Discrepant Mass Estimates in the Cluster of Galaxies Abell 1689

TL;DR

This study compares X-ray and lensing mass estimates of galaxy cluster Abell 1689, revealing discrepancies likely due to projection effects and instrumental biases, with implications for understanding cluster mass profiles.

Contribution

It provides a detailed analysis of mass estimate discrepancies in Abell 1689, highlighting the role of projection effects and instrumental biases in X-ray measurements.

Findings

X-ray mass estimates are 30-50% lower than lensing within 200 h-1 kpc.

At larger radii, X-ray and lensing mass profiles agree.

Instrumental absorption affects temperature measurements in Chandra observations.

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 suggest a steeper profile than what is found from the X-ray analysis, while still in agreement with the mass at large radii. 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. We also find that the previously reported high hard-band to broad-band 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.