Andromeda XXI

This large satellite of the Andromeda Galaxy (M31) has a half-light radius of roughly 1 kpc.

Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, [Fe/H]=-1.8 when plotted in a color-magnitude diagram.

Like other dwarf spheroidal galaxies, Andromeda XXI shows no sign of current star formation and appears to have had 90% of its stars formed 5.8 billion years ago. Its central dark matter density is lower than expected from the ΛCDM model, but this could be explained if it lost most of its mass in a previous tidal stripping event or had experience tidal shocks from an eccentric orbit.

Although moderately bright (M_V = −9.1 ± 0.3), it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.