A simulation-based inference of the Milky Way merger history

Sante, A, Font, AS ORCID: 0000-0001-8405-9883, Kawata, D, Makinen, TL and Grand, RJJ ORCID: 0000-0001-9667-1340 (2026) A simulation-based inference of the Milky Way merger history. Monthly Notices of the Royal Astronomical Society, 550 (2). ISSN 0035-8711

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Abstract

Accreted stars in the Milky Way (MW) preserve information about their galaxies of origin in their chemo-dynamical
properties. In this study, we use the chemo-dynamical signatures in the merger debris to approximate the posterior distribution of the properties of disrupted satellites at their accretion. Adopting a simulation-based inference framework, we train an ensemble of normalizing flows using samples of merger debris from the Auriga simulations. Applying this methodology to a local sample of accreted stars in the MW, we infer the lookback times, stellar and halo masses, and halo mass merger ratios of several known accretion events: Gaia-Enceladus-Sausage (GES), Helmi streams, Heracles, I’itoi, LMS-1/Wukong, Sagittarius, Sequoia, and Thamnos. Our predictions for accretion time and mass align with previous estimates and follow the expected stellar mass–metallicity relation across redshifts. The total accreted stellar mass from these events is predicted to be 2.2+1.1 −0.6 × 109 M, with the GES and Sagittarius dwarfs being the largest contributors. The total accreted stellar mass from all disrupted progenitors is 1.4+1.1−0.4 × 109 M, which is consistent with previous mass
measurements of this component. We provide a prediction for the evolution of the MW halo mass until the accretion
of Sagittarius (z ≈ 1): We find that the halo mass growth from the time of its first merger (z ≈ 5) to z ≈ 2 significantly
exceeds the total mass of the known progenitors accreted during that interval, suggesting the presence of unidentified substructures. Our estimate of the Galaxy halo mass after the Sagittarius merger, but priorto the accretion of the Magellanic Clouds, is 5.9+1.4 −1.1 × 1011 M.

Item Type:	Article
Uncontrolled Keywords:	software: machine learning; software: simulations; Galaxy: halo; Galaxy: kinematics and dynamics; Galaxy: stellar content; astro-ph.GA; astro-ph.GA; 5109 Space Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; 51 Physical Sciences; Stem Cell Research; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QA Mathematics > QA76 Computer software Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	12 June 2026
Date of first compliant Open Access:	16 September 2026
Date Deposited:	16 Sep 2026 08:07
Last Modified:	16 Sep 2026 08:07
DOI or ID number:	10.1093/mnras/stag1162
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29440

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