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DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor

Dey, Arjun; Najita, Joan R.; Koposov, S.E.; Josephy-Zack, J.; Maxemin, Gabriel; Bell, Eric F.; Poppett, C.; Patel, E.; Beraldo e Silva, L.; Raichoor, A.; Schlegel, D.; Lang, D.; Meisner, A.; Myers, Adam D.; Aguilar, J.; Ahlen, S.; Allende Prieto, C.; Brooks, D.; Cooper, A.P.; Dawson, K.S.; de la Macorra, A.; Doel, P.; Font-Ribera, A.; Garcia-Bellido, Juan; Gontcho, S. Gontcho A; Guy, J.; Honscheid, K.; Kehoe, R.; Kisner, T.; Kremin, A.; Landriau, M.; Le Guillou, L.; Levi, Michael E.; Li, T.S.; Martini, Paul; Miquel, R.; Moustakas, J.; Nie, Jundan; Palanque-Delabrouille, N.; Prada, F.; Schlafly, E.F.; Sharples, Ray M.; Tarle, Gregory; Ting, Yuan-Sen; Tyas, L.; Valluri, M.; Wechsler, Risa H.; Zou, H.

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Arjun Dey

Joan R. Najita

S.E. Koposov

J. Josephy-Zack

Gabriel Maxemin

Eric F. Bell

C. Poppett

E. Patel

L. Beraldo e Silva

A. Raichoor

D. Schlegel

D. Lang

A. Meisner

Adam D. Myers

J. Aguilar

S. Ahlen

C. Allende Prieto

D. Brooks

A.P. Cooper

K.S. Dawson

A. de la Macorra

P. Doel

A. Font-Ribera

Juan Garcia-Bellido

S. Gontcho A Gontcho

J. Guy

K. Honscheid

R. Kehoe

T. Kisner

A. Kremin

M. Landriau

L. Le Guillou

Michael E. Levi

T.S. Li

Paul Martini

R. Miquel

J. Moustakas

Jundan Nie

N. Palanque-Delabrouille

F. Prada

E.F. Schlafly

Gregory Tarle

Yuan-Sen Ting

L. Tyas

M. Valluri

Risa H. Wechsler

H. Zou


We present DESI observations of the inner halo of M31, which reveal the kinematics of a recent merger - a galactic immigration event - in exquisite detail. Of the 11,416 sources studied in 3.75 hours of on-sky exposure time, 7,438 are M31 sources with well measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the NE Shelf and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1 - 2 Gyr ago. Significant numbers of metal-rich stars are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be log10MNFW(<125kpc)/M⊙=11.78+0.13−0.10. The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.


Dey, A., Najita, J. R., Koposov, S., Josephy-Zack, J., Maxemin, G., Bell, E. F., …Zou, H. (2023). DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor. Astrophysical Journal, 944(1), Article 1.

Journal Article Type Article
Acceptance Date Oct 26, 2022
Online Publication Date Feb 8, 2023
Publication Date Feb 10, 2023
Deposit Date Oct 27, 2022
Publicly Available Date Oct 28, 2022
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 944
Issue 1
Article Number 1
Related Public URLs


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Copyright Statement
Original content from this work may be used under the terms<br /> of the Creative Commons Attribution 4.0 licence. Any further<br /> distribution of this work must maintain attribution to the author(s) and the title<br /> of the work, journal citation and DOI

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