Gas expulsion in MOND: the possible origin of diffuse globular clusters and ultra-faint dwarf galaxies

TL;DR

This study uses N-body simulations within Milgromian dynamics (MOND) to explore how gas expulsion affects the evolution and survival of star clusters, potentially explaining the origins of diffuse globular clusters and ultra-faint dwarf galaxies.

Contribution

It demonstrates that star clusters can remain bound with very low star formation efficiency in MOND and reveals how their density profiles depend on initial conditions, offering insights into observed low-density systems.

Findings

Clusters can survive with SFE as low as 2.5% in MOND.

Surviving clusters in MOND bind more mass than in Newtonian gravity.

Diffuse initial clusters experience less size expansion after gas expulsion.

Abstract

We study the evolution of star clusters located in the outer regions of a galaxy undergoing a sudden mass loss through gas expulsion in the framework of Milgromian dynamics (MOND) by means of N-body simulations. We find that, to leave a bound star cluster, the star formation efficiency (SFE) of an embedded cluster dominated by deep MOND gravity can be reduced down to $2.5\%$. For a given SFE, the star clusters that survive in MOND can bind a larger fraction of mass compared to the Newtonian dynamics. Moreover, the more diffuse the embedded cluster is, the less substantial the size expansion of the final star cluster is. The density profiles of a surviving star cluster are more cuspy in the centre for more massive embedded clusters, and the central density profiles are flatter for less massive embedded clusters or for lower SFE. This work may help to understand the low concentration and extension of the distant low-density globular clusters (GCs) and ultra-faint and diffuse satellite galaxies around the Milky Way.