Systematics in lensing reconstruction: dark matter rings in the sky?

Abstract

Context. Non-parametric lensing methods are a useful way of reconstructing the lensing mass of a cluster without making assumptions about the way the mass is distributed in the cluster. These methods are particularly powerful in the case of galaxy clusters with a large number of constraints. The advantage of not assuming implicitly that the luminous matter follows the dark matter is particularly interesting in those cases where the cluster is in a non-relaxed dynamical state. On the other hand, non-parametric methods have several limitations that should be taken into account carefully. Aims. We explore some of these limitations and focus on their implications for the possible ring of dark matter around the galaxy cluster CL0024+17. Methods. We project three background galaxies through a mock cluster of known radial profile density and obtain a map for the arcs (θ map). We also calculate the shear field associated with the mock cluster across the whole field of view (3.3 arcmin). Combining the positions of the arcs and the two-direction shear, we perform an inversion of the lens equation using two separate methods, the biconjugate gradient, and the quadratic programming (QADP) to reconstruct the convergence map of the mock cluster. Results. We explore the space of the solutions of the convergence map and compare the radial density profiles to the density profile of the mock cluster. When the inversion matrix algorithms are forced to find the exact solution, we encounter systematic effects resembling ring structures, that clearly depart from the original convergence map. Conclusions. Overfitting lensing data with a non-parametric method can produce ring-like structures similar to the alleged one in CL0024.

Structure of Abell 1995 from optical and X-ray data: a galaxy cluster with an elongated radio halo

“…Moreover, this disagrees with the results found for other extensively studied clusters, such as the Bullet Cluster (e.g., Markevitch et al 2002) or clusters CL 0152-1357 and MS 1054-0321 (Jee et al 2005a,b), where a coincidence is found between the galaxy and dark matter distributions. Nevertheless, the peculiarity of A1995 is not so extreme as in other clusters, such as CL 0024+17, where evidence was found for a dark matter ring (Jee et al 2007) without a obvious galaxy counterpart (Qin et al 2008; but see also Ponente & Diego 2011, who suggest that dark matter ringlike structures could be due to systematics in lensing reconstruction). In any case, the observed dichotomy between the dark matter and galaxy distributions makes A1995 an appealing target for future studies.…”

GEOMETRIC CORROBORATION OF THE EARLIEST LENSED GALAXY AT z ≃ 10.8 FROM ROBUST FREE-FORM MODELLING

“…Here we outline briefly WSLAP+ for the mass reconstruction and refer the reader for details of its implementation in our previous papers (Diego et al 2005a(Diego et al , 2005b(Diego et al , 2007(Diego et al , 2015aPonente & Diego 2011;Sendra et al 2014).…”

The universal Einstein radius distribution from 10 000 SDSS clusters

“…Diego et al 2005; Coe et al 2008; Merten et al 2009) are novel in the sense that they are assumption free and highly flexible (e.g. Coe et al 2010; Ponente & Diego 2011), and methods based directly on arc morphologies yield high‐resolution results (see also Grillo et al 2010). The parametric method presented here is simply aimed to produce the critical curves in an automated way based on simple physical considerations (and thus is capable of finding multiple images as we have shown constantly before), and constitutes another important step towards the ability to deduce the lensing properties of clusters in large sky surveys in an automated way, so that we aim now to present the first observationally deduced, universal distribution of Einstein radii.…”