Simulations of cluster ultra-diffuse galaxies in MOND

TL;DR

This study uses MOND simulations to explore the dynamics of ultra-diffuse galaxies in the Coma cluster, suggesting they are likely on their first infall and out-of-equilibrium, which explains their high velocity dispersions.

Contribution

It provides the first detailed MOND simulations of UDGs in a galaxy cluster, analyzing their infall and tidal effects to explain observed velocity dispersions.

Findings

Tidal effects alone do not raise velocity dispersions to observed levels.

UDGs on first infall can maintain high velocity dispersions without disruption.

Out-of-equilibrium state is necessary to reconcile observations with MOND predictions.

Abstract

Ultra-diffuse galaxies (UDGs) in the Coma cluster have velocity dispersion profiles that are in full agreement with the predictions of Modified Newtonian Dynamics (MOND) in isolation. However, the external field effect (EFE) from the cluster seriously deteriorates this agreement. It has been suggested that this could be related to the fact that UDGs are out-of-equilibrium objects whose stars have been heated by the cluster tides or that they recently fell onto the cluster on radial orbits, such that their velocity dispersion may not reflect the EFE at their instantaneous distance from the cluster center. Here, we simulate UDGs within the Coma cluster in MOND, using the Phantom of Ramses (\textsc{por}) code, and show that if UDGs are initially at equilibrium within the cluster, tides are not sufficient to increase their velocity dispersions to values as high as the observed ones. On the other hand, if they are on a first radial infall onto the cluster, they can keep high velocity dispersions without being destroyed until their first pericentric passage. We conclude that, without alterations such as a screening of the EFE in galaxy clusters or much higher baryonic masses than currently estimated, in the MOND context UDGs must be out-of-equilibrium objects on their first infall onto the cluster.