DISCREPANT MASS ESTIMATES IN THE CLUSTER OF GALAXIES ABELL 1689

Abstract

We present a new mass estimate of a well studied gravitational lensing cluster, Abell 1689, from deep Chandra observations with a total exposure of 200 ks. Within r = 200 h−1 kpc, the X-ray mass estimate is systematically lower than that of lensing by 30%–50%. At r> 200 h−1 kpc, the mass density profiles from X-ray and weak lensing methods give consistent results. The most recent weak lensing work suggests a steeper profile than what is found from the X-ray analysis, while still in agreement with the mass at large radii. Fitting the total mass profile to a Navarro–Frenk–White model, we find M200 = (1.16+0.45−0.27) × 1015 h−1 M with a concentration, c200 = 5.3+1.3−1.2, using nonparametric mass modeling. With parametric profile modeling, we find M200 = (0.94+0.11−0.06)×1015 h−1 M and c200 = 6.6+0.4−0.4. This is much lower compared to masses deduced from the combined strong and weak lensing analysis. Previous studies have suggested that cooler small-scale structures can bias X-ray temperature measurements or that the northern part of the cluster is disturbed. We find these scenarios unlikely to resolve the central mass discrepancy since the former requires 70%–90 % of the space to be occupied by these cool structures, and excluding the northern substructure does not significantly affect the total mass profiles. A more plausible explanation is a projection effect. Assuming that the gas temperature and density profiles have a prolate symmetry, we can bring the X-ray mass estimate into a closer agreement with that of lensing. We also find that the