Distant Echoes of the Milky Way’s Last Major Merger

Chandra, Vedant; Naidu, Rohan P.; Conroy, Charlie; Ji, Alexander P.; Rix, Hans-Walter; Bonaca, Ana; Cargile, Phillip A.; Han, Jiwon Jesse; Johnson, Benjamin D.; Ting (丁源森), Yuan-Sen; Woody, Turner; Zaritsky, Dennis

doi:10.3847/1538-4357/accf13

Distant Echoes of the Milky Way’s Last Major Merger

Journal Article · Thu Jun 29 00:00:00 EDT 2023 · The Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/accf13· OSTI ID:2425400

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Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Kavli Institute for Astrophysics and Space Research
Univ. of Chicago, IL (United States); Univ. of Chicago, IL (United States). Kavli Inst. for Cosmological Physics (KICP)
Max Planck Inst. fuer Astronomie, Heidelberg (Germany)
Carnegie Observatories, Pasadena, CA (United States)
Australian National Univ., Weston, ACT (Australia); Australian National Univ., Acton, ACT (Australia)
Univ. of Arizona, Tucson, AZ (United States). Steward Observatory

The majority of the Milky Way’s stellar halo consists of debris from our galaxy’s last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from the GSE have been kinematically and chemically studied in the inner 30 kpc of our galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to 100 kpc. We obtain follow-up spectra of stars in two strong overdensities—including the previously identified outer Virgo Overdensity—and find them to be relatively metal rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming 60–90 kpc counterparts to the 15–20 kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from 50 to 100 kpc, in the same plane as the Sagittarius Stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our galaxy’s stellar halo.

View Accepted Manuscript (DOE)

Research Organization:: US Department of Energy (USDOE), Washington, DC (United States). Office of Science, Sloan Digital Sky Survey (SDSS)

Sponsoring Organization:: National Aeronautics and Space Administration (NASA); National Science Foundation (NSF); USDOE

OSTI ID:: 2425400

Journal Information:: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 951; ISSN 0004-637X

