Baryonic mass budgets in the central regions of the Bullet Cluster and their consistency with strong lensing in MOND
Phys. Rev. D 114, 023001 – Published 1 July, 2026
DOI: https://doi.org/10.1103/6zrp-q7c4
Abstract
Strong lensing observations of the Bullet Cluster have traditionally been regarded as definitive evidence for the existence of dark matter, thereby posing a major challenge to the Milgromian dynamics (MOND) framework. The offset between the lensing mass and the x-ray gas centroids indicates the presence of a substantial amount of unseen mass in the vicinity of the brightest cluster galaxy (BCG). However, the high metallicities observed in the intracluster x-ray gas and in the massive early-type member galaxies suggest a past epoch dominated by massive stars, which have since evolved into stellar remnants. Their mass contribution is naturally incorporated in the integrated galaxy-wide initial mass function (IGIMF) theory, which predicts a significantly higher baryonic mass in early-type galaxies than the canonical IMF. In this work, we re-estimate the baryonic masses of the three BCG-centred core regions of the Bullet Cluster using recent JWST photometry and compare them with MOND strong-lensing masses. The IGIMF masses are derived for constant- and for (self-) enriched-metallicity stellar population synthesis models. These constitute, respectively, the lower and the upper limits of the IGIMF estimated mass, though the latter is the more realistic self-consistent scenario. Our results show that the MOND strong lensing masses of all cores fall between the IGIMF masses derived from the constant- and the enriched-metallicity stellar models. These results suggest that the baryonic mass budget is consistent with MOND requirements from strong-lensing observations in the core regions of the Bullet Cluster. However, the physical viability of this scenario also depends on the spatial distribution and dynamical behavior of the remnant population, which remain to be established. Independently of the validity of MOND, there is now an emerging perspective regarding dark matter models: to account for the observations, they seem to require less dark matter than previously anticipated.
