Testing light-unaffiliated mass clumps in MACS 0416 on galaxy and galaxy-cluster scales using the JWST^⋆

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
² School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
³ Faculty of Mathematics and Physics, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia
⁴ UHasselt – Flanders Make, Digital Future Lab, Wetenschapspark 2, B-3590 Diepenbeek, Belgium

^⋆⋆ Corresponding author: marceau.limousin@lam.fr

Received: 18 June 2025
Accepted: 20 August 2025

Abstract

Light-unaffiliated mass clumps (LUMCs), i.e. dark matter (DM) components without any stellar counterparts, have been reported in strong-lensing mass reconstructions of MACS 0416, both on galaxy and galaxy-cluster scales. On a galaxy-cluster scale, the most recent LENSTOOL parametric mass reconstruction based on 303 spectroscopically confirmed multiple images features a LUMC in the south of the cluster. On galaxy scale, the most recent GRALE non-parametric mass reconstruction based on 237 multiple images features two LUMCs, M1 and M2. Given the implications of these findings in the context of structure formation and evolution, we tested these features parametrically using the LENSTOOL code. First, we show that a mass model in which each large-scale DM component introduced in the modelling is associated with a stellar counterpart can reproduce the 303 multiple images, removing the need for any cluster-scale LUMC in MACS 0416. We then updated the GRALE non-parametric mass reconstruction using the 303 multiple images, finding that one of the two galaxy-scale LUMCs, M1, is no longer significant, while M2 remains. We tested M2 by explicitly including it in our parametric model, at the position and with the mass inferred from our updated GRALE model. We find that the inclusion of this LUMC does not improve the global root mean square (RMS), but mildly improves locally the RMS for one multiple image located close to M2. Besides, the preferred mass for M2 corresponds to the lowest mass allowed by the adopted prior. If we allow the mass of M2 to reach 0, then LENSTOOL converges to this null value, consistently rejecting M2. We present a detailed comparison of parametric and non-parametric models in the M2 area. It appears that both approaches show very similar surface mass density at this location, with a 5–6% difference between the mass maps. The difference is that GRALE favours a distinct mass substructure, while LENSTOOL favours a more diffuse mass distribution. We were able to propose a parametric mass model without including any LUMCs, providing further evidence of DM being associated with light in galaxy clusters. Finally, further investigations into the mass distribution at the M2 location are necessary. In this paper, we present two new mass models and associated products based on the 303 multiple images that will be hosted at the Strong Lensing Cluster Atlas Data Base at the Laboratoire d’Astrophysique de Marseille.