ALBERT

Description: This paper presents 52 X-ray bright galaxy clusters selected within the 11 deg 2 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 T X  = 2 keV), luminosities (median L X, 500 = 5 x 10 43  erg s –1 ) and total mass estimates (median 5 10 13 h –1 M ) are measured, adapting to the specific signal-to-noise regime of XMM -LSS observations. Particular care is taken in deriving the sample selection function by means of realistic simulations reproducing the main characteristics of XMM observations. The redshift distribution of clusters shows a deficit of sources when compared to the cosmological expectations, regardless of whether Wilkinson Microwave Anisotropy Probe -9 or Planck -2013 cosmic microwave background 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) 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 L X - T X 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 L X - T X - z space, properly accounting for selection biases. We also 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. We provide in a table the catalogue of 52 clusters together with their measured properties.