Ultra faint dwarf galaxies: an arena for testing dark matter versus modified gravity

TL;DR

Ultra-faint dwarf galaxies challenge modified gravity theories because their luminosity-velocity dispersion correlation weakens significantly, supporting the dark matter hypothesis which predicts such stochasticity.

Contribution

The paper proposes using ultra-faint dwarf galaxies to distinguish between dark matter and modified gravity scenarios based on their luminosity-velocity dispersion correlation.

Findings

Strong correlation in non-UFD galaxies (-0.688)

Weak correlation in UFD galaxies (-0.077)

Potential for future data to clarify the nature of the correlation

Abstract

The scenario consistent with a wealth of observations for the missing mass problem is that of weakly interacting dark matter particles. However, arguments or proposals for a Newtonian or relativistic modified gravity scenario continue to be made. A distinguishing characteristic between the two scenarios is that dark matter particles can produce a gravitational effect, in principle, without the need of baryons while this is not the case for the modified gravity scenario where such an effect must be correlated with the amount of baryonic matter. We consider here ultra-faint dwarf (UFD) galaxies as a promising arena to test the two scenarios based on the above assertion. We compare the correlation of the luminosity with the velocity dispersion between samples of UFD and non-UFD galaxies, finding a significant loss of correlation for UFD galaxies. For example, we find for 28 non-UFD galaxies a strong correlation coefficient of -0.688 which drops to -0.077 for the 23 UFD galaxies. Incoming and future data will determine whether the observed stochasticity for UFD galaxies is physical or due to systematics in the data. Such a loss of correlation (if it is to persist) is possible and consistent with the dark matter scenario for UFD galaxies but would constitute a new challenge for the modified gravity scenario.