  Understanding galaxy formation involves look-back and fossil-record studies
of distant and nearby galaxies, respectively. Debris trails in our Galaxy's
spheroidal halo of old stars provide evidence of "bottom-up" formation via
tidal disruption/merging of dwarf satellite galaxies, but it is difficult to
study our Galaxy's large-scale structure from within. Studies of our
neighbouring Andromeda galaxy have concluded that its spheroid contains
chemically enriched ("metal-rich") stars out to a radius of 30 kiloparsecs with
an exponential r^1/4 fall-off in density thereby resembling a galactic "bulge".
Were Andromeda's true halo to be found, our detailed yet global view of its
stellar dynamics, substructure, chemical abundance, and age distribution would
directly constrain hierarchical halo formation models. We report here on the
discovery of a hitherto elusive halo of metal-poor Andromeda stars, distinct
from its bulge, with a power-law brightness profile extending beyond r = 160
kiloparsecs. This is 3 - 5 times larger than any previously mapped Andromeda
spheroidal/disk component. Together, the Galactic and Andromeda halos span >1/3
of the distance between them, suggesting that stars occupy a substantial volume
fraction of our Local Group, and possibly most galaxy groups.
