The XMM-LSS survey: the Class 1 cluster sample over the initial 5 deg2 and its cosmological modelling

Abstract

We present a sample of 29 galaxy clusters from the XMM‐LSS survey over an area of some 5 deg2 out to a redshift of z= 1.05. The sample clusters, which represent about half of the X‐ray clusters identified in the region, follow well‐defined X‐ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X‐ray luminosities and temperatures as well as masses, obtained from dedicated spatial and spectral fitting. The cluster distribution peaks around z= 0.3 and T= 1.5 keV, half of the objects being groups with a temperature below 2 keV. Our LX–T(z) relation points towards self‐similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self‐similar evolution, our number density estimates up to z= 1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well‐monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the LX–T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10 deg2 has the potential to exclude the non‐evolution hypothesis, but those constraints on more refined intracluster medium models will probably be limited by the large intrinsic dispersion of the LX–T relation, whatever be the sample size. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to σ8, Ωm, LX–T(z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass–flux and mass–count rate relations as well as a comparison between the XMM‐LSS mass sensitivity and that of forthcoming Sunyaev–Zel'dovich surveys.

Scaling Relations for Galaxy Clusters: Properties and Evolution

“…For this reason no clear consensus has been reached on the evolution of the X-ray scaling relations, despite a number of observational studies carried out in the past decade (e.g. Vikhlinin et al 2002Vikhlinin et al , 2009aEttori et al 2004b;Kotov & Vikhlinin 2005;Maughan et al 2006;Maughan 2007;Maughan et al 2012;O'Hara et al 2006;Morandi et al 2007;Branchesi et al 2007;Pacaud et al 2007;Andreon et al 2011;Reichert et al 2011).…”

“…This bias is commonly known as Malmquist bias and should be taken into account in both the scaling relation calibration and the cosmological analysis based on such relations. Some works in which this bias has been properly taken into account in the interpretation of scaling relations are Ikebe et al (2002), Stanek et al (2006), Pacaud et al (2007), Pratt et al (2009),Vikhlinin et al (2009a. Mittal et al (2011) even applied individual Malmquist bias corrections for SCC, WCC, and NCC clusters.…”

Observational probes of cosmic acceleration

“…Turning to X-rays, the REFLEX, XXL, and eROSITA curves all show the increase of mass threshold with redshift characteristic of flux-limited surveys. The XXL selection is that of Valageas et al (2011) scaled to match the observed density of C1 clusters in the XMM-LSS field (Pacaud et al, 2007), while the eROSITA threshold represents a flux limit ≈ 4 × 10 −14 erg s −1 , corresponding to ≈ 50 photon counts (Pillepich et al, 2012). The mass limit is higher by a factor of ≈ 3 for clusters reaching 300 photon counts.…”

Covariance matrices for halo number counts and correlation functions

“…We show the mass detection probabilities as a function of redshift in the left hand panel of Fig density of this population is ∼6 deg −2 . This complex selection function F(M, z), which differs from a simple mass or X-ray flux threshold (see also Pacaud et al 2006Pacaud et al , 2007, is readily included in our formalism through a redefinition of the halo mass function, n(M, z) → F(M, z)n(M, z). -The Dark Energy Survey (DES) is an optical imaging survey to cover 5000 deg 2 with the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory 6 .…”

“…Indeed, according to the standard cosmological model, the large-scale structures of the present Universe have formed through the amplification by gravitational instability of small almost-Gaussian primordial fluctuations (Peebles 1980). Then, from observations of the recent Universe, such as galaxy surveys (Cole et al 2005;Tegmark et al 2006), cluster surveys (Evrard 1989;Oukbir & Blanchard 1992;Pacaud et al 2007), weak-lensing studies (Massey et al 2007;Munshi et al 2008), or measures of baryon acoustic oscillations (Eisenstein et al , 2005, one can derive constraints on the cosmological parameters (e.g., the mean matter and dark energy contents) and on the properties of the initial perturbations (e.g., possible deviations from Gaussianity). Moreover, one can check whether these structures have really formed through this gravitational instability process.…”