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The growth of brightest cluster galaxies (BCGs) is closely related to the properties of their host cluster. We present evidence for dry mergers as the dominant source of BCG mass growth at z ≲ 1 in the XXL 100 brightest cluster sample. We use the global red sequence, Hα emission and mean star formation history to show that BCGs in the sample possess star formation levels comparable to field ellipticals of similar stellar mass and redshift. XXL 100 brightest clusters are less massive on average than those in other X-ray selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display high central gas concentration, rendering inefficient the growth of BCGs via star formation resulting from the accretion of cool gas. Using measures of the relaxation state of their host clusters, we show that BCGs grow as relaxation proceeds. We find that the BCG stellar mass corresponds to a relatively constant fraction 1 per cent of the total cluster mass in relaxed systems. We also show that, following a cluster scale merger event, the BCG stellar mass lags behind the expected value from the Mcluster–MBCG relation but subsequently accretes stellar mass via dry mergers as the BCG and cluster evolve towards a relaxed state. [ABSTRACT FROM AUTHOR]

Traditionally, galaxy clusters have been expected to retain all the material accreted since their formation epoch. For this reason, their matter content should be representative of the Universe as a whole, and thus their baryon fraction should be close to the Universal baryon fraction b= m. We make use of the sample of the 100 brightest galaxy clusters discovered in the XXL Survey to investigate the fraction of baryons in the form of hot gas and stars in the cluster population. Since it spans a wide range of mass (1013-1015 M⊙) and redshift (0:05-1:1) and benefits from a large set of multiwavelength data, the XXL-100-GC sample is ideal for measuring the global baryon budget of massive halos. We measure the gas masses of the detected halos and use a mass-temperature relation directly calibrated using weak-lensing measurements for a subset of XXL clusters to estimate the halo mass. We find that the weak-lensing calibrated gas fraction of XXL-100-GC clusters is substantially lower than was found in previous studies using hydrostatic masses. Our best-fit relation between gas fraction and mass reads fgas;500 = 0:055+0:007 -0:006 M500=1014 M⊙ 0:21+0:11 -0:10. The baryon budget of galaxy clusters therefore falls short of the Universal baryon fraction by about a factor of two at r500;MT. Our measurements require a hydrostatic bias 1-b = MX=MWL = 0:72+0:08 -0:07 to match the gas fraction obtained using lensing and hydrostatic equilibrium, which holds independently of the instrument considered. Comparing our gas fraction measurements with the expectations from numerical simulations, we find that our results favour an extreme feedback scheme in which a significant fraction of the baryons are expelled from the cores of halos. This model is, however, in contrast with the thermodynamical properties of observed halos, which might suggest that weak-lensing masses are overestimated. In light of these results, we note that a mass bias 1 - b = 0:58 as required to reconcile Planck cosmic microwave background and cluster counts should translate into an even lower baryon fraction, which poses a major challenge to our current understanding of galaxy clusters. [ABSTRACT FROM AUTHOR]

Context: The XXL Survey is the largest homogeneous and contiguous survey carried out with XMM-Newton. Covering an area of 50 deg² distributed over two fields, it primarily investigates the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution. Aims: Given its depth and sky coverage, XXL is particularly suited to systematically unveiling the clustering of X-ray clusters and to identifying superstructures in a homogeneous X-ray sample down to the typical mass scale of a local massive cluster. Methods: A friends-of-friends algorithm in three-dimensional physical space was run to identify large-scale structures. In this paper we report the discovery of the highest redshift supercluster of galaxies found in the XXL Survey. We describe the X-ray properties of the clusters members of the structure and the optical follow-up. Results: The newly discovered supercluster is composed of six clusters of galaxies at a median redshift z ~ 0:43 and distributed across ~300×150 (10×5 Mpc) on the sky. This structure is very compact with all the clusters residing in oneXMMpointing; for this reason this is the first supercluster discovered with the XXL Survey. Photometric redshifts from the CFHTLS (Canada-France-Hawaii Telescope Legacy Survey) data release T0007 placed the supercluster at an approximate redshift of zphot ~ 0:45; subsequent spectroscopic follow-up with WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a median redshift of z ~ 0:43. An estimate of the X-ray mass and luminosity of this supercluster returns values of 1:7 × 1015 M⊙ and of 1:68 × 1044 erg s-1, respectively, and a total gas mass of Mgas = 9:3 × 1013 M⊙. These values put XLSSC-e at the average mass range of superclusters; its appearance, with two members of equal size, is quite unusual with respect to other superclusters and provides a unique view of the formation process of a massive structure. [ABSTRACT FROM AUTHOR]

