Deep Gemini/GMOS imaging of an extremely isolated globular cluster in
  the Local Group
  We report on deep imaging of a remote M31 globular cluster, MGC1, obtained
with Gemini/GMOS. Our colour-magnitude diagram for this object extends ~5
magnitudes below the tip of the red giant branch and exhibits features
consistent with an ancient metal-poor stellar population, including a long,
well-populated horizontal branch. The red giant branch locus suggests MGC1 has
a metal abundance [M/H] ~ -2.3. We measure the distance to MGC1 and find that
it lies ~160 kpc in front of M31 with a distance modulus of 23.95 +/- 0.06.
Combined with its large projected separation of 117 kpc from M31 this implies a
deprojected radius of Rgc = 200 +/- 20 kpc, rendering it the most isolated
known globular cluster in the Local Group by some considerable margin. We
construct a radial brightness profile for MGC1 and show that it is both
centrally compact and rather luminous, with Mv = -9.2. Remarkably, the cluster
profile shows no evidence for a tidal limit and we are able to trace it to a
radius of at least 450 pc, and possibly as far as ~900 pc. The profile exhibits
a power-law fall-off with exponent -2.5, breaking to -3.5 in its outermost
parts. This core-halo structure is broadly consistent with expectations derived
from numerical models, and suggests that MGC1 has spent many gigayears in
isolation.
