The X-ray/SZ view of the virial region. II. Gas mass fraction. (arXiv:1301.0624v1 [astro-ph.CO])

The X-ray/SZ view of the virial region. II. Gas mass fraction. (arXiv:1301.0624v1 [astro-ph.CO]):
Several recent studies used the hot gas fraction of galaxy clusters as a
standard ruler to constrain dark energy, which provides competitive 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. Combining the SZ thermal pressure from Planck and
the X-ray gas density from ROSAT, we measured for the first time the average
gas fraction (fgas) 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
fgas around the mean value at different radii and its dependence on the
cluster's temperature. The gas mass fraction increases with radius and reaches
the cosmic baryon fraction close to R200. At R200, we measure
fgas,200=0.176+/-0.009. 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. In 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. We
find that fgas estimates in NCC systems slightly disagree with the cosmic value
approaching R200. 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, cool-core clusters are found to provide reliable
constraints on fgas at overdensities >200, which makes them suitable for
cosmological studies.