Pegasus V -- a newly discovered ultra-faint dwarf galaxy on the outskirts of Andromeda

Abstract

We report the discovery of an ultra-faint dwarf in the constellation of Pegasus. Pegasus V (Peg V) was initially identified in the public imaging data release of the DESI Legacy Imaging Surveys and confirmed with deep imaging from Gemini/GMOS-N. The colour-magnitude diagram shows a sparse red giant branch (RGB) population and a strong over-density of blue horizontal branch stars. We measure a distance to Peg V of 𝐷 = 692 +33 −31 kpc, making it a distant satellite of Andromeda with 𝑀 𝑉 = −6.3 ± 0.2 and a half-light radius of 𝑟 half = 89 ± 41 pc. It is located ∼ 260 kpc from Andromeda in the outskirts of its halo. The RGB is well-fit by a metal-poor isochrone with [Fe/H]= −3.2, suggesting it is very metal poor. This, combined with its blue horizontal branch could imply that it is a reionisation fossil. This is the first detection of an ultra-faint dwarf outside the deep Pan-Andromeda Archaeological Survey area, and points to a rich, faint satellite population in the outskirts of our nearest neighbour.