Accreted versusin situMilky Way globular clusters

Abstract

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 data base of ages and metallicities for 93 Milky Way GCs from literature deep colour–magnitude data. The age–metallicity relation for 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 GCs 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 six to eight dwarf galaxies, were accreted to build the Milky Way GC system we see today.

Field/isolated lenticular galaxies with high SN values: the case of NGC 4546 and its globular cluster system

“…An interpretation of this observation could be that NGC 4546 has a relatively quiescent merger history. This is because it is thought that many, if not most, metal-poor GCs are gained from satellite accretion (see e.g., Forbes & Bridges 2010;Choksi et al 2018). This scenario is supported by the case of NGC 1277, a galaxy that is strongly suspected of having undergone very few mergers because it completely lacks blue GCs (a so-called "red nugget galaxy"; Beasley et al 2018).…”

The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

“…Key to that enterprise is to discern which of the Galactic GCs were formed in situ and which were accreted. In the last decade, the availability of precise ages (Marín-Franch et al 2009;VandenBerg et al 2013) has led to the discovery of the bifurcation in the age-metallicity relation of Galactic GCs (Marín-Franch et al 2009;Forbes & Bridges 2010;Leaman et al 2013), which, combined with results from high-resolution hydrodynamical cosmological simulations of Milky Way analogues, has further constrained the origin the Galactic GC system (Kruijssen et al 2019;Myeong et al 2019). Furthermore, the advent of the Gaia survey (Gaia Collaboration et al 2018), and the resulting precise 6D phase-space information have made possible a much better characterization of the properties of the Galactic GC system.…”

The Relationships Among Compact Stellar Systems: A Fresh View of Ultracompact Dwarfs

“…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 galaxy accretion. Galaxy-to-galaxy variations in the prevalence of these two populations (e.g., NGC 1399 versus M87; Mieske et al 2006a) might then reveal differences in star formation history and galaxy assembly (cf.…”

“…These interpretations can be connected to a broad framework for different modes of galaxy assembly (e.g., Carollo et al 2007;Zolotov et al 2009;Forbes & Bridges 2010;Oser et al 2010;Arnold et al 2011;Font et al 2011). The blue UCDs formed from stripped nuclei can be considered part of the outer halo build-up by accretion of low-mass infalling satellites at relatively late epochs, depositing accompanying GCs and field stars as well.…”