Complex Structure of Galaxy Cluster A1689: Evidence for a Merger from X-Ray Data?

A1689 is a galaxy cluster at z=0.183 for which previous measurements of its mass by using various techniques gave discrepant results. We present a new detailed measurement of the mass with the data based on X-ray observations with the European Photon Imaging Camera aboard the XMM-Newton Observatory, determined by using an unparameterized deprojection technique. Fitting the total mass profile to a Navarro-Frenk-White model yields halo concentration c=7.2^+1.6_-2.4 and r₂₀₀=1.13+/-0.21h^-1Mpc, corresponding to a mass that is less than half of that found from gravitational lensing. Adding to the evidence of substructure from optical observations, X-ray analysis shows a highly asymmetric temperature profile and a nonuniform redshift distribution, implying large-scale relative motion of the gas. A lower than expected gas mass fraction f_gas=0.072+/-0.008 (for a flat ΛCDM cosmology) suggests a complex spatial and/or dynamical structure. We also find no sign of any additional absorbing component previously reported on the basis of the Chandra data, confirming the XMM-Newton low-energy response by using data from ROSAT.