Revisiting the missing mass problem in MOND for nearby galaxy clusters

TL;DR

This study re-evaluates the baryonic mass in galaxy clusters within MOND using the IGIMF theory, finding that baryonic matter accounts for most of the dynamical mass, thus reducing the missing mass problem.

Contribution

It introduces the use of the integrated galaxy-wide initial mass function (IGIMF) to better estimate stellar and remnant masses in galaxy clusters under MOND.

Findings

Baryonic mass accounts for at least 88% of the MOND dynamical mass.

Using IGIMF reduces the missing mass problem in galaxy clusters.

Remnant kick velocities are constrained, supporting MOND's viability.

Abstract

In the framework of Milgromian dynamics (MOND), galaxy clusters are known to exhibit a residual missing mass problem, with the baryonic mass falling short of the dynamical mass by about a factor of two. The baryon content of clusters is dominated by the intracluster medium (ICM), while the stellar contribution depends sensitively on the assumed stellar initial mass function (IMF). We re-evaluate the stellar and remnant masses in galaxy clusters by adopting the integrated galaxy-wide initial mass function (IGIMF) theory, which accounts for the dependence of the IMF on galaxy properties and star formation histories. Massive elliptical galaxies, characterized by high metallicities and short formation timescales, are inferred to form with top-heavy IMFs, leading to a substantial population of stellar remnants. Using observational data from WINGS and 2MASS for 46 nearby (z < 0.1) galaxy clusters, we compute stellar, remnant, and intracluster light masses and combine them with previously derived ICM masses. The resulting total baryonic masses are compared to MOND dynamical masses inferred from hydrostatic equilibrium. We find that the baryonic mass in stars, remnants and the ICM accounts for at least $88^{+5+2}_{-4-1}\%$ of the MOND dynamical mass. This constrains the kick velocities of the remnants and substantially alleviates the missing mass problem for galaxy clusters in MOND.