TY - JOUR
T1 - Distant Echoes of the Milky Way’s Last Major Merger
AU - Chandra, Vedant
AU - Naidu, Rohan P.
AU - Conroy, Charlie
AU - Ji, Alexander P.
AU - Rix, Hans Walter
AU - Bonaca, Ana
AU - Cargile, Phillip A.
AU - Han, Jiwon Jesse
AU - Johnson, Benjamin D.
AU - Ting, Yuan Sen
AU - Woody, Turner
AU - Zaritsky, Dennis
N1 - Funding Information:
We thank the referee for constructive feedback that significantly improved the manuscript. V.C. gratefully acknowledges a Peirce Fellowship from Harvard University. We thank Morgan Fouesneau, Rene Andrae, David W. Hogg, Tom Donlon, Guilherme Limberg, and Will Cerny for insightful conversations, and Vasily Belokurov for detailed feedback. We acknowledge Kevin Schlaufman for inspiring the “ECHOS” in the title of this work. We are grateful to the staff at Las Campanas Observatory—including Yuri Beletsky, Carla Fuentes, Jorge Araya, Hugo Rivera, Alberto Pastén, Roger Leiton, Matías Díaz, and Carlos Contreras—for their invaluable assistance. C.C. and P.C. acknowledge support from NSF grant NSF AST-2107253. Support for this work was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51515.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
Funding Information:
We thank the referee for constructive feedback that significantly improved the manuscript. V.C. gratefully acknowledges a Peirce Fellowship from Harvard University. We thank Morgan Fouesneau, Rene Andrae, David W. Hogg, Tom Donlon, Guilherme Limberg, and Will Cerny for insightful conversations, and Vasily Belokurov for detailed feedback. We acknowledge Kevin Schlaufman for inspiring the “ECHOS” in the title of this work. We are grateful to the staff at Las Campanas Observatory—including Yuri Beletsky, Carla Fuentes, Jorge Araya, Hugo Rivera, Alberto Pastén, Roger Leiton, Matías Díaz, and Carlos Contreras—for their invaluable assistance. C.C. and P.C. acknowledge support from NSF grant NSF AST-2107253. Support for this work was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51515.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
Funding Information:
This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope; LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. This research has made extensive use of NASA’s Astrophysics Data System Bibliographic Services. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.
Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.
PY - 2023/7/1
Y1 - 2023/7/1
N2 - 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.
AB - 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.
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U2 - 10.3847/1538-4357/accf13
DO - 10.3847/1538-4357/accf13
M3 - Article
AN - SCOPUS:85164010703
SN - 0004-637X
VL - 951
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 26
ER -