Dents in the mirror: a novel probe of dark matter substructure in galaxy clusters from the astrometric asymmetry of lensed arcs

ABSTRACT Astrometric perturbations of lensed arcs behind galaxy clusters have been recently suggested as promising probes of small-scale ($\lesssim 10^9 {\rm M}_{\odot }$) dark matter substructure. Populations of cold dark matter (CDM) subhaloes, predicted in hierarchical structure formation theory, can break the symmetry of arcs near the critical curve, leading to positional shifts in the observed images. We present a novel statistical method to constrain the average subhalo mass fraction ($f_{\rm sub}$) in clusters that takes advantage of this induced positional asymmetry. Focusing on CDM, we extend a recent semi-analytic model of subhalo tidal evolution to accurately simulate realistic subhaloes within a cluster-scale host. We simulate the asymmetry of lensed arcs from these subhalo populations using approximate Bayesian computation. Using mock data, we demonstrate that our method can reliably recover the simulated $f_{\rm sub}$ to within 68 per cent confidence interval (CI) in 73 per cent of cases, regardless of the lens model, astrometric precision, and image morphology. We show that the constraining power of our method is optimized for larger samples of well-observed arcs, ideal for recent James Webb Space Telescope observations of cluster lenses. As a preliminary test, we apply our method to the MACSJ0416 Warhol arc and AS1063 System 1. For Warhol, we constrain the upper limit on $\log f_{\rm sub} \lt-3.48^{+1.00}_{-0.91}$, while for AS1063 System 1, we constrain $\log f_{\rm sub} = -2.44^{+0.61}_{-0.86}$ (both at 68 per cent CI), consistent with CDM predictions. We elaborate on our method’s limitations and its future potential to place stringent constraints on dark matter properties in cluster environments.