Bridging Arcs to Shear : Multiscale Dark Matter Mapping in Galaxy Clusters with Strong and Weak Gravitational Lensing

Abstract

Matter in the Universe is inferred to be dominated by a collisionless, non-baryonic component, dark matter, which accounts for roughly 80% of the total matter content. Since dark matter emits no electromagnetic radiation, its presence must be probed indirectly via its gravitational interactions with baryonic matter. To date, direct detection experiments have yielded null results, and gravitational lensing, where the light from background sources is deflected and distorted by an intervening mass distribution, is one of the primary tools used to probe dark matter indirectly. Galaxy clusters, as the most massive observable structures, assemble via hierarchical accretion and merger events. Their large dark matter content makes them ideal to map dark matter. In dense cluster cores, strong lensing produces multiple, highly distorted images (or arcs) of background galaxies; at larger radii, weak lensing induces coherent shear patterns in the shapes of distant galaxies. By combining both regimes, clusters' projected mass distribution can be reconstructed from their core and up to several megaparsecs.

In this thesis, two contrasting systems are examined via a combined strong and weak lensing analysis. First, MACSJ1423 (z = 0.545), a highly relaxed system, its mass distribution has been reconstructed using deep imaging from the Hubble Space Telescope (HST). No significant substructure is detected in the cluster, consistent with a dynamically mature state. Second, Abell2744 (z = 0.308), a major merger, is analysed with joint HST and James Webb Space Telescope (JWST) observations. The resulting mass distribution yields six massive substructures (including its core). The abundance and spatial distribution of these substructures are sensitive to the properties of dark matter, providing an observational test of dark matter to possibly distinguish between cold, warm and self-interacting dark matter candidates. The potential of the subhalo mass function to put constraints on these dark matter candidates is discussed, and plans for future comparisons with cosmological simulations are outlined.

Finally, the SLICE (Strong LensIng and Cluster Evolution) survey illustrates the next generation of strong and weak lensing studies. A new mass model for MACSJ0027, derived from JWST imaging, is presented as a proof of concept, and the challenges of combining ultra-deep, multi-wavelength data with sophisticated lensing algorithms are discussed. These developments marks a new era of precision cosmology and dark matter characterisation.