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We present a detailed comparison of the Milky Way (MW) globular cluster (GC) kinematics with the 25 MW-mass cosmological simulations from the E-MOSAICS project. While the MW falls within the kinematic distribution of GCs spanned by the simulations, the relative kinematics of its metal-rich (⁠|$[\rm {Fe}/\rm {H}] \gt -1.2$|⁠) versus metal-poor (⁠|$[\rm {Fe}/\rm {H}] \lt -1.2$|⁠) and inner (r < 8 kpc) versus outer (r > 8 kpc) populations are atypical for its mass. To understand the origins of these features, we perform a comprehensive statistical analysis of the simulations, and find 18 correlations describing the assembly of L * galaxies and their dark matter haloes based on their GC population kinematics. The correlations arise because the orbital distributions of accreted and in situ GCs depend on the masses and accretion redshifts of accreted satellites, driven by the combined effects of dynamical fraction, tidal stripping, and dynamical heating. Because the kinematics of in situ /accreted GCs are broadly traced by the metal-rich/metal-poor and inner/outer populations, the observed GC kinematics are a sensitive probe of galaxy assembly. We predict that relative to the population of L * galaxies, the MW assembled its dark matter and stellar mass rapidly through a combination of in situ star formation, more than a dozen low-mass mergers, and 1.4 ± 1.2 early (⁠|$z$| = 3.1 ± 1.3) major mergers. The rapid assembly period ended early, limiting the fraction of accreted stars. We conclude by providing detailed quantitative predictions for the assembly history of the MW. [ABSTRACT FROM AUTHOR]

The ages and metallicities of globular clusters (GCs) are known to be powerful tracers of the properties of their progenitor galaxies, enabling their use in determining the merger histories of galaxies. However, while useful in separating GCs into individual accretion events, the orbits of GC groups themselves have received less attention as probes of their progenitor galaxy properties. In this work, we use simulations of galaxies and their GC systems from the MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE project to explore how the present-day orbital properties of GCs are related to the properties of their progenitor galaxies. We find that the orbits of GCs deposited by accretion events are sensitive to the mass and merger redshift of the satellite galaxy. Earlier mergers and larger galaxy masses deposit GCs at smaller median apocentres and lower total orbital energy. The orbital properties of accreted groups of GCs can therefore be used to infer the properties of their progenitor galaxy, though there exists a degeneracy between galaxy mass and accretion time. Combining GC orbits with other tracers (GC ages, metallicities) will help to break the galaxy mass/accretion time degeneracy, enabling stronger constraints on the properties of their progenitor galaxy. In situ GCs generally orbit at lower energies (small apocentres) than accreted GCs, however they exhibit a large tail to high energies and even retrograde orbits (relative to the present-day disc), showing significant overlap with accreted GCs. Applying the results to Milky Way GCs groups suggests a merger redshift z  ∼ 1.5 for the Gaia Sausage/Enceladus and z > 2 for the 'low-energy'/ Kraken group, adding further evidence that the Milky Way had two significant mergers in its past. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: Gaia -Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered 'low-energy' GCs, which provide an excellent match to the predicted properties of the enigmatic galaxy 'Kraken'. The five galaxies cover a narrow stellar mass range [ M ⋆ = (0.6–4.6) × 108 M⊙], but have widely different accretion redshifts (⁠|$\mbox{$z_{\rm acc}$}=0.57\!-\!2.65$|⁠). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with stellar and virial mass ratios of |$\mbox{$r_{M_\star }$}=1$|⁠ : |$31^{+34}_{-16}$| and |$\mbox{$r_{M_{\rm h}}$}=1$|⁠ : |$7^{+4}_{-2}$|⁠ , respectively. The progenitors match the z  = 0 relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. Even though these progenitors likely were the Milky Way's most massive accretion events, they contributed a total mass of only log (M ⋆, tot/M⊙) = 9.0 ± 0.1, similar to the stellar halo. This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. We conclude by organizing these accretion events into the most detailed reconstruction to date of the Milky Way's merger tree. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) have been posited, alongside dwarf galaxies, as significant contributors to the field stellar population of the Galactic halo. In order to quantify their contribution, we examine the fraction of halo stars formed in stellar clusters in the suite of 25 present-day Milky Way-mass cosmological zoom simulations from the E-MOSAICS project. We find that a median of 2.3 and 0.3 per cent of the mass in halo field stars formed in clusters and GCs, defined as clusters more massive than 5 × 103 and 105 M⊙, respectively, with the 25–75th percentiles spanning 1.9–3.0 and 0.2–0.5 per cent being caused by differences in the assembly histories of the host galaxies. Under the extreme assumption that no stellar cluster survives to the present day, the mass fractions increase to a median of 5.9 and 1.8 per cent. These small fractions indicate that the disruption of GCs plays a subdominant role in the build-up of the stellar halo. We also determine the contributed halo mass fraction that would present signatures of light-element abundance variations considered to be unique to GCs, and find that clusters and GCs would contribute a median of 1.1 and 0.2 per cent, respectively. We estimate the contributed fraction of GC stars to the Milky Way halo, based on recent surveys, and find upper limits of 2–5 per cent (significantly lower than previous estimates), suggesting that models other than those invoking strong mass loss are required to describe the formation of chemically enriched stellar populations in GCs. [ABSTRACT FROM AUTHOR]

