Abell 611. II. X-ray and strong lensing analyses

We present the results of our analyses of the X-ray emission and of the strong lensing systems in the galaxy cluster Abell 611 [z = 0.288]. This cluster is an optimal candidate for a comparison of the mass reconstructions obtained through X-ray and lensing techniques, because of its very relaxed dynamical appearance and its exceptional strong lensing system. We infer the X-ray mass estimate deriving the density and temperature profile of the intra-cluster medium within the radius r ≃ 700 kpc through a non-parametric approach, taking advantage of the high spatial resolution of a Chandra observation. Assuming that the cluster is in hydrostatic equilibrium and adopting a matter density profile, we can recover the total mass distribution of Abell 611 via the X-ray data. Moreover, we derive the total projected mass in the central regions of Abell 611 performing a parametric analysis of its strong lensing features through the publicly available analysis software Lenstool. As a final step we compare the results obtained with both methods. We derive a good agreement between the X-ray and strong lensing total mass estimates in the central regions where the strong lensing constraints are present (i.e. within the radius r ≃ 100 kpc), while a marginal disagreement is found between the two mass estimates when extrapolating the strong lensing results in the outer spatial range. We suggest that in this case the X-ray/strong lensing mass disagreement can be explained by an incorrect estimate of the relative contributions of the baryonic component and of the dark matter, caused by the intrinsic degeneracy between the different mass components in the strong lensing analysis. We discuss the effect of some possible systematic errors that influence both mass estimates. We find a slight dependence of the measurements of the X-ray temperatures (and therefore of the X-ray total masses) on the background adopted in the spectral analysis, with total deviations on the value of M₂₀₀ of the order of the 1σ statistical error. The strong lensing mass results are instead sensitive to the parameterisation of the galactic halo mass in the central regions, in particular to the modelling of the brightest cluster galaxy (BCG) baryonic component, which induces a significant scatter in the strong lensing mass results.