# Discussing the first velocity dispersion profile of an ultra-diffuse galaxy in MOND

## TL;DR

This study models the velocity dispersion profile of the ultra-diffuse galaxy Dragonfly 44 using MOND, finding good agreement near the center but some deviations in the outer regions, suggesting possible environmental effects or measurement errors.

## Contribution

It provides the first velocity dispersion profile of an ultra-diffuse galaxy in MOND, testing predictions in different gravitational environments.

## Key findings

- Excellent MOND fit near galaxy center
- Outer region shows two-sigma deviation from MOND prediction
- Potential implications for galaxy's distance and environmental effects

## Abstract

Using Jeans modelling, we calculate the velocity dispersion profile of the ultra-diffuse galaxy (UDG) Dragonfly 44 in MOND. For the nominal mass-to-light ratio from the literature and an isotropic profile, the agreement with the data is excellent near the center of the galaxy. However, in modified gravity, close to the cluster core, the gravitational environment should bring the galaxy back towards the Newtonian behavior. This success of the isolated MOND prediction for the central velocity dispersion could then mean that the galaxy is at large distances ($\gg5$ Mpc) from the cluster core, as hinted by the fact that close-by UDGs share similar velocities with a dispersion well below that of the cluster itself. There is however a two-sigma tension in the outer part of the UDG due to an increase of the observed dispersion profile with respect to the flat MOND prediction. This deviation could simply be a measurement error. Other possibilities could be, for a UDG far from the cluster, a higher-than-nominal baryonic mass with a tangentially anisotropic dispersion profile, or even a sign of a dark baryonic halo. If the UDG is closer to the cluster core, the deviation could be a sign that it is in the process of disruption.