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.
Highlights
Cosmological models of structure formation predict that galaxies are built up via the hierarchical accretion and merger of many smaller subsystems over time
Our resulting colour–magnitude diagram (CMD) displays features consistent with those of an ancient stellar population, including a long horizontal branch (HB) populated across the instability strip
By fitting isochrones calculated by several different groups, as well as fiducial sequences observed for Galactic globular clusters, we determine MGC1 to be a metal-poor object with [M/H] ≈ −2.3
Summary
Cosmological models of structure formation predict that galaxies are built up via the hierarchical accretion and merger of many smaller subsystems over time. M31 is of particular significance as it is the nearest large galaxy to our own, and possesses the most extensive globular cluster system in the Local Group This galaxy has been targeted by a number of wide-field imaging surveys in recent years and these have revealed a wealth of lowbrightness substructure in its outer regions (e.g. Ibata et al 2001; Ferguson et al 2002; Ibata et al 2007; McConnachie et al 2009) as well as many new members of its halo globular cluster system, extending to very large radii (e.g. Huxor et al 2005; Mackey et al 2006, 2007; Huxor et al 2008). These measurements are consistent with the systemic velocity of M31 (vr = −300 km s−1), suggesting that MGC1 is a bona fide member of M31’s outer halo globular cluster system In this regard, it is of considerable significance as very few such objects have yet been discovered – there are only three other known globular clusters in M31 with Rp 70 kpc (Mackey et al 2007; Huxor et al 2008). We present results from deep Gemini follow-up imaging of this intriguing cluster, with the particular goals of (i) obtaining an accurate distance measurement in order to understand properly its position in the M31 system, (ii) obtaining a precise estimate of its metal abundance and (iii) making a thorough analysis of its structure
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