The remnants of galaxy formation from a panoramic survey of the region around M31.
In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than M(v) = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.
McConnachie
Alan W
AW
NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, Canada V9E 2E7. alan.mcconnachie@nrc-cnrc.gc.ca
Irwin
Michael J
MJ
Ibata
Rodrigo A
RA
Dubinski
John
J
Widrow
Lawrence M
LM
Martin
Nicolas F
NF
Côté
Patrick
P
Dotter
Aaron L
AL
Navarro
Julio F
JF
Ferguson
Annette M N
AM
Puzia
Thomas H
TH
Lewis
Geraint F
GF
Babul
Arif
A
Barmby
Pauline
P
Bienaymé
Olivier
O
Chapman
Scott C
SC
Cockcroft
Robert
R
Collins
Michelle L M
ML
Fardal
Mark A
MA
Harris
William E
WE
Huxor
Avon
A
Mackey
A Dougal
AD
Peñarrubia
Jorge
J
Rich
R Michael
RM
Richer
Harvey B
HB
Siebert
Arnaud
A
Tanvir
Nial
N
Valls-Gabaud
David
D
Venn
Kimberly A
KA