We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space-and ground-based data with constant or cosmologically-varying shear fields. The simplest (control) experiment included parametric galaxies with a realistic distribution of signal-to-noise, size, and ellipticity, and a complex point spread function (PSF). The other experiments tested the additional impact of realistic galaxy morphology, multiple exposure imaging, and the uncertainty about a spatially-varying PSF; the last two questions will be explored in Paper II. The 24 participating teams competed to estimate lensing shears to within systematic error tolerances for upcoming Stage-IV dark energy surveys, making 1525 submissions overall. GREAT3 saw considerable variety and innovation in the types of methods applied. Several teams now meet or exceed the targets in many of the tests conducted (to within the statistical errors). We conclude that the presence of realistic galaxy morphology in simulations changes shear calibration biases by ∼ 1 per cent for a wide range of methods. Other effects such as truncation biases due to finite galaxy postage stamps, and the impact of galaxy type as measured by the Sérsic index, are quantified for the first time. Our results generalize previous studies regarding sensitivities to galaxy size and signal-to-noise, and to PSF properties such as seeing and defocus. Almost all methods' results support the simple model in which additive shear biases depend linearly on PSF ellipticity.

We review the methods adopted to reconstruct the mass profiles in X-ray luminous galaxy clusters. We discuss the limitations and the biases affecting these measurements and how these mass profiles can be used as cosmological proxies.

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 M 200 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.