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 [ \mbox { $M _ { \star } $ } = ( 0.6 { - } 4.6 ) \times 10 ^ { 8 } ~ { } \mbox { M$ { } _ { \odot } $ } ] , but have widely different accretion redshifts ( \mbox { $z _ { 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 _ { 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 _ { \star,tot } / \mbox { M$ { } _ { \odot } $ } ) } = 9.0 \pm 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 organising these accretion events into the most detailed reconstruction to date of the Milky Way ’ s merger tree .