A new look at massive clusters: weak lensing constraints on the triaxial dark matter haloes of A1689, A1835 and A2204

Measuring the three-dimensional (3D) distribution of mass on galaxy cluster scales is a crucial test of the Lambda cold darkmatter (Lambda CDM) model, providing constraints on the nature of darkmatter. Recentwork investigatingmass distributions of individual galaxy clusters (e. g. Abell 1689) using weak and strong gravitational lensing has revealed potential inconsistencies between the predictions of structure formation models relating halo mass to concentration and those relationships as measured in massive clusters. However, such analyses employ simple spherical halo models while a growing body of work indicates that triaxial 3D halo structure is both common and important in parameter estimates. We recently introduced a Markov Chain Monte Carlo method to fit fully triaxial models to weak lensing data that gives parameter and error estimates that fully incorporate the true shape uncertainty present in nature. In this paper we apply that method to weak lensing data obtained with the ESO/MPG Wide Field Imager for galaxy clusters A1689, A1835 and A2204, under a range of Bayesian priors derived from theory and from independent X-ray and strong lensing observations. For Abell 1689, using a simple strong lensing prior we find marginalized mean parameter values M-200 = (0.83 +/- 0.16) x 10(15) h(-1) M-circle dot and C = 12.2 +/- 6.7, which are marginally consistent with the mass-concentration relation predicted in Lambda CDM. With the same strong lensing prior we find for Abell 1835 M-200 = (0.67 +/- 0.22) x 10(15) h(-1) M-circle dot and C = 7.1(-7.1)(+10.6), and using weak lensing information alone find for Abell 2204 M-200 = (0.50 +/- 0.19) x 10(15) h(-1) M-circle dot and C = 7.1 +/- 6.2. The large error contours that accompany our triaxial parameter estimates more accurately represent the true extent of our limited knowledge of the structure of galaxy cluster lenses, and make clear the importance of combining many constraints from other theoretical, lensing (strong, flexion), or other observational (X-ray, Sunyaev-Zeldovich, dynamical) data to confidently measure cluster mass profiles. If we assume CDM to be correct and apply a mass-concentration prior derived from CDM structure formation simulations, we find {M-200 = (0.99 +/- 0.18) x 10(15) h(-1) M-circle dot; C = 7.7 +/- 2.1}, {M-200 = (0.68 +/- 0.19) x 10(15) h(-1) M-circle dot; C = 4.4 +/- 1.6} and {M-200 = (0.45 +/- 0.13) x 10(15) h(-1) M-circle dot; C = 5.0 +/- 1.7} for A1689, A1835 and A2204, respectively.