Comparing masses in literature (CoMaLit) – I. Bias and scatter in weak lensing and X-ray mass estimates of clusters

The first building block for using galaxy clusters in astrophysics and cosmology is an accurate determination of their mass, which can be estimated with weak lensing (WL) determinations or X-ray analyses assuming hydrostatic equilibrium (HE). By comparing the two mass proxies in well observed samples of rich clusters, we determined the intrinsic scatters, σWL ∼15 per cent for WL masses and σHE ∼25 per cent for HE masses. The certain assessment of the bias is hampered by differences as large as ∼40 per cent in either WL or HE mass estimates reported by different groups. If the scatter in the mass proxy is not considered, the slope of any scaling relation ‘mass–observable ’ is biased towards shallower values, whereas the intrinsic scatter of the scaling is over-estimated.