Determine whether Gep I is a genuine cluster or a transient dark-subhalo-captured overdensity

Measure the velocity dispersion of Gep I to determine whether it is a genuine long-lived star cluster or a transient stellar overdensity produced by Andromeda XXV stars temporarily captured by a dark matter subhalo.

Background

The photometric and structural properties of Gep I are consistent both with a genuine, extended globular cluster and with simulations in which a dark matter subhalo temporarily captures field stars inside a dwarf spheroidal galaxy. The current data contain no kinematic information, so they cannot test whether Gep I has an anomalously high velocity dispersion. Such a measurement would discriminate between a long-lived bound cluster and a transient subhalo-associated stellar overdensity.

References

While Gep~I is mostly likely a genuine star cluster, the current data do not exclude the tantalising possibility that it consists of And~XXV stars temporarily captured by a dark subhalo orbiting within the dSph's potential well.

— The solitary star cluster of the Andromeda XXV dwarf spheroidal  (2608.26959 - Crociati et al., 27 Aug 2026) in Abstract; Section 3.1; Summary and conclusions