Kinematically isolating intracluster light in AS1063: a distinct stellar population tracing the dark matter core

We present a technique using integral-field spectroscopy to kinematically isolate intracluster light (ICL) from the brightest cluster galaxy (BCG), enabling insights into their contrasting stellar populations and the inner dark matter halo profile. Applying this technique to MUSE data of Abell S1063, we find that both components are old (~9 Gyr), but the ICL is more metal-poor than the BCG, implying distinct progenitor pathways. Within 100 kpc, the ICL comprises 55 per cent of the BCG+ICL stellar mass - substantially more than inferred from photometric decompositions. The BCG and ICL possess distinct velocity distributions, which remain nearly constant with radius, with an ICL velocity dispersion more than twice that of the BCG, and a line-of-sight velocity offset of 86 ± 18 km/s. Two-dimensional Sérsic fits yield an ICL centroid offset of 5.2 ± 1.5 kpc from the BCG. These phase-space offsets are indicative of the BCG oscillating in a broad potential minimum. Furthermore, the ICL surface-brightness profile matches that of strong-lensing estimates assuming a cored halo. Simulations indicate that the inner halo profile varies systematically for different dark matter models. Applied to larger samples, our method therefore offers a complementary route to testing the nature of dark matter, while simultaneously revealing the stellar origins of the ICL.