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Kraken reveals itself -- the merger history of the Milky Way reconstructed with the E-MOSAICS simulations
Astrophysics of Galaxies · Submitted 2 Mar 2020
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim We conclude by organising these accretion events into the most detailed reconstruction to date of the Milky Way's merger tree.
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The abstract, sentence by sentence
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: {\it 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 [$M_\star=(0.6{-}4.6)\times10^8~{\rm M}_\odot$], but have widely different accretion redshifts ($z_{\rm acc}=0.57{-}2.65$). 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 $r_{M_\star}=1$:$31^{+34}_{-16}$ and $r_{M_{\rm h}}=1$:$7^{+4}_{-2}$, 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 $\log{(M_{\rm \star,tot}/{\rm M}_\odot)}=9.0\pm0.1$, 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.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. | background 0.99 | 0.13 | 0.04 | asserted 1.00 |
| s1 | 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. | contribution 0.94 | 0.61 | 0.03 | asserted 0.96 |
| s2 | 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. | method 1.00 | 0.93 | 0.02 | asserted 0.98 |
| s3 | 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. | method 1.00 | 0.80 | 0.03 | asserted 1.00 |
| s4 | We apply the network to Galactic GCs associated with five progenitors: {\it 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'. | result 0.77 | 0.89 | 0.15 | asserted 0.95 |
| s5 | The five galaxies cover a narrow stellar mass range [$M_\star=(0.6{-}4.6)\times10^8~{\rm M}_\odot$], but have widely different accretion redshifts ($z_{\rm acc}=0.57{-}2.65$). | result 1.00 | 0.89 | 0.05 | asserted 1.00 |
| s6 | 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 $r_{M_\star}=1$:$31^{+34}_{-16}$ and $r_{M_{\rm h}}=1$:$7^{+4}_{-2}$, respectively. | result 0.88 | 0.89 | 0.30 | hedged possibility 0.95 |
| s7 | The progenitors match the $z=0$ relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. | result 0.91 | 0.86 | 0.06 | hedged possibility 0.73 |
| s8 | Even though these progenitors likely were the Milky Way's most massive accretion events, they contributed a total mass of only $\log{(M_{\rm \star,tot}/{\rm M}_\odot)}=9.0\pm0.1$, similar to the stellar halo. | result 0.98 | 0.90 | 0.12 | asserted 0.65 |
| s9 | This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. | interpretation 1.00 | 0.79 | 0.24 | asserted 0.86 |
| s10 | We conclude by organising these accretion events into the most detailed reconstruction to date of the Milky Way's merger tree. | contribution 0.97 | 0.93 | 0.28 | asserted 0.97 |
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The paper
Original abstract
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: {\it 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 [$M_\star=(0.6{-}4.6)\times10^8~{\rm M}_\odot$], but have widely different accretion redshifts ($z_{\rm acc}=0.57{-}2.65$). 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 $r_{M_\star}=1$:$31^{+34}_{-16}$ and $r_{M_{\rm h}}=1$:$7^{+4}_{-2}$, 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 $\log{(M_{\rm \star,tot}/{\rm M}_\odot)}=9.0\pm0.1$, 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.
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References
How later papers used it
151 later papers cite it
1 later paper by other authors is listed below: 1 used its data or values. 2 more papers by the same authors are listed: 2 used its data or values.
Uses its data or values 3 of 16 papers with this label · 2 by the same authors
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Used the ages of the eight members selected by another work and this paper, which provided consistent evidence for considering them purely associated to Sagittarius.
Globular cluster candidates in the Sagittarius dwarf galaxy (2021) · 0.80 · paraphrased: the licence does not allow quoting
By the same authors
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For the true origin labels we use the classification by [ref] (as revised by [ref] for Pal 1 and NGC 6441) based on the GC ages, metallicities, and orbits.
In-situ or accreted? Using deep learning to infer the origin of extragalactic globular clusters from observables (2023) (same authors) · Testing the model on the Milky Way GCs · 0.98
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Adopted 2 Gyr as its fiducial value based on this paper, which used all available information on the Kraken GCs to infer about 1.7 Gyrs as the difference between GSE and Kraken's accretion epochs.
