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

Abstract

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.

A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater*

“…Most of these are "young halo" clusters, for which it has been pointed out that their concentration is lower (i.e., large core radii) than for old halo clusters and bulge/ diskclusters. This structural property is also found for clusters in external dwarf galaxies (Mackey & Gilmore 2004).…”

Globular clusters of the Local Group – statistical classification

“…The robustness of the classification is tested by taking a few bootstrap samples generated from the original one. The classification differs from that by Zinn (1985) and is in agreement with Zinn (1993) and Mackey & Gilmore (2004). For MW and M 31 three clusters, disc, inner halo, and outer halo GCs are found.…”

“…The statistical dimensionality is two in every situation which is less than those of elliptical galaxies (Santiago & Djorgovski 1992). This procedure is completely different from the method of studying two point correlation or studying the properties of GCs with respect to a single parameter like colour or HB morphology as done by previous authors (Zinn 1985(Zinn , 1993Mackey & Gilmore 2004). As the present set up is multivariate, it is quite likely to carry out the analysis by multivariate methods.…”

“…This is concluded from the kinematic properties, absence of agemetallicity correlation and metallicity gradient etc in Cluster 3. This is also suggested by Mackey & Gilmore (2004) on the basis of HB morphology of the Galactic halo GCs and those in the neighbouring dwarf galaxies. The CA for M 31 GCs are shown in Table 12 and Fig.…”

Kinematic imprint of clumpy disk formation on halo objects

“…Composite models are also possible. Mackey & Gilmore (2004) have estimated that nearly half of the mass of the Galactic stellar halo is from external accretion and the other half is from an internal origin. other runs with different settings of the clump mass, number, and the initial radii (see Sect.…”