Function Reiprich & Boehringer 2001 Look at the large error bars!! We want to do much better. Things will change drastically with future surveys. Especially as a probe of dark energy eqn `of state “w”

Jan 2004 ICGC – 04, Kochi Upcoming Cluster Surveys SZ-surveys: Planck : 2008(?) 7,000/8,000 – 30,000/40,000 SPT : 2005(?) 20,000-30,000 ACT : late 2004 few thousands APEX : middle 2004 few thousands X-Ray surveys DUET : Unsuccesful 20,000-30,000 DUO : very +ve report, late 2004 ~10,000 XMM-LSS : 2004+, ~1000(?) Cluster surveys will be a thrust area for some time to come!

Jan 2004 ICGC – 04, Kochi From observations to detecting clusters: an example Diego & SM 04

Jan 2004 ICGC – 04, Kochi The Cluster Redshift Distribution Cluster redshift distribution probes: 1) volume-redshift relation 2) abundance evolution Volume Element Abundance Mass Selection Function VolumeAbundance 3) cluster structure and evolution. One can get dN/dz if we can get the redshifts of the detected clusters

Jan 2004 ICGC – 04, Kochi Sensitivity of Cluster Redshift Distribution to Dark Energy Equation of State Increasing w keeping  E fixed hasthe following effects Volume effect Growth Effect Fig courtesy Joe Mohr It decreases volume surveyed It decreases growth rate of perturbations

Jan 2004 ICGC – 04, Kochi Potential for different methods to constrain `w’ Levine, Shultz & White 2002 Complimentary & Highly Competetive

Jan 2004 ICGC – 04, Kochi Dual Nature of Galaxy Clusters It’s critical- for almost any analysis- to keep in mind that clusters are young objects and yet as a population they exhibit striking regularity Statistical studies of (x-ray flux limited samples of) galaxy clusters reveal that more than half exhibit merger signatures. There’s lot of substructures. Statistical studies of (x-ray flux limited samples of) galaxy clusters reveal regularity Existence of simple flux-mass relation

Jan 2004 ICGC – 04, Kochi Self-Calibration in SZ surveys Two surveys, the South Pole Telescope Survey and the Planck all sky survey (yielding > clusters), contain enough information to constrain the interesting cosmological parameters and solve for the structure of galaxy clusters simultaneously! Assumptions required: 1. Hierarchichal structure formation is correct 2. A mass-X-ray luminosity relation exists (or a mass-SZE luminosity relation exists) 3. Crude redshift estimates are available for each cluster detected in the survey Survey mm  tot w 88 hn bb NormSlope Priorsflat Planck %0.004 SPT %0.005 SM & J. Mohr 2003

Jan 2004 ICGC – 04, Kochi Self-Calibration : Continued A Caveat: What if there is “evolution” ?? Everything is lost ! Or is it? Self Calibration can be regained (by adding complimentary info) ! 1. Need to do follow-up mass estimates of a few clusters 2. Cluster power spectrum, P(k) Not impressive by themselves, when combined with dN/dz they do wonders

Jan 2004 ICGC – 04, Kochi An Example: Planck All Sky Cluster Survey N clusters ~ SM & J. Mohr, 2004

Jan 2004 ICGC – 04, Kochi How well do we get `w’ ? SurveyOn its own + P(k)+100 clusters follow- up + Both SPT Planck  error around w=-1, normalized over 9 other parameters Note: WMAPext gives ~ 0.11 To pin down dark energy we must have cluster surveys + (CMB+SNe)

Jan 2004 ICGC – 04, Kochi So what did I say, Upcoming large yield cluster surveys would unveil a new era of doing cosmology with clusters. It has the promise to become 4 th pillar of precision cosmology along with CMB, SNe and weak lensing. As an example, we have shown that these surveys provide us an opportunity to probe the enigma of dark energy with high precision (the other great way is to do weak lensing tomography) Most importantly, we’ve seen that even with uncertainties in cluster physics, `self-calibration’ in these surveys makes clusters an important tool in precision cosmology ( “w” to few percents, certainly competetive or maybe even better than what can be done with CMB or SNe alone. )