Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group |
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| Authors: | 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 |
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| Institution: | Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ;Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA;Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany;Department of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL;Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH;Observatoire Astronomique, Universitéde Strasbourg, CNRS, 11, rue de l'Université, F-67000 Strasbourg, France;Institute of Astronomy, School of Physics, A29, University of Sydney, NSW 2006, Australia;NRC Herzberg Institute for Astrophysics, 5071 West Saanich Road, Victoria, British Columbia V9E 2E7, Canada |
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| Abstract: | 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 R p= 117 kpc from M31, this implies a deprojected radius of R gc= 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. |
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| Keywords: | globular clusters: general galaxies: individual: M31 |
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