The α-element abundances of the globular cluster (GC) and field star populations of galaxies encode information about the formation of each of these components. We use the E-MOSAICS cosmological simulations of ∼ L * galaxies and their GCs to investigate the [α/Fe]–[Fe/H] distribution of field stars and GCs in 25 Milky Way–mass galaxies. The [α/Fe]–[Fe/H] distribution of GCs largely follows that of the field stars and can also therefore be used as tracers of the [α/Fe]–[Fe/H] evolution of the galaxy. Due to the difference in their star formation histories, GCs associated with stellar streams (i.e. which have recently been accreted) have systematically lower [α/Fe] at fixed [Fe/H]. Therefore, if a GC is observed to have low [α/Fe] for its [Fe/H] there is an increased possibility that this GC was accreted recently alongside a dwarf galaxy. There is a wide range of shapes for the field star [α/Fe]–[Fe/H] distribution, with a notable subset of galaxies exhibiting bimodal distributions, in which the high [α/Fe] sequence is mostly comprised of stars in the bulge, a high fraction of which are from disrupted GCs. We calculate the contribution of disrupted GCs to the bulge component of the 25 simulated galaxies and find values between 0.3 and 14 per cent, where this fraction correlates with the galaxy's formation time. The upper range of these fractions is compatible with observationally inferred measurements for the Milky Way, suggesting that in this respect the Milky Way is not typical of L*galaxies, having experienced a phase of unusually rapid growth at early times. [ABSTRACT FROM AUTHOR]

We use the age–metallicity distribution of 96 Galactic globular clusters (GCs) to infer the formation and assembly history of the Milky Way (MW), culminating in the reconstruction of its merger tree. Based on a quantitative comparison of the Galactic GC population to the 25 cosmological zoom-in simulations of MW-mass galaxies in the E-MOSAICS project, which self-consistently model the formation and evolution of GC populations in a cosmological context, we find that the MW assembled quickly for its mass, reaching {25, 50} per cent of its present-day halo mass already at z = {3, 1.5} and half of its present-day stellar mass at z = 1.2. We reconstruct the MW's merger tree from its GC age–metallicity distribution, inferring the number of mergers as a function of mass ratio and redshift. These statistics place the MW's assembly rate among the 72th–94th percentile of the E-MOSAICS galaxies, whereas its integrated properties (e.g. number of mergers, halo concentration) match the median of the simulations. We conclude that the MW has experienced no major mergers (mass ratios >1:4) since z ∼ 4, sharpening previous limits of z ∼ 2. We identify three massive satellite progenitors and constrain their mass growth and enrichment histories. Two are proposed to correspond to Sagittarius (a few 108 M⊙) and the GCs formerly associated with Canis Major (⁠|${\sim }10^9\, \mbox{M$_\odot $}$|⁠). The third satellite has no known associated relic and was likely accreted between z = 0.6 and 1.3. We name this enigmatic galaxy Kraken and propose that it is the most massive satellite (⁠|$M_*\sim 2\times 10^9\, \mbox{M$_\odot $}$|⁠) ever accreted by the MW. We predict that ∼40 per cent of the Galactic GCs formed ex situ (in galaxies with masses M * = 2 × 107– |$2\times 10^9\, \mbox{M$_\odot $}$|⁠), with 6 ± 1 being former nuclear clusters. [ABSTRACT FROM AUTHOR]