Self-Interacting Dark Matter, Core Collapse and the Galaxy-Galaxy Strong Lensing Discrepancy

Abstract: Gravitational lensing by galaxy clusters has emerged as a powerful tool to probe the standard Cold Dark Matter (CDM) paradigm of structure formation in the Universe. Despite the remarkable explanatory power of CDM on large scales, tensions with observations on small scales have been reported. Recent studies find that the observational cross-section for Galaxy-Galaxy Strong Lensing (GGSL) in clusters exceeds the CDM prediction by more than an order of magnitude, and persists even after rigorous examination of various possible systematics. We investigate the impact of modifying the internal structure of cluster dark matter sub-halos on GGSL and report that altering the inner density profile, given by $r^{\gamma}$, to steeper slopes with $\gamma > 2.5$ can alleviate the GGSL discrepancy. Deviating from the $\gamma \sim 1.0$ cusps that CDM predicts, these steeper slopes could arise in models of self-interacting dark matter undergoing core collapse. Our results motivate additional study of sub-halo core collapse in dense cluster environments.