Reverse engineering the Milky Way

Abstract

The ages, metallicities, alpha-elements and integrals of motion of globular clusters (GCs) accreted by the Milky Way from disrupted satellites remain largely unchanged over time. Here we have used these conserved properties in combination to assign 76 GCs to 5 progenitor satellite galaxies -one of which we dub the Koala dwarf galaxy. We fit a leaky-box chemical enrichment model to the age-metallicity distribution of GCs, deriving the effective yield and the formation epoch of each satellite. Based on scaling relations of GC counts we estimate the original halo mass, stellar mass and mean metallicity of each satellite. The total stellar mass of the 5 accreted satellites contributed around 10 9 M in stars to the growth of the Milky Way but over 50% of the Milky Way's GC system. The 5 satellites formed at very early times and were likely accreted 8-11 Gyr ago, indicating rapid growth for the Milky Way in its early evolution. We suggest that at least 3 satellites were originally nucleated, with the remnant nucleus now a GC of the Milky Way. Eleven GCs are also identified as having formed ex-situ but could not be assigned to a single progenitor satellite.

Predicting accreted satellite galaxy masses and accretion redshifts based on globular cluster orbits in the E-MOSAICS simulations

“…Massari et al (2019) also found evidence for a population of GCs in the Milky Way at low energies (the L-E group) which do not appear to have formed in situ in the Galaxy (the GCs have ages and metallicities consistent with the 'accreted' or 'satellite' branch 1 ), and are not connected to previously known merger events. This GC population is plausibly consistent with the proposed Kraken (Kruijssen et al 2019b) accretion event, which was predicted based on number of GCs in the 'satellite' branch of the Milky Way GC age-metallicity distribution (see also Forbes 2020;Kruijssen et al 2020).…”

The accreted nuclear clusters of the Milky Way

“…Myeong et al (2019) suggest it is associated with the Sequoia accretion event based on the actions, inclination and eccentricity of its orbit, while MKH19 suggest it is associated with G-E/S based on the binding energy of its orbit within the MW. Similarly, Forbes (2020) assigned Cen and NGC 1851 as the NSCs of Sequoia and G-E/S, respectively.…”

“…MKH19 unambiguously assign NGC 6273 to the 'low energy' group of GCs. This group has subsequently been associated with the Kraken merger event (Kruijssen et al 2019b(Kruijssen et al , 2020Pfeffer et al 2020;Forbes 2020). Recently, Horta et al (in preparation) have reported the discovery of the stellar component of Kraken, based on a combination of abundances and kinematics, lending further support to the existence of this relatively massive accreted galaxy.…”

“…Additionally, they found a collection of GCs, which they term the "Low-energy" group, without (at the time) any known accretion event. This group is likely to be associated with the Kraken merger event, an early and relatively massive accretion event that has been postulated based on the kinematics and age-metallicity distribution of GCs (Kruijssen et al 2019a(Kruijssen et al ,b, 2020Pfeffer et al 2020;Forbes 2020).…”

Linking globular cluster formation at low and high redshift through the age–metallicity relation in E-MOSAICS

“…Marín-Franch et al 2009;Forbes & Bridges 2010;Leaman, VandenBerg & Mendel 2013). This shallower branch has been demonstrated to be made up of accreted clusters (Kruijssen et al 2019b;Massari, Koppelman & Helmi 2019;Forbes 2020), and it also appears as a separate branch for metal-poor GCs in the α-Fe plane (e.g. Horta et al 2020c).…”