Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo

By A. Mackey, G. Gilmore. 2004

174 citations recorded by the source corpus.

We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies. This is done by means of a detailed comparison between the ‘old halo’, ‘young halo’ and ‘bulge/disc’ subsystems defined by Zinn and the globular clusters in the Large Magellanic Cloud, Small Magellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies. We first use high-quality photometry from Hubble Space Telescope images to derive a complete set of uniform measurements of horizontal branch (HB) morphology in the external clusters. We also compile structural and metallicity measurements for these objects and update the data base of such measurements for the Galactic globular clusters, including new calculations of HB morphology for 11 objects. Using these data together with recent measurements of globular cluster kinematics and ages we examine the characteristics of the three Galactic cluster subsystems. Each is quite distinct in terms of their spatial and age distributions, age–metallicity relationships, and typical orbital parameters, although we observe some old halo clusters with ages and orbits more similar to those of young halo objects. In addition, almost all of the Galactic globular clusters with large core radii fall into the young halo subsystem, while the old halo and bulge/disc ensembles are characterized by compact clusters. We demonstrate that the majority of the external globular clusters are essentially indistinguishable from the Galactic young halo objects in terms of HB morphology, but ∼20–30 per cent of external clusters have HB morphologies most similar to the Galactic old halo clusters. We further show that the external clusters have a distribution of core radii which very closely matches that for the young halo objects. The old halo distribution of core radii can be very well represented by a composite distribution formed from ∼83–85 per cent of objects with structures typical of bulge/disc clusters, and ∼15–17 per cent of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 young halo clusters and 15–17 per cent of the old halo clusters (10–12 objects) are of external origin. Based on cluster number counts, we estimate that the Galaxy may have experienced approximately seven merger events with cluster-bearing dwarf-spheroidal-type galaxies during its lifetime, building up ∼45–50 per cent of the mass of the Galactic stellar halo. Finally, we identify a number of old halo objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed dwarf galaxy in Canis Major fall into this category.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/6210af6e66d25487dfc5c1ee6f405fccce20ad13

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Citations in the corpus

Accreted versus in situ Milky Way globular clusters (2010, background): “On the basis of their spatial distribution and sizes, Mackey & Gilmore (2004) suggested that ∼16%, or ∼11, of the old halo GCs were also accreted.”
The stellar halo of the Galaxy (2008, background): “This is also supported by the work of Mackey & Gilmore (2004), who conclude that all young halo clusters (with [Fe/H] −0.8 dex and red HB morphology, more common in the outer halo) and ∼ 15% of the old halo clusters ([Fe/H] −0.8 dex and blue HB morphology) are of external origin.”
A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies (2005, methodology): “To be consistent with the Lee & Carney analysis, we have plotted the clusters according to their classification in Mackey & Gilmore (2004).”
THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. IX. HORIZONTAL BRANCH MORPHOLOGY AND THE SECOND PARAMETER PHENOMENON (2009, background): “Mackey & Gilmore (2004) summarize our current knowledge of the GC populations in the Galaxy and its satellites in the HB morphology–metallicity diagram.”
ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY (2010, result): “…effect in the Sgr globular clusters is in marked contrast to the globular cluster systems of the LMC and Fornax (e.g., Zinn 1993b; Buonanno et al. 1999; Mackey & Gilmore 2004), which show moderate/strong influences of a second parameter on their respective HB types at a given metallicity.”
The Sausage Globular Clusters (2018, background): “The young halo clusters can extend to large radii and are all believed to have been accreted (see e.g., Mackey & Gilmore 2004; Mackey & van den Bergh 2005).”
Globular Cluster System and Milky Way Properties Revisited (2005, background): “Mackey & Gilmore (2004) estimate a lower limit of 50% for the destruction rate over the last Hubble time, an intermediate value between those of Gnedin & Ostriker (1997) and Mackey & van den Bergh (2005).”
Galactic stellar haloes in the CDM model (2009, background): “There is evidence for multiple kinematic and chemical subdivisions within the Galactic globular cluster population (e.g. Searle & Zinn 1978; Frenk & White 1980; Zinn 1993; Mackey & Gilmore 2004, and references therein).”
The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, methodology): “…yields and metallicity calibrations are uncertain at a similar level (Schaye 19 In order to match the total mass of the stellar halo, Mackey & Gilmore (2004) do assume lower satellite masses than those of our inferred progenitors, with a typical mass scale in logarithmic mass space…”
The Luminosity Function of the Milky Way Satellites (2007, background): “Moving outwards (16 D 32 kpc), the globular clusters belong to the halo and may have been accreted (Mackey & Gilmore 2004).”
Kinematics of SDSS subdwarfs: structure and substructure of the Milky Way halo (2009, background): “Previous authors have questioned whether subsets of these four clusters may be associated (e.g. Dinescu, Girard & van Altena 1999; Palma, Majewski & Johnston 2002; Mackey & Gilmore 2004), but it appears that no one to date has attempted to explicitly associate all four clusters.”
The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way (2012, background): “Mackey & Gilmore (2004) have discussed the ages of the different populations of GCs.”