Tracing Structure: Shape and Centroid Deviations in 39 Strong Lensing Clusters as a Test of Cluster Formation Predictions

TL;DR

This study tests structure formation predictions of $Λ$CDM by comparing how well the baryonic components (BCG, ICL, ICM) trace the DM halo, as represented by the Core Lensing Mass (CLM), in 39 strong-lensing clusters. It employs ellipse-fitting on multi-wavelength data (HST for BCG/ICL, Chandra for ICM, and lensing-derived CLM) to measure shapes and centroids, and uses ICM morphology to classify dynamical state. The results show that misalignments arise from cluster assembly, relaxation, and merger histories, with the ICL most closely aligned with the DM halo (as traced by CLM) in both position angle and centroid, and with the ICL/CLM being more elliptical on average than the ICM and BCG. These findings provide empirical constraints on how well $Λ$CDM-based predictions match the internal structure of clusters and the fidelity of different tracers in mapping the gravitational potential.

Abstract

Strong lensing galaxy clusters provide a unique and powerful way to test simulation-derived structure predictions that follow from $Λ$ Cold Dark Matter ($Λ$CDM) cosmology. Specifically, the relative alignments of the dark matter (DM) halo, stars, and hot intracluster gas in these clusters offer insights into how well theoretical structure predictions hold. We measure the position angles, ellipticities, and locations/centroids of the brightest cluster galaxy (BCG), the Intracluster Light (ICL), the hot Intracluster Medium (ICM), and the Core Lensing Mass (CLM) for a sample of strong lensing galaxy clusters from the Sloan Giant Arcs Survey (SGAS). We measure the shapes (position angles and ellipticities) and centroids of these distributions using ellipse-fitting methods applied to different datasets: HST WFC3 imaging for the BCG and ICL, Chandra X-ray observations for the ICM, and strong-lensing mass reconstructions for the CLM. Additionally, we incorporate ICM morphological measures to classify the dynamical state of the cluster sample. Using this multi-component approach, we constrain the shape and centroids of these distributions in this sample and evaluate the different observable components in terms of their ability to trace the gravitational potential of their respective clusters. We find that misalignments between cluster components can be explained by astrophysical processes related to cluster assembly, relaxation, and merger histories. We find that the ICL is most closely aligned with its host DM halo, as traced by the CLM distribution, in both position angle and centroid. Additionally, we find that on average the ICL and CLM are more elliptical than the ICM and BCG.

Figures (1)

• Figure 1: Visualization of the cluster components considered in this work (the CLM, ICL, BCG, and ICM) for an example galaxy cluster, J0957+0509. Each panel displays a red ellipse representing the measured shape and centroid. Center panel: a $5"\times5"$ representative color (gri) Subaru image of the field Oguri2012 for a scale comparison of the different cluster components. Top-left: the minimum $\chi^2$ mass map generated from the lensing profile derived in Sharon2020. Top-right: HST WFC3/IR F160W imaging data Sharon2020 tuned to highlight the ICL distribution. Bottom-Left: Chandra data displaying the ICM Gassis2025 with an arrow pointing to the ICM model derived using CIAO Sherpa. Bottom-Right: HST WFC3/IR F160W imaging data Sharon2020 tuned to highlight the BCG distribution with an arrow pointing to the BCG model derived using GALFITPeng2002.