Evidence from Disrupted Halo Dwarfs that $r$-process Enrichment via Neutron Star Mergers is Delayed by $\gtrsim500$ Myrs (2021) (same authors) · 0.90 · paraphrased: the licence does not allow quoting
How the literature uses it
35 arXiv papers cite this in 84 sentences
Papers citing it · 61 sentences without a confident label
Uses its data or values 3 papers
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As our fiducial value we adopt 2 Gyr based on [ref] who used all available information on the Kraken GCs (age, metallicity, dynamics) to infer $≈1.7$ Gyrs as the difference between GSE and Kraken's accretion epochs.
Evidence from Disrupted Halo Dwarfs that $r$-process Enrichment via Neutron Star Mergers is Delayed by $\gtrsim500$ Myrs · Mass & star-formation duration · 0.90
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A total of $∼55$ known accreted GCs can be associated to the 5 known GC-bearing accretion events [ref] .
The kinematics of globular cluster populations in the E-MOSAICS simulations and their implications for the assembly history of the Milky Way · Comparison with constraints on the assembly history of the MW and $L^*$ galaxies · 0.92
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Finally, [ref] estimate the stellar mass of the Kraken to be $∼2×10^8 M_⊙$, or approximately 70% of the mass of GE/S.
Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy · The Properties of the IGS Progenitor · 0.98
Compares with it 5 papers
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As a sanity check, based on this method, Sgr would have an inferred stellar mass of $≈3×10^8 M_⊙$, which is in excellent agreement with various literature estimates that span $≈2-6 ×10^8 M_⊙$ [ref] , and within a factor of 2 of our adopted value ($6×10^8 M_⊙$) from [ref] .
Live Fast, Die $\alpha$-Enhanced: The Mass-Metallicity-$\alpha$ Relation of the Milky Way's Disrupted Dwarf Galaxies · Disrupted galaxy sample · 0.96
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This is in excellent agreement with recent estimates from GSE's GC age-metallicity relation ($≈2-4×10^8 M_⊙$, [ref] ), star counts of [Fe/H]$<-1$, $e>0.7$ APOGEE red giants ($≈2-5×10^8 M_⊙$, [ref] ), and the integrated SFR of a chemical evolution model ($≈6×10^8 M_⊙$, [ref] ).
Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure · Gaia-Sausage-Enceladus (GSE) · 0.96
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It is possible that four of these stars belong to the dwarf galaxy Kraken and their mean ages are estimated to be $9.2_-1.7^+1.9$ Gyr in agreement with estimates from [ref] .
Age Determination of Galaxy Merger Remnant Stars using Asteroseismology · Conclusion · 0.89
Cites it as support 1 paper
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Post Gaia-DR2 almost all halo GCs (those at high-energy, that are not on disk-like orbits) have been associated with phase-space structures seen in stars, strongly suggesting they were accreted along with some dwarf Galaxy [ref] .
Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure · The Ex-Situ Halo · 0.97
Cites it as background 4 papers
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Recently it has become clear that many, if not most of the GCs are associated with accreted galaxies in the halo [ref] .
A Diffuse Metal-Poor Component of the Sagittarius Stream Revealed by the H3 Survey · Sagittarius Globular Clusters · 0.86
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There are several studies denoting stars and globular clusters to galaxy remnants such as [ref] .
Age Determination of Galaxy Merger Remnant Stars using Asteroseismology · Ex-situ · 0.82
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There are several other suggested merger events present in the MW's history, these events tend to be either lower in stellar mass or at higher redshifts and are not very well characterised [ref] .
Observing EAGLE galaxies with JWST: predictions for Milky Way progenitors and their building blocks · Introduction · 0.80
What it cites · 33 sentences without a confident label
Uses data or values from 1 paper
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In this paper, we use the groups of GCs identified by [ref] as having been accreted from the same satellite progenitors to determine the masses and accretion redshifts of these five galaxies.
Origin of the system of globular clusters in the Milky Way · Introduction · 0.98
Compares with 2 papers
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[ref] estimate that was accreted $∼10$ ago, whereas [ref] estimate a range of $8-11$ ago, and [ref] provide an upper limit on the accretion redshift of $<1.5$ (or $<9.5$).
Co-formation of the Galactic disc and the stellar halo · Accretion redshifts · 0.95
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The remaining 10 satellites are expected to have masses below $∼10^8$ (or otherwise they would have contributed $5$ GCs, which would plausibly have been identified), implying that the total accreted stellar mass is likely to be close to the sum of the five satellites discussed here (which is on the low side of, but consistent with the predictions for Milky Way-mass galaxies by [ref] and [ref] , who find $1-5×10^9$).
Empirical constraints on the formation of early-type galaxies · Satellite accretion history and in-situ growth · 0.95
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