In hierarchical cosmological models1, 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 disk2,3. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history4, 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 Mv = -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.
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We thank the entire staff at the Canada–France–Hawaii Telescope for taking the data, for initial processing with Elixir and for their continuing support throughout this project. A.M.N.F. and A.D.M. are supported by a Marie Curie Excellence Grant from the European Commission under contract MCEXT-CT-2005-025869. G.F.L. thanks the Australian Nuclear Science and Technology Organisation (ANSTO) for supporting his involvement in PAndAS through its Access to Major Research Facilities Program (AMRFP). R.M.R. acknowledges grant AST-0709479 from the National Science Foundation, and grants GO-9453, GO-10265 and GO-10816 from the Space Telescope Science Institute. The image of M33 overlaid in Fig. 1 is reproduced by courtesy of T. A. Rector and M. Hanna.
Author Contributions All authors assisted in the development and writing of the paper. In addition, A.W.M. is the Principal Investigator of PAndAS; M.J.I. led the data processing effort; R.A.I. was the Principal Investigator of an earlier CFHT MegaPrime/MegaCam survey, which PAndAS builds on (which included S.C.C., A.M.N.F., M.J.I., G.F.L., N.F.M., A.W.M. and N.T.); J.D., L.M.W. modelled the M31–M33 interaction; N.F.M. had a lead role in the study of the dwarf galaxies; P.C. assisted with constructing the luminosity function; and A.L.D. developed the theoretical isochrones.
This animated movie depicts a model of the interaction of M31 and M33 highlighting the excitation of tidal tails as the plausible origin of the extensions of the M33 stellar disk and gaseous warp. The tidal stripping of the M33 stellar halo is not the likely origin of the observed extensions.
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