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

TL;DR

This study presents deep Gemini/GMOS imaging of the isolated M31 globular cluster MGC1, revealing its ancient, metal-poor stellar population, large radius, and a core-halo structure indicative of long-term isolation in the Local Group.

Contribution

First detailed imaging and analysis of the most isolated known globular cluster in the Local Group, providing insights into its structure, composition, and evolutionary history.

Findings

MGC1 is approximately 160 kpc in front of M31.

The cluster has a large radius of at least 450 pc, possibly up to 900 pc.

It shows a core-halo profile consistent with long-term isolation.

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 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.