Low-mass disc galaxies and the issue of stability: MOND vs dark matter

TL;DR

This study compares MOND and dark matter models in explaining the rotation curves and stability of bulgeless galaxies, finding that dark matter models can fit observations with adjusted parameters, while MOND struggles to account for stability without high stellar velocity dispersions.

Contribution

The paper provides a new observational test for MOND using bar stability analysis in bulgeless galaxies, highlighting the need for precise velocity dispersion measurements.

Findings

Dark matter models can match rotation curves and stability with adjusted mass-to-light ratios.

MOND fits are poor in some galaxies, possibly due to noncircular motions in gas.

Most galaxies require high stellar velocity dispersions for MOND stability, inconsistent with observations.

Abstract

We analyse the rotation curves and gravitational stability of a sample of six bulgeless galaxies for which detailed images reveal no evidence for strong bars. We explore two scenarios: Newtonian dark matter models and MOdified Newtonian Dynamics (MOND). By adjusting the stellar mass-to-light ratio, dark matter models can match simultaneously both the rotation curve and bar-stability requirements in these galaxies. To be consistent with stability constraints, in two of these galaxies, the stellar mass-to-light ratio is a factor of ~1.5-2 lower than the values suggested from galaxy colours. In contrast, MOND fits to the rotation curves are poor in three galaxies, perhaps because the gas tracer contains noncircular motions. The bar stability analysis provides a new observational test to MOND. We find that most of the galaxies under study require abnormally-high levels of random stellar motions to be bar stable in MOND. In particular, for the only galaxy in the sample for which the line-of-sight stellar velocity dispersion has been measured (NGC 6503), the observed velocity dispersion is not consistent with MOND predictions because it is far below the required value to guarantee bar stability. Precise measurements of mass-weighted velocity dispersions in (unbarred and bulgeless) spiral galaxies are crucial to test the consistency of MOND.