Publisher:: IOP PublishingCopyright Statement

Country of Publication:: United States

Language:: English

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The Milky Way’s Shell Structure Reveals the Time of a Radial Collision Donlon, Thomas; Newberg, Heidi Jo; Sanderson, Robyn The Astrophysical Journal, Vol. 902, Issue 2 https://doi.org/10.3847/1538-4357/abb5f6	journal	October 2020
Quantifying the Impact of the Large Magellanic Cloud on the Structure of the Milky Way’s Dark Matter Halo Using Basis Function Expansions Garavito-Camargo, Nicolás; Besla, Gurtina; Laporte, Chervin F. P. The Astrophysical Journal, Vol. 919, Issue 2 https://doi.org/10.3847/1538-4357/ac0b44	journal	September 2021
Reconstructing the Last Major Merger of the Milky Way with the H3 Survey Naidu, Rohan P.; Conroy, Charlie; Bonaca, Ana The Astrophysical Journal, Vol. 923, Issue 1 https://doi.org/10.3847/1538-4357/ac2d2d	journal	December 2021
The Mass of the Milky Way from the H3 Survey Shen, Jeff; Eadie, Gwendolyn M.; Murray, Norman The Astrophysical Journal, Vol. 925, Issue 1 https://doi.org/10.3847/1538-4357/ac3a7a	journal	January 2022
Radialization of Satellite Orbits in Galaxy Mergers Vasiliev, Eugene; Belokurov, Vasily; Evans, N. Wyn The Astrophysical Journal, Vol. 926, Issue 2 https://doi.org/10.3847/1538-4357/ac4fbc	journal	February 2022
Reconstructing the Disrupted Dwarf Galaxy Gaia-Sausage/Enceladus Using Its Stars and Globular Clusters Limberg, Guilherme; Souza, Stefano O.; Pérez-Villegas, Angeles The Astrophysical Journal, Vol. 935, Issue 2 https://doi.org/10.3847/1538-4357/ac8159	journal	August 2022
Gastro Library. I. The Simulated Chemodynamical Properties of Several Gaia–Sausage–Enceladus-like Stellar Halos Amarante, João A. S.; Debattista, Victor P.; Beraldo E. Silva, Leandro The Astrophysical Journal, Vol. 937, Issue 1 https://doi.org/10.3847/1538-4357/ac8b0d	journal	September 2022
The Poor Old Heart of the Milky Way Rix, Hans-Walter; Chandra, Vedant; Andrae, René The Astrophysical Journal, Vol. 941, Issue 1 https://doi.org/10.3847/1538-4357/ac9e01	journal	December 2022
The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging Schlafly, Edward F.; Meisner, Aaron M.; Green, Gregory M. The Astrophysical Journal Supplement Series, Vol. 240, Issue 2 https://doi.org/10.3847/1538-4365/aafbea	journal	February 2019
Abundance Estimates for 16 Elements in 6 Million Stars from LAMOST DR5 Low-Resolution Spectra Xiang, Maosheng; Ting, Yuan-Sen; Rix, Hans-Walter The Astrophysical Journal Supplement Series, Vol. 245, Issue 2 https://doi.org/10.3847/1538-4365/ab5364	journal	December 2019
The Seventeenth Data Release of the Sloan Digital Sky Surveys: Complete Release of MaNGA, MaStar, and APOGEE-2 Data No authors listed The Astrophysical Journal Supplement Series, Vol. 259, Issue 2, p. 35 https://doi.org/10.3847/1538-4365/ac4414	journal	March 2022
The >100 kpc Distant Spur of the Sagittarius Stream and the Outer Virgo Overdensity, as Seen in PS1 RR Lyrae Stars Sesar, Branimir; Hernitschek, Nina; Dierickx, Marion I. P. The Astrophysical Journal, Vol. 844, Issue 1 https://doi.org/10.3847/2041-8213/aa7c61	journal	July 2017
They Might Be Giants: An Efficient Color-based Selection of Red Giant Stars Conroy, Charlie; Bonaca, Ana; Naidu, Rohan P. The Astrophysical Journal, Vol. 861, Issue 2 https://doi.org/10.3847/2041-8213/aacdf1	journal	July 2018
A Gaia -Enceladus Analog in the EAGLE Simulation: Insights into the Early Evolution of the Milky Way Bignone, Lucas A.; Helmi, Amina; Tissera, Patricia B. The Astrophysical Journal, Vol. 883, Issue 1 https://doi.org/10.3847/2041-8213/ab3e0e	journal	September 2019
Timing the Early Assembly of the Milky Way with the H3 Survey Bonaca, Ana; Conroy, Charlie; Cargile, Phillip A. The Astrophysical Journal, Vol. 897, Issue 1 https://doi.org/10.3847/2041-8213/ab9caa	journal	July 2020
Discovery of Magellanic Stellar Debris in the H3 Survey Zaritsky, Dennis; Conroy, Charlie; Naidu, Rohan P. The Astrophysical Journal Letters, Vol. 905, Issue 1 https://doi.org/10.3847/2041-8213/abcb83	journal	December 2020
The Magellanic Stream at 20 kpc: A New Orbital History for the Magellanic Clouds Lucchini, Scott; D’Onghia, Elena; Fox, Andrew J. The Astrophysical Journal Letters, Vol. 921, Issue 2 https://doi.org/10.3847/2041-8213/ac3338	journal	November 2021
The Unmixed Debris of Gaia-Sausage/Enceladus in the Form of a Pair of Halo Stellar Overdensities Perottoni, Hélio D.; Limberg, Guilherme; Amarante, João A. S. The Astrophysical Journal Letters, Vol. 936, Issue 1 https://doi.org/10.3847/2041-8213/ac88d6	journal	August 2022
On the Solar Velocity Drimmel, Ronald; Poggio, Eloisa Research Notes of the AAS, Vol. 2, Issue 4 https://doi.org/10.3847/2515-5172/aaef8b	journal	October 2018

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Cosmological Simulations of Stellar Halos with Gaia Sausage–Enceladus Analogs: Two Sausages, One Bun?

Journal Article · Sun Apr 13 20:00:00 EDT 2025 · The Astrophysical Journal · OSTI ID:2555914

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