Testing Modified Newtonian Dynamics with Rotation Curves of Dwarf and Low Surface Brightness Galaxies

TL;DR

This study tests modified Newtonian dynamics (MOND) using rotation curves of dwarf and low surface brightness galaxies, finding MOND explains most but not all cases, and exploring how the acceleration constant varies with galaxy properties.

Contribution

The paper provides an empirical assessment of MOND with a new sample of galaxies and investigates the correlation between the acceleration constant and galaxy surface brightness.

Findings

MOND explains the rotation curves of about 75% of the sample.

Lower surface brightness galaxies tend to have lower fitted a_0 values.

The average fitted a_0 is approximately 0.7×10^-8 cm s^-2, slightly lower than previous estimates.

Abstract

Dwarf and low surface brightness galaxies are ideal objects to test modified Newtonian dynamics (MOND), because in most of these galaxies the accelerations fall below the threshold below where MOND supposedly applies. We have selected from the literature a sample of 27 dwarf and low surface brightness galaxies. MOND is successful in explaining the general shape of the observed rotation curves for roughly three quarters of the galaxies in the sample presented here. However, for the remaining quarter, MOND does not adequately explain the observed rotation curves. Considering the uncertainties in distances and inclinations for the galaxies in our sample, a small fraction of poor MOND predictions is expected and is not necessarily a problem for MOND. We have also made fits taking the MOND acceleration constant, a_0, as a free parameter in order to identify any systematic trends. We find that there appears to be a correlation between central surface brightness and the best-fit value of a_0, in the sense that lower surface brightness galaxies tend to have lower a_0. However, this correlation depends strongly on a small number of galaxies whose rotation curves might be uncertain due to either bars or warps. Without these galaxies, there is less evidence of a trend, but the average value we find for a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies. Such lower fitted values of a_0 could occur if external gravitational fields are important.