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 Grale, 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 free-form methods like Grale. We used two different sets of multiple image 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). For the very well-constrained central $\sim$100 kpc region of the cluster we made detailed comparison of the Grale reconstructed lensing mass and stellar mass retrieved by the Spectral Energy Distribution (SED) fitting software FAST++. We found a light-unaccompanied mass peak in this region, whose existence, while tentative, is favored by the distribution of nearby images that are local maxima in the Fermat potential. However, further tests, using different methodologies are needed to confirm the reality of this feature. If it shown to be real, this light-unaccompanied mass peak is consistent with dark matter self-interaction cross-section $\sigma \lesssim 1$cm$^2$/g, while being in tension with larger cross-sections.