The XMM-LSS survey: the Class 1 cluster sample over the extended 11 deg² and its spatial distribution

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This paper presents 52 X-ray bright galaxy clusters selected within the 11 deg² XMM-LSS survey. 51 of them have spectroscopic redshifts (0.05<z<1.06), one is identified at z phot=1.9, and all together make the high-purity "Class 1" (C1) cluster sample of the XMM-LSS, the highest density sample of X-ray selected clusters with a monitored selection function. Their X-ray fluxes, averaged gas temperatures (median Tₓ=2 keV), luminosities (median Lₓ,₅₀₀=5×10⁴³ ergs/s) and total mass estimates (median 5×10¹³ h⁻¹ M) are measured, adapting to the specific signal-to-noise regime of XMM-LSS observations. The redshift distribution of clusters shows a deficit of sources when compared to the cosmological expectations, regardless of whether WMAP-9 or Planck-2013 CMB parameters are assumed. This lack of sources is particularly noticeable at 0.4 z 0.9. However, after quantifying uncertainties due to small number statistics and sample variance we are not able to put firm (i.e. >3 σ) constraints on the presence of a large void in the cluster distribution. We work out alternative hypotheses and demonstrate that a negative redshift evolution in the normalization of the Lₓ-Tₓ relation (with respect to a self-similar evolution) is a plausible explanation for the observed deficit. We confirm this evolutionary trend by directly studying how C1 clusters populate the Lₓ-Tₓ-z space, properly accounting for selection biases. We point out that a systematically evolving, unresolved, central component in clusters and groups (AGN contamination or cool core) can impact the classification as extended sources and be partly responsible for the observed redshift distribution.[abridged]