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Andromeda, the cannibal

An international team of researchers, with one from Paris Observatory, has just observed the Andromeda galaxy (M31) and her partner, the Triangulum galaxy (M33). Using the Canada-France-Hawaii telescope (CFHT), they detected the presence of a large stellar halo around Andromeda. This halo would have been torn off from M33 at the time of its passage near M31, a few billion years ago. This observation consolidates the scenario of galaxy growth by cannibalism and shows that the actual size of certain galaxies is much larger than the discs observed. This result was published in Nature on September 3, 2009.

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The current scenario of galaxy evolution proposes that they become increasingly massive by absorption of smaller galaxies. This galactic cannibalism, constituting the hierarchical scenario, causes gravitational effects (tides) at the time of the successive passages of the small galaxies close to the massive ones before a final merger. Stars and gas are torn off and dispersed on large scales around the central and satellite galaxies, forming a huge stellar halo. Observing these haloes allows to better understand the hierarchical scenario. However, galaxies in general are very distant and it is very difficult to observe these faint gas and stellar haloes.

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Figure 1
Optical image (stars) of the disturbances around M31 and M33. The Moon appears on the same scale at the top left, as well as the traditional images of the visible discs of the galaxies. One sees there perfectly the important dissemination of stars along tidal streams produced by the interaction between these two galaxies and other dwarf galaxies. The circles in dotted lines around Andromeda and M33 correspond to respective distances of 900.000 and 300.000 light-years. © Alan McConnachie and PAndAS collaboration.

The Andromeda galaxy, M31, is the nearest spiral and has a companion galaxy, M33. The interacting system is studied within the framework of the Pan-Andromeda Archeological Survey (PAndAS) to detect the presence of stellar halos ejected by the interaction between M31 and M33. The international team(A) uses since 2008 the MegaCam camera on the 3.6m telescope of the CFH.

The observations highlight the presence of many stars drawing extremely wide luminous structures around Andromeda. These stars could not be formed by Andromeda but are the consequence of the tidal effects tearing off gas and stars of M33 and other dwarf galaxies at the time of their passages close to Andromeda. Moreover, the deformations observed of the gas discs in M31 and M33 are probably produced by the passage of M33 close to M31.

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Figure 2
Stroboscopic projection, obtained from the numerical simulation of the passage of M33 near M31. The simulation suggests that M33 will soon be absorbed by Andromeda. © John Dubinski and PAndAS collaboration.

The researchers used a simulation model to reproduce the encounter between the two galaxies. M33 passed near Andromeda a few billion years ago at a minimal distance of 40 kpc. The tidal effects deformed M33 tearing off gas and stars; the debris are attracted by M31 while the discs of Andromeda and M33 are deformed by the passage of the companion. This scenario reproduces perfectly what has just been observed around Andromeda by the PAndAS program.

This observation allows to better understand the hierarchical scenario of galaxy formation, and in particular the number of dwarf galaxies around massive ones. These new observations indeed show, and for the first time, that the observed number of dwarf galaxies around massive ones is not so far from theoretical predictions.

Le film de la simulation

(A) The team is made of: Alan W. McConnachie (Herzberg, Ca), Michael J. Irwin (Cambridge, UK), Rodrigo A. Ibata (Strasbourg, Fr), John Dubinski (Toronto, Ca), Lawrence M. Widrow (Queens, Ca), Nicolas F. Martin (Heidelberg, De), Patrick Cote (Herzberg, Ca), Aaron L. Dotter (Victoria, Ca), Julio F. Navarro (Victoria, Ca), Annette M. N. Ferguson (Edimbourgh, UK), Thomas H. Puzia (Herzberg, Ca), Geraint F. Lewis (Sydney, Au), Arif Babul (Victoria, Ca), Pauline Barmby (Ontario, Ca), Olivier Bienayme (Strasbourg, Fr), Scott C. Chapman (Cambridge, UK), Robert Cockcroft (Ontario, Ca), Michelle L. M. Collins (Cambridge, UK), Mark A. Fardal (Amherst, USA), William E. Harris (Ontario, Ca), Avon Huxor (Bristol, UK), A. Dougal Mackey (Edimbourgh, UK), Jorge Penarrubia (Cambridge, UK), R. Michael Rich (Los Angeles, USA), Harvey B. Richer (Vancouver, Ca), Arnaud Siebert (Strasbourg, Fr), Nial Tanvir (Leicester, UK), David Valls-Gabaud (Meudon, Fr), Kimberly A. Venn (Victoria, Ca)

Press release from CNRS


  • The remnants of galaxy formation from a panoramic survey of the region around M31 Mc Connachie et al: Nature, 3 Septembre 2009


David Valls-Gabaud (Observatoire de Paris, GEPI, et CNRS)

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