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Traditionally, galaxy clusters have been expected to retain all the materialaccreted since their formation epoch. For this reason, their matter contentshould be representative of the Universe as a whole, and thus their baryonfraction should be close to the Universal baryon fraction. We make use of thesample of the 100 brightest galaxy clusters discovered in the XXL Survey toinvestigate the fraction of baryons in the form of hot gas and stars in thecluster population. We measure the gas masses of the detected halos and use amass--temperature relation directly calibrated using weak-lensing measurementsfor a subset of XXL clusters to estimate the halo mass. We find that theweak-lensing calibrated gas fraction of XXL-100-GC clusters is substantiallylower than was found in previous studies using hydrostatic masses. Our best-fitrelation between gas fraction and mass reads $f_{\rmgas,500}=0.055_{-0.006}^{+0.007}\left(M_{\rm500}/10^{14}M_\odot\right)^{0.21_{-0.10}^{+0.11}}$. The baryon budget of galaxyclusters therefore falls short of the Universal baryon fraction by about afactor of two at $r_{\rm 500}$. Our measurements require a hydrostatic bias$1-b=M_X/M_{\rm WL}=0.72_{-0.07}^{+0.08}$ to match the gas fraction obtainedusing lensing and hydrostatic equilibrium. Comparing our gas fractionmeasurements with the expectations from numerical simulations, our resultsfavour an extreme feedback scheme in which a significant fraction of thebaryons are expelled from the cores of halos. This model is, however, incontrast with the thermodynamical properties of observed halos, which mightsuggest that weak-lensing masses are overestimated. We note that a mass bias$1-b=0.58$ as required to reconcile Planck CMB and cluster counts shouldtranslate into an even lower baryon fraction, which poses a major challenge toour current understanding of galaxy clusters. [Abridged]

The XXL Survey. XIII. Baryon content of the bright cluster sample