1. Cosmology with the XMM Cluster Survey (XCS)
Martin Sahlén, University of Sussex
with
Pedro Viana (Porto), Andrew Liddle, Kathy Romer (PI)
and others (XCS Consortium)
COSMO ’07, University of Sussex, 22 August 2007

2. M. Sahlén - Cosmology with the XCS
Outline
Why Galaxy Clusters?
From Theory to Predictions:
Simulation and Observation
The XMM Cluster Survey
Forecasts
Status and Conclusions
M. Sahlén - Cosmology with the XCS

3. M. Sahlén - Cosmology with the XCS
Why Galaxy Clusters?
Galaxy clusters: largest grav. bound objects, hot intracluster gas – bremsstrahlung (X-ray)
Cluster abundance exponentially sensitive to σ8 and ΩM → good constraining power
Probe structure formation; constraints complementary to CMB, SNIa, etc.
Knop et al. 2003
Allen et al. 2004
M. Sahlén - Cosmology with the XCS

4. Why Galaxy Clusters Again?
Explain different scenarios.
Getting L-T evolution and/or scatter wrong -> get constraints completely wrong
WHY TWO CONSTRAINTS GET OMEGA_M SO GOOD???
M. Sahlén - Cosmology with the XCS

5. Theoretical Components
n(M, z) – comoving number density of clusters
M(O, z) – relation between halo mass and direct observable
dV/dz – cosmic volume evolution
fsel(O,z) – probability of detecting a given cluster
Uncertainties in observables and relations
Define O_lim, number density w redshift
M. Sahlén - Cosmology with the XCS

6. Dynamics of Cluster Science
THEORY
SIMULATIONS
FITTING FORMULAE
IC’s
Particle physics
Gravita-tional theory
Cosmo-dynamics
Cluster dynamics
Mass function Jenkins et al. 2001
Bias
Sheth & Tormen 1999
Halo conc.
Neto et al. 2007
e.g. Virgo Hubble Volume
e.g. XCS
Mass-observable relations
Muanwong et al. 2006
OBSERVATIONS

7. M. Sahlén - Cosmology with the XCS
Mass Function
Jenkins mass function; Jenkins et al. 2001
Pδ(k) - PS of density contrast; depends on primordial PS, transfer function and perturbation growth suppression factor
Primordial spectrum specified, transfer function and growth factor determined by cosmology
Here: parameterisation for σ(R); Viana & Liddle 1996
M. Sahlén - Cosmology with the XCS

8. Mass-Observable/Scaling Relations
Luminosity-Temperature
Mass-Temperature
Evolution (γ, δ, η, ν)
Self-similar γ = 1/2, η = 1/3
Scatter (σlogL, σlogT)
Self-calibration and follow-up
Explain L-T: local L-T and a redshift dependence. Self-sim case, power-law case, scatter in local.
Same for M-T
The scaling relation parameters can be constrained simultaneously with cosmological params: self-calibration – cluster surveys have this capability. Follow-up direct observations too.
e.g. Levine et al. 2002, Lima & Hu 2004, 2005, Majumdar & Mohr 2004
M. Sahlén - Cosmology with the XCS

9. XMM Cluster Survey (XCS)
Mining XMM-Newton images
X-ray temperature, flux, redshift
2 keV < T < 8 keV, zmax = 1.45
500 □˚
Important ‘pathfinder’
survey for e.g. SPT,
E-ROSITA, and DES
(XCS collaboration)
M. Sahlén - Cosmology with the XCS

10. M. Sahlén - Cosmology with the XCS
Selection Function
Calculated using cluster detection pipeline with mock clusters - numerically very intensive to compute (months)
Dependencies include:
Halo model
X-ray spectrum
Detector characteristics
Cosmology
M. Sahlén - Cosmology with the XCS

11. Detecting Mock Clusters
Mock cluster added to image
Original XMM-Newton image
Original source detection
Mock source detection
M. Sahlén - Cosmology with the XCS

12. From Theory to Predictions
N-body/hydrodynamic simulations - full non-linear treatment, necessary!
Mass function, mass-observable relations, etc. calibrated to simulations/observations
Selection function: simulations using the detection pipeline
Used along with cosmology to make predictions
M. Sahlén - Cosmology with the XCS

13. Expected Number Counts
Full XCS;
ΩM = 0.3
ΩΛ = 0.7
σ8 = 0.8
Self-sim M-T
Scatter:
σlogL = 0.3
σlogT = 0.1
Explain axes. Self-sim M-T.
Just self-sim does not add that much, Scatter adds significantly.
M. Sahlén - Cosmology with the XCS

14. Expected Constraints, Full XCS
Fiducial:
ΩM = 0.3
ΩΛ = 0.7
σ8 = 0.8
Self-similar M-T
No scatter
500 □˚
2 keV < T < 8 keV
0.1 < z < 1
Flat universe
Flat universe – folding in WMAP.
No scatter. Self-sim M-T good description of data.
Two evolution cases. Linear excellent fit to self-similar.
XCS SCAL REL
Spt: also need smthg like xcs for calibration
Erosetta approved xray survey, not as sensitive or spatial res as xcs, kind of xcs full sky, xcs pathfinder, can NOT meas temperature! Need to calibrate scal rel to pther data.
planck all sky same no, higher mass, low z, calibration to xmm survey (can’t use xcs)
des quarter-half sky clusters, optical, mass-optical relation is problem – xcs ppl helping with des for this – if calibration ok, can do DE evoln. Requires xcs!
M. Sahlén - Cosmology with the XCS

15. M. Sahlén - Cosmology with the XCS
Systematic Biases
Explain different scenarios.
Getting L-T evolution and/or scatter wrong -> get constraints completely wrong
WHY TWO CONSTRAINTS GET OMEGA_M SO GOOD??? Does not appear to be coincidence but we are looking into how general these results are.
M. Sahlén - Cosmology with the XCS

16. M. Sahlén - Cosmology with the XCS
Status
XCS DR1
168 □˚
Exp. ~ clusters with >500 photons and T > 2 keV
166 candidates, 119 confirmed with redshift, BUT clusters with T < 2 keV not excluded yet
Results expected late 2008
Full XCS
500 □˚
Exp. ~ clusters with >500 photons and T > 2 keV
Results expected 2010
M. Sahlén - Cosmology with the XCS

17. M. Sahlén - Cosmology with the XCS
Conclusions
Cluster cosmology can be modeled using N-body/hydrodynamic simulation results tuned to observations
A comprehensive MCMC forecasting and data analysis code has been developed
M. Sahlén - Cosmology with the XCS

18. M. Sahlén - Cosmology with the XCS
Conclusions
XCS DR1 (2008): σ8 to 15%, ΩM to 25% (~WMAP3)
Full XCS (2010): σ8 and ΩM to 5%
Knowledge of L-T scatter and evolution necessary (self-calibration/follow-up)
XCS is a key step for cluster surveys
Allowing curvature reduces by about 5-10 percentage points.
M. Sahlén - Cosmology with the XCS

19. M. Sahlén - Cosmology with the XCS
Ongoing Work
Self-calibration
Temperature & redshift errors
Future/reference surveys
M. Sahlén - Cosmology with the XCS