Within the framework of hierarchical structure formation, large spiral galaxies like the Milky Way or Andromeda arose from the merger of many small galaxies and protogalaxies. Later in their evolution, spiral galaxies become the dominant component in such mergers, cannibalizing smaller systems that fall within their sphere of influence. The complete destruction of the victim is usually progressive, and may take several orbits. However, the stellar debris from the destroyed dwarf galaxy follows a similar orbital trajectory to the progenitor, which is likely to have started life far away from the place of its final demise, and so the tidally disrupted matter tends to be deposited over a broad range in distance from the larger galaxy. Over time, with the accumulation of many such mergers, large galaxies develop an extensive stellar and dark-matter 'halo', the latter being by far the most massive component of the galaxy. Meanwhile, part of the (dissipative) gas component of the smaller galaxies feeds the growth of the disk of the larger galaxy. This is seen in numerical simulations of galaxy formation, which result in galactic haloes comprising clumps of dark matter. If this prediction is correct, then haloes should possess significant substructure—in contrast to previous suggestions, which predict the dark and luminous components of haloes to be distributed smoothly.

Andromeda or M31 galaxy is our Galaxy's "big sister", twice as large but otherwise very similar. It is the nearest large galaxy, lying only 2.2 million light-years away. Astronomers have known for some years that our own Galaxy is a cannibal. Its outer parts are threaded with tell-tale streams of stars from small galaxies it has engulfed.

The first sensitive panoramic wide field imaging survey of M31 using the Wide Field Camera on the Isaac Newton Telescope has unambiguously revealed the presence of a giant stellar stream within M31's halo. The source of the stream is likely to be either, or both, of the peculiar dwarf galaxies M32 and NGC205, close companions of M31, which may have lost a substantial amount of stars, gas and dust due to their tidal interactions with the massive host galaxy. The broad agreement