The formation and assembly history of the Milky Way revealed by its globular cluster population

By J. Kruijssen, J. Pfeffer, M. Reina-Campos, Robert A. Crain, Nate Bastian Heidelberg, Ljmu. Monthly notices of the Royal Astronomical Society, 2018

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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 (⁠∼109M⊙). 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 ∗ ∼2×10 9 M ⊙) ever accreted by the MW. We predict that ∼40 per cent of the Galactic GCs formed ex situ (in galaxies with masses M* = 2 × 107–2×109M⊙), with 6 ± 1 being former nuclear clusters.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/e36b2d38d62bb112057f52c392ac7e848ab5d23c

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Citations in the corpus

Nuclear star clusters (2009, methodology): “Using a different approach to simulate the formation of massive clusters based on a suite of “zoomed-in” SPH simulations, Kruijssen et al. (2019) suggest ∼6 stripped NSCs in the Milky Way.”
The biggest splash (2019, methodology): “In a pioneering study, Kruijssen et al. (2019) claim to have identified three significant early mergers, although note that their assignment of individual GCs to particular events does not use any orbital information.”
Measuring Radial Orbit Migration in the Galactic Disk (2018, background): “We assume it to be fixed at −1 dex, a choice supported by the age–metallicity relation of globular clusters at low Galactocentric radii (e.g., Kruijssen et al. 2018).”