Accreted versus in situ Milky Way globular clusters

By D. Forbes, T. Bridges. 2010

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A large, perhaps dominant fraction, of the Milky Way’s halo is thought to be built-up by the accretion of smaller galaxies and their globular cluster (GC) systems. Here we examine the Milky Way’s GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Way’s GCs reveals two distinct tracks – one with near constant old age of �12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with the young track and we speculate that they may have been accreted along with their host dwarf galaxy, whose nucleus may survive as a GC. Finally, we suggest that 27-47 GCs (about 1/4 of the entire system), from 6-8 dwarf galaxies, were accreted to build the Milky Way GC system we seen today.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/e834363e2c08edfb2fce435bba6ccaac29e51ce9

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Citations in the corpus

Globular clusters as the relics of regular star formation in ‘normal’ high-redshift galaxies (2015, methodology): “Likewise, recent work (e.g.Forbes & Bridges 2010; Georgiev et al. 2010; Conroy, Loeb & Spergel 2011) seems to rule out that GC formation is the result of a high Jeans mass following recombination (Peebles & Dicke 1968), thermal instabilities in hot galaxy haloes (Fall & Rees 1985), or star…”
The bifurcated age–metallicity relation of Milky Way globular clusters and its implications for the accretion history of the galaxy (2013, background): “…or HB properties of the clusters with their position in the Galaxy to infer which ones may have been accreted (e.g., Mackey & van den Bergh 2005; Forbes & Bridges 2010) Having the luxury of the split AMR and orbital data that enable us to classify the GCs into in-situ disk and halo systems, we…”
THE RELATIONSHIPS AMONG COMPACT STELLAR SYSTEMS: A FRESH VIEW OF ULTRACOMPACT DWARFS (2011, background): “Such distinctions are reminiscent of the inner and outer halo GCs of the Milky Way (e.g., Forbes & Bridges 2010; Dotter et al. 2011), and suggest that there may be generically two populations of UCDs: one associated with massive (and therefore metal-rich) bulge formation, and another with dwarf…”
ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY (2010, methodology): “The Age-Metallicity Relation and Horizontal Branch Morphology of Sgr Globular Clusters In Figure 12 (left panel), we plot the ages and metallicities (drawn from tabulations by Marin-Franch et al. 2009, and Forbes & Bridges 2010) of our eleven high/moderate/low confidence Sgr globular clusters.”
The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, methodology): “Contrary to Sagittarius, its putative nuclear cluster NGC 2808 (Forbes & Bridges 2010; Carballo-Bello et al. 2018) does not have a metallicity spread (Car- retta 2015) and as a result it traces the age-metallicity evolution of its host at {τ/Gyr, [Fe/H]} = {10.90 ± 0.57,−1.14}.”
The Sausage Globular Clusters (2018, background): “5 shows age versus metallicity for the Sausage GCs, as well as 7 GCs that have been claimed as associates of the the Sagittarius (Sgr), specifically Terzan 7, Terzan 8, Arp 2, Pal 12, NGC 4147, NGC 6715 and Whiting 1 (Forbes & Bridges 2010).”
Globular cluster systems in nearby dwarf galaxies – III. Formation efficiencies of old globular clusters (2010, unknown): “This is consistent with the recent age-metallicity relation (AMR) derived for those systems (Forbes & Bridges 2010; Sharina et al. 2010).”
Origin of the system of globular clusters in the Milky Way (2019, methodology): “…errors of .1 Gyr; e.g. Marín-Franch et al. 2009; VandenBerg et al. 2013) and homogeneous metallicity measurements (Carretta et al. 2009) led to the discovery that the age–metallicity relation (AMR) of Galactic GCs is bifurcated (Marín-Franch et al. 2009; Forbes & Bridges 2010; Leaman et al. 2013).”
GLOBULAR CLUSTERS IN THE OUTER GALACTIC HALO: NEW HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS IMAGING OF SIX GLOBULAR CLUSTERS AND THE GALACTIC GLOBULAR CLUSTER AGE–METALLICITY RELATION (2011, methodology): “The comparisons with Sgr and the MCs indicate that a collection of dwarf galaxies with a range of AMRs are likely to have contributed to the Galactic GC population, consistent with the findings of Forbes & Bridges (2010).”
Evidence for two early accretion events that built the Milky Way stellar halo (2019, background): “…with major accretion events – in particular, the Sagittarius (Sgr) GCs (Terzan 7, Terzan 8, Arp 2, Pal 12, NGC 4147, NGC 6715, and Whiting 1, Forbes & Bridges 2010, and marked as diamonds) or the Sausage GCs (as listed in Table 1 of Myeong et al. 2018d, and marked as upside-down…”
Evidence from the H3 Survey That the Stellar Halo Is Entirely Comprised of Substructure (2020, background): “Pal 1 also does not lie on the in situ branch of MW GCs in the age–metallicity relation (Forbes & Bridges 2010; Forbes 2020; Kruijssen et al. 2020; but see Massari et al. 2019).”
Proper motions and dynamics of the Milky Way globular cluster system fromGaiaDR2 (2018, background): “The now available accurate kinematic information complements the age and metallicity measurements and will help to better distinguish accreted cluster from those formed in situ (e.g., Forbes & Bridges 2010).”