The X-ray/SZ view of the virial region II. Gas mass fraction

Aims. Several recent studies used the hot gas fraction of galaxy clusters as a standard ruler to constrain dark energy, which provides results compared to other techniques. This method, however, relies on the assumption that the baryon fraction in clusters agrees with the cosmic value Omega(b)/Omega(m), and does not differ from one system to another. We test this hypothesis by measuring the gas mass fraction over the entire cluster volume in a sample of local clusters. Methods. Combining the Sunyaev-Zel'dovich thermal pressure from Planck and the X-ray gas density from ROSAT, we measured for the first time the average gas fraction (f(gas)) out to the virial radius and beyond in a large sample of clusters. We also obtained azimuthally-averaged measurements of the gas fraction for 18 individual systems, which we used to compute the scatter of f(gas) around the mean value at different radii and its dependence on the cluster's temperature. Results. The gas mass fraction increases with radius and reaches the cosmic baryon fraction close to R-200. At R-200, we measure f(gas,200) = 0.176 +/- 0.009 (0.166 +/- 0.012 for the subsample of 18 clusters in common between Planck and ROSAT). We find significant differences between the baryon fraction of relaxed, cool-core (CC) systems and unrelaxed, non-cool core (NCC) clusters in the outer regions. On average, the gas fraction in NCC clusters slightly exceeds the cosmic baryon fraction, while in CC systems the gas fraction converges to the expected value when accounting for the stellar content, without any evidence for variations from one system to another. Conclusions. We find that f(gas) estimates in NCC systems slightly disagree with the cosmic value approaching R-200. This result could be explained either by a violation of the assumption of hydrostatic equilibrium or by an inhomogeneous distribution of the gas mass. Conversely, CC clusters are found to provide reliable constraints on f(gas) at overdensities Delta > 200, which makes them suitable for cosmological studies.