Build-up and survival of the disc: from numerical models of galaxy formation to the Milky Way
Abstract
We study the build-up and survival of angular momentum in the stellar disc using a statistical suite of cosmological simulations of Milky Way-mass galaxies. Our results show that stellar kinematics at z=0 rarely recover the true times of disc spin-up, due to the disruptive impact of massive radial merger events. The proto-disc (i.e. Aurora) and kicked-up disc stars (the Splash) become indistinguishable at low metallicities, and the local fraction of kicked-up disc stars remains < 20 per cent even after major mergers. In contrast, observations from Gaia and legacy surveys reveal that Galactic α-rich populations as old as τ =13.5 Gyr show significant rotation, with median η > 0.75. This places strong constraints on the total merger ratio between the proto-Milky Way and its last significant merger (Gaia-Sausage-Enceladus, GSE), favouring minor mergers with mass ratios < 1:4. We present the age–metallicity relation for the stellar halo and estimate the interaction epoch at τ spin−up≃ τ GSE∼ 11 Gyr. We note an abrupt dearth of halo and Splash stars after a lookback time of 10 Gyr, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range −0.8<[Fe/H]< −0.3 share a formation time of τ starburst∼ 11 Gyr, which we interpret as a signature of a starburst triggered by the first pericentric interaction of the GSE. This is remarkable corroboration between our GSE interaction and starburst times of τ GSE=τ starburst ∼ 11 Gyr.
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BibTeX
@article{orkney2026buildup,
title = {{Build-up and survival of the disc: from numerical models of galaxy formation to the Milky Way}},
author = {Matthew Orkney and Chervin Laporte},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = jun,
doi = {10.1093/mnras/staf2154},
eprint = {2509.09576},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05855816v1},
note = {CC BY},
}
RIS
TY - JOUR
TI - Build-up and survival of the disc: from numerical models of galaxy formation to the Milky Way
AU - Matthew Orkney
AU - Chervin Laporte
PY - 2026
DA - 2026/06/05
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/staf2154
UR - https://hal.science/hal-05855816v1
AB - We study the build-up and survival of angular momentum in the stellar disc using a statistical suite of cosmological simulations of Milky Way-mass galaxies. Our results show that stellar kinematics at z=0 rarely recover the true times of disc spin-up, due to the disruptive impact of massive radial merger events. The proto-disc (i.e. Aurora) and kicked-up disc stars (the Splash) become indistinguishable at low metallicities, and the local fraction of kicked-up disc stars remains < 20 per cent even after major mergers. In contrast, observations from Gaia and legacy surveys reveal that Galactic α-rich populations as old as τ =13.5 Gyr show significant rotation, with median η > 0.75. This places strong constraints on the total merger ratio between the proto-Milky Way and its last significant merger (Gaia-Sausage-Enceladus, GSE), favouring minor mergers with mass ratios < 1:4. We present the age–metallicity relation for the stellar halo and estimate the interaction epoch at τ _spin−up≃ τ _GSE∼ 11 Gyr. We note an abrupt dearth of halo and Splash stars after a lookback time of 10 Gyr, marking the end of the merger interaction. Finally, we show that Globular Clusters in the metallicity range −0.8<[Fe/H]< −0.3 share a formation time of τ _starburst∼ 11 Gyr, which we interpret as a signature of a starburst triggered by the first pericentric interaction of the GSE. This is remarkable corroboration between our GSE interaction and starburst times of τ _GSE=τ _starburst ∼ 11 Gyr.
SN - 0035-8711
SN - 1365-2966
ER -