SLICE -- Combining Strong Lensing and X-ray in AC\,114. Insights into the Merger Scenario

TL;DR

AC 114 is a historically important strong-lensing cluster whose mass and merger state are re-examined through a fully joint strong-lensing and X-ray analysis using JWST SLICE data alongside Chandra and XMM-Newton observations. The study doubles the number of strong-lensing constraints with new JWST-imaged systems and conjugated sub-spots, and explicitly models the X-ray–emitting gas with three dPIE components within Lenstool to perform a combined SL+X-ray fit. The resulting mass model reveals a unimodal dark matter halo centred on the BCG with a large core radius of $r_{ m core}=83 ext{ kpc}$, and identifies AC 114 b as a NW companion whose gas has largely been stripped, signaling a major merger in a late post-collisional phase. The work demonstrates the value of fully integrated SL+X-ray modelling for complex, dynamically active clusters in the JWST era and sets the stage for future high-resolution gas/kinematic diagnostics with missions like XRISM and Euclid.

Abstract

AC114 is a historically significant galaxy cluster, being one of the first strong lensing clusters detected from the ground in the early 1990s, prior to the launch of the HST. Despite this early prominence, no detailed lensing analyses have been carried out for more than fifteen years. We here study this cluster using JWST imaging obtained as part of the SLICE program, complemented by archival HST and X-ray observations. JWST data reveal ten new multiply imaged systems and enable the identification of conjugate substructures in several of the sixteen systems, significantly increasing the number of strong lensing constraints. Using these data, we construct a parametric mass model with Lenstool and extend it by explicitly incorporating the Chandra data in a combined strong lensing+X-ray fit. Our best-fit model reproduces the multiple images with an RMS of 0.4" while simultaneously matching the X-ray data. The dark matter distribution is unimodal and centered on the brightest cluster galaxy, with a large core radius of 83+-5kpc, consistent with values reported in other strong lensing clusters. The strong lensing constraints require the inclusion of an external shear component which position angle points unambiguously towards a nearby (~1Mpc), well defined mass concentration at the same redshift in the North-West, for which we propose the naming AC114b. The spatial coverage of the XMM-Newton data encompasses the whole structure, allowing us to probe the X-ray properties of the companion cluster and the thermodynamics of AC114, providing evidence for a major merger, in line with previous signatures seen in Chandra, radio and optical spectroscopic data. Our results shed new light on the merging scenario, revealing a major merger caught in a late post-collisional phase, where AC114 is the dominant system and Ac114b has likely been stripped of its hot gas.

Figures (9)

• Figure 1: Core of AC 114 from JWST (F322W, F150W) and HST (F814W) data. We show in red the multiple images known before JWST, and in cyan the one discovered thanks to the JWST data. We draw in white the critical curve for $z=1.87$, the redshift of systems 1 and 2.
• Figure 2: Surroundings of AC 114 from Legacy Survey DR10 data, revealing AC 114 b, dominated by BCG$_2$, located at 210$\arcsec$ (972 kpc) from the BCG. The white dashed circle centred on the BCG corresponds to the SL region of AC 114. We show the contours of the X-ray count maps from Chandra and XMM-Newton in cyan and in magenta, respectively. The field size is 392$\arcsec$$\times$392$\arcsec$. 100$\arcsec$ corresponds to 463 kpc.
• Figure 3: csmoothChandra X-ray contours are shown in cyan. The position of the BCG is marked by a yellow circle. Left: Mass contours derived from fitting the X-ray data only are shown in red. The gas distribution is modeled as a superposition of three dPIE profiles, whose positions are indicated by red circles. Right: Mass contours, shown in white, correspond to different models of the gas distribution used in the combined SL+X-ray fits. The positions of the dPIE components for each realisation are indicated by white circles.
• Figure 4: Corner plots showing the posterior distributions of the main cluster parameters for the SL+X-ray fit (black) and for the SL only fit (violet). We also show in green the results obtained by fitting the SL constraints while including the description of the X-ray component derived in Section 4.2.1 from the X-ray only fit. The RMS is equal to 0.43$\arcsec$ in all cases.
• Figure 5: Left: 2D projected total mass map. White contours correspond to the DM component, cyan to the X-ray gas, blue to the BCG, and magenta to the galaxy scale perturbers. The green contour corresponds to the total mass. For clarity, we show only one. Its similarity with the white contour highlights that the DM component dominates the overall shape of the total mass map. Right: Corresponding 1D mass profiles for the total mass (green), the X-ray gas mass (cyan), the dark matter component (black), the BCG (blue), and the galaxy-scale perturbers (magenta). The vertical dashed line corresponds to the extent of the SL region.