Reconstructing the Last Major Merger of the Milky Way with the H3 Survey

Rohan P. Naidu*, Charlie Conroy, Ana Bonaca, Dennis Zaritsky, Rainer Weinberger, Yuan Sen Ting, Nelson Caldwell, Sandro Tacchella, Jiwon Jesse Han, Joshua S. Speagle, Phillip A. Cargile

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    86 Citations (Scopus)

    Abstract

    Several lines of evidence suggest that the Milky Way underwent a major merger at z ∼ 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of ≈500 simulations we find that a GSE with M ∗ = 5 × 108 M o˙, M DM = 2 × 1011 M o˙ best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE's outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at ≈35° to the plane and connects GSE's apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a "double-break"density profile with breaks at both ≈15-18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain ≈10% of GSE's stars. The retrograde (radial) GSE debris originates from its outer (inner) disk - exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (-0.04 ± 0.01 dex r50-1). These simulations imply that GSE delivered ≈20% of the Milky Way's present-day dark matter and ≈50% of its stellar halo.

    Original languageEnglish
    Article number92
    JournalAstrophysical Journal
    Volume923
    Issue number1
    DOIs
    Publication statusPublished - 10 Dec 2021

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