An excursion into the core of the cluster lens Abell 1689

Abstract

Abell 1689 is a well studied cluster of galaxies and one of the largest gravitational lens systems ever observed. We have obtained a reconstruction of the cluster Abell 1689 using G , a free-form lens inversion method that relies exclusively on the multiple image data. Non-inclusion of any data related to cluster member galaxies ensures an unbiased measure of the mass distribution, which is the most notable feature of this method. We used two different sets of multiply lensed systems from the available strong lensing data -one containing only the secure systems (107 images), and the other containing all available systems, only excluding some very non-secure systems (151 images). Both the reconstructions produced similar mass distributions whose circularly symmetric radial profiles are well fit with the Navarro-Frenk-White (NFW) profile with concentration parameter values, 𝑐 ∼ 6.8. For the very well-constrained central ∼100 kpc region of the cluster we made detailed comparison of the G reconstructed lensing mass and stellar mass retrieved by the Spectral Energy Distribution (SED) fitting software ++. We found an offset in the light peak of the Brightest Cluster Galaxy (BCG) and its associated lensing mass peak, of about 10 kpc. We also found a light-unaccompanied mass peak in this region, whose location agrees with features retrieved by some of the earlier reconstructions using different methodologies. Both the light-unaccompanied mass peak and the BCG offset are consistent with dark matter self-interaction cross-section 𝜎 1cm 2 /g, while the mass peak is in tension with larger cross-sections.