We report on deep imaging of a remote M31 globular cluster , MGC1 , obtained with Gemini/GMOS .
Our colour-magnitude diagram for this object extends \sim 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 ] \approx - 2.3 .
We measure the distance to MGC1 and find that it lies \sim 160 kpc in front of M31 with a distance modulus \mu = 23.95 \pm 0.06 .
Combined with its large projected separation of R _ { { p } } = 117 kpc from M31 this implies a deprojected radius of R _ { { gc } } = 200 \pm 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 M _ { V } = -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 \sim 900 pc .
The profile exhibits a power-law fall-off with exponent \gamma = -2.5 , breaking to \gamma = -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 .