Mackey, A. D.Ferguson, A. M. N.Irwin, M. J.Martin, N. F.Huxor, A. P.Tanvir, N. R.Chapman, S. C.Ibata, R. A.Lewis, G. F.McConnachie, A. W.2014-03-122014-03-122009-09-08Mackey, A. D., A. M. N. Ferguson, M. J. Irwin, N. F. Martin, et al. 2009. "Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group." Monthly Notices of the Royal Astronomical Society 401:533.0035-8711http://hdl.handle.net/10222/45378http://dx.doi.org/10.1111/j.1365-2966.2009.15678.xWe 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.article401533