Context: The XXL Survey is the largest survey carried out by XMM-Newton. Covering an area of 50 deg², the survey contains ~450 galaxy clusters out to a redshift ~2 and to an X-ray flux limit of ~5 × 10-15 erg s-1 cm-2. This paper is part of the first release of XXL results focussed on the bright cluster sample. Aims: We investigate the scaling relation between weak-lensing mass and X-ray temperature for the brightest clusters in XXL. The scaling relation discussed in this article is used to estimate the mass of all 100 clusters in XXL-100-GC. Methods: Based on a subsample of 38 objects that lie within the intersection of the northern XXL field and the publicly available CFHTLenS shear catalog, we derive the weak-lensing mass of each system with careful considerations of the systematics. The clusters lie at 0.1 < z < 0.6 and span a temperature range of T ⋍ 1-5 keV. We combine our sample with an additional 58 clusters from the literature, increasing the range to T ⋍ 1-10 keV. To date, this is the largest sample of clusters with weak-lensing mass measurements that has been used to study the mass-temperature relation. Results: The mass-temperature relation fit (M ∝ Tb) to the XXL clusters returns a slope b = 1.78+0.37 -0.32 and intrinsic scatter σlnM|T ⋍ 0.53; the scatter is dominated by disturbed clusters. The fit to the combined sample of 96 clusters is in tension with selfsimilarity, b = 1.67 ± 0.12 and σlnM|T ⋍ 0.41. Conclusions: Overall our results demonstrate the feasibility of ground-based weak-lensing scaling relation studies down to cool systems of ~1 keV temperature and highlight that the current data and samples are a limit to our statistical precision. As such we are unable to determine whether the validity of hydrostatic equilibrium is a function of halo mass. An enlarged sample of cool systems, deeper weak-lensing data, and robust modelling of the selection function will help to explore these issues further. [ABSTRACT FROM AUTHOR]

Galaxy clusters and groups are important cosmological probes and giant cosmic laboratories for studying galaxy evolution. Much effort has been devoted to understanding how and when baryonic matter cools at the centre of potential wells. However, a clear picture of the effciency with which baryons are converted into stars is still missing. We present the K-band luminosity-halo mass relation, LK;500 - M500;WL, for a subsample of 20 of the 100 brightest clusters in the XXL Survey observed with WIRCam at the Canada-France-Hawaii Telescope (CFHT). For the first time, we have measured this relation via weak-lensing analysis down to M500;WL = 3:5 × 1013 M⊙. This allows us to investigate whether the slope of the LK - M relation is different for groups and clusters, as seen in other works. The clusters in our sample span a wide range in mass, M500;WL = 0:35-12:10 × 1014 M⊙, at 0 < z < 0:6. The K-band luminosity scales as log10(LK;500=1012 L⊙) / ⊙ log10(M500;WL=1014 M⊙) with ⊙ = 0:85+0:35 -0:27 and an intrinsic scatter of ln LKjM = 0:37+0:19 -0:17. Combining our sample with some clusters in the Local Cluster Substructure Survey (LoCuSS) present in the literature, we obtain a slope of 1:05+0:16 -0:14 and an intrinsic scatter of 0:14+0:09 -0:07. The flattening in the LK - M seen in previous works is not seen here and might be a result of a bias in the mass measurement due to assumptions on the dynamical state of the systems. We also study the richness-mass relation and find that group-sized halos have more galaxies per unit halo mass than massive clusters. However, the brightest cluster galaxy (BCG) in low-mass systems contributes a greater fraction to the total cluster light than BCGs do in massive clusters; the luminosity gap between the two brightest galaxies is more prominent for group-sized halos. This result is a natural outcome of the hierarchical growth of structures, where massive galaxies form and gain mass within low-mass groups and are ultimately accreted into more massive clusters to become either part of the BCG or one of the brighter galaxies. [ABSTRACT FROM AUTHOR]