Understanding the internal dynamics of elliptical galaxies without non-baryonic dark matter

TL;DR

This paper explores alternative explanations for the internal dynamics of elliptical galaxies, challenging the necessity of non-baryonic dark matter by considering modified gravity and stellar effects, supported by analysis of galaxy data.

Contribution

It demonstrates that the observed velocity dispersions in early-type galaxies can be explained without dark matter by applying MOND or accounting for binary stars, with some residual discrepancies.

Findings

Velocity dispersions can be explained without dark matter using MOND or binary star effects.

Moderate discrepancies remain, but no non-baryonic dark matter is required.

Variations in stellar initial mass function and non-equilibrium dynamics are relevant.

Abstract

Assuming virial equilibrium and Newtonian dynamics, low-mass early-type galaxies have larger velocity dispersions than expected from the amount of baryons they contain. The conventional interpretation of this finding is that their dynamics is dominated by non-baryonic matter. However, there is also strong evidence that many low-mass early-type galaxies formed as tidal dwarf galaxies, which would contain almost no dark matter. Using an extensive catalogue of early-type galaxies, we therefore discuss how the internal dynamics of early-type galaxies in general can be understood by replacing the assumption of non-baryonic dark matter with two alternative assumptions. The first assumption is that Milgromian dynamics (i.e., MOND) is valid, which changes the effective gravitational force in the weak-field limit. The second assumption is that binary stars affect the observed line-of-sight velocity dispersions. Some moderate discrepancies between observed and predicted velocity dispersions remain also when these effects are implemented. Nevertheless, the observed velocity dispersions in early-type galaxies can then easily be explained without invoking the presence of non-baryonic dark matter in them, but with already documented variations of the galaxy-wide stellar initial mass function and non-equilibrium dynamics in some of the low-mass early-type galaxies.