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 Ω b /Ω 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 (10 13 −10 15 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 f gas,500 = 0.055 +0.007 −0.006 M 500 /10 14 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 r 500,MT . Our measurements require a hydrostatic bias 1 − b = M X /M WL = 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.

Context. The XXL Survey is the largest homogeneous survey carried out with XMM-Newton. Covering an area of 50 deg 2 , the survey contains several hundred galaxy clusters out to a redshift of ∼2 above an X-ray flux limit of ∼5 × 10 −15 erg cm −2 s −1 . This paper belongs to the first series of XXL papers focusing on the bright cluster sample. Aims. We investigate the luminosity-temperature (LT) relation for the brightest clusters detected in the XXL Survey, taking fully into account the selection biases. We investigate the form of the LT relation, placing constraints on its evolution. Methods. We have classified the 100 brightest clusters in the XXL Survey based on their measured X-ray flux. These 100 clusters have been analysed to determine their luminosity and temperature to evaluate the LT relation. We used three methods to fit the form of the LT relation, with two of these methods providing a prescription to fully take into account the selection effects of the survey. We measure the evolution of the LT relation internally using the broad redshift range of the sample. Results. Taking fully into account selection effects, we find a slope of the bolometric LT relation of B LT = 3.08±0.15, steeper than the self-similar expectation (B LT = 2). Our best-fit result for the evolution factor is E(z)1.64±0.77 , fully consistent with "strong self-similar" evolution where clusters scale self-similarly with both mass and redshift. However, this result is marginally stronger than "weak selfsimilar" evolution, where clusters scale with redshift alone. We investigate the sensitivity of our results to the assumptions made in our fitting model, finding that using an external LT relation as a low-z baseline can have a profound effect on the measured evolution. However, more clusters are needed in order to break the degeneracy between the choice of likelihood model and mass-temperature relation on the derived evolution.

We test the assumption of hydrostatic equilibrium in an X-ray luminosity selected sample of 50 galaxy clusters at 0.15 < z < 0.3 from the Local Cluster Substructure Survey (LoCuSS). Our weak-lensing measurements of M 500 control systematic biases to sub-4 per cent, and our hydrostatic measurements of the same achieve excellent agreement between XMM-Newton and Chandra. The mean ratio of X-ray to lensing mass for these 50 clusters is β X = 0.95 ± 0.05, and for the 44 clusters also detected by Planck, the mean ratio of Planck mass estimate to LoCuSS lensing mass is β P = 0.95±0.04. Based on a careful like-for-like analysis, we find that LoCuSS, the Canadian Cluster Comparison Project (CCCP), and Weighing the Giants (WtG) agree on β P ≃ 0.9 − 0.95 at 0.15 < z < 0.3. This small level of hydrostatic bias disagrees at ∼ 5σ with the level required to reconcile Planck cosmology results from the cosmic microwave background and galaxy cluster counts.

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Context. The XXL Survey is the largest survey carried out by XMM-Newton. Covering an area of 50 deg 2 , 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 ∝ T b ) to the XXL clusters returns a slope b = 1.78−0.32 and intrinsic scatter σ ln M|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 σ ln M|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.Key words. gravitational lensing: weak -X-rays: galaxies: clusters -galaxies: groups: general -galaxies: clusters: general The Master catalogue is available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via