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 \ } \% 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 \sim 4 , sharpening previous limits of z \sim 2 .
We identify three massive satellite progenitors and constrain their mass growth and enrichment histories .
Two are proposed to correspond to Sagittarius ( few 10 ^ { 8 } ~ { } \mbox { M$ { } _ { \odot } $ } ) 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 – 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 \sim 40 \% of the Galactic GCs formed ex-situ ( in galaxies with masses M _ { * } = 2 \times 10 ^ { 7 } – 2 \times 10 ^ { 9 } ~ { } \mbox { M$ { } _ { \odot } $ } ) , with 6 \pm 1 being former nuclear clusters .