Cosmology with Cluster Surveys

1. 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

2. 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 ICGC – 04, Kochi

3. Galaxy clusters are the most massive, collapsed structures in the universe. They contain galaxies, hot, ionized gas (107-8K) 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: HST Chandra Image of Zw3158 SZE X-ray What Are Galaxy Clusters? • X-ray emission • Sunyaev-Zel’dovich Effect • Gravitational lensing ICGC – 04, Kochi

4. The Basics of SZ Effect ICGC – 04, Kochi

5. 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) ICGC – 04, Kochi

6. Reiprich & Boehringer 2001 Example: Local Abundance/Mass Function 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” ICGC – 04, Kochi

7. 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! ICGC – 04, Kochi

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

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

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

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

12. 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 Dual Nature of Galaxy Clusters Existence of simple flux-mass relation ICGC – 04, Kochi

13. Two surveys, the South Pole Telescope Survey and the Planck all sky survey (yielding > 20000 clusters), contain enough information to constrain the interesting cosmological parameters and solve for the structure of galaxy clusters simultaneously! Self-Calibration in SZ surveys • Assumptions required: • Hierarchichal structure formation is correct • A mass-X-ray luminosity relation exists (or a mass-SZE luminosity relation exists) • Crude redshift estimates are available for each cluster detected in the survey SM & J. Mohr 2003 ICGC – 04, Kochi

14. Self-Calibration : Continued A Caveat: What if there is “evolution” ?? Or is it? Everything is lost ! 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 ICGC – 04, Kochi

15. An Example: Planck All Sky Cluster Survey Nclusters ~ 22000 SM & J. Mohr, 2004 ICGC – 04, Kochi

16. How well do we get `w’ ? 1s 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) ICGC – 04, Kochi

17. 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 4th 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. ) ICGC – 04, Kochi