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[superscript –1] kpc, the X-ray mass estimate is systematically lower than that of lensing by 30%-50%. At r>200 h[superscript –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 M [subscript 200] = (1.16[superscript +0.45] [subscript –0.27]) × 10[superscript 15] h[superscript –1] M ☉ with a concentration, c 200 = 5.3[superscript +1.3] [subscript –1.2], using nonparametric mass modeling. With parametric profile modeling, we find M 200 = (0.94[superscript +0.11] –0.06) × 10[superscript 15] h[superscript –1] M ☉ and c 200 = 6.6[superscript +0.4] [subscript –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 previously reported high hard-band to broadband temperature ratio in A1689, and many other clusters observed with Chandra, may be resulting from the instrumental absorption that decreases 10%-15% of the effective area at ~1.75 keV. Caution must be taken when analyzing multiple spectral components under this calibration uncertainty.