Pegasus V -- a newly discovered ultra-faint dwarf galaxy on the
  outskirts of Andromeda
  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 $M_V=-6.3\pm0.2$ and a half-light radius of $r_{\rm
half}=89\pm41$~pc. It is located $\sim260$~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.
