Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group

A. D. MacKey, A. M.N. Ferguson, M. J. Irwin, N. F. Martin, A. P. Huxor, N. R. Tanvir, S. C. Chapman, R. A. Ibata, G. F. Lewis, A. W. McConnachie

Research output: Contribution to journalArticle (Academic Journal)peer-review

39 Citations (Scopus)


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 mag 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 μ = 23.95 ± 0.06. Combined with its large projected separation of Rp = 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 γ = -2.5, breaking to γ = -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.

Original languageEnglish
Pages (from-to)533-546
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 1 Jan 2010


  • Galaxies: individual: M31
  • Globular clusters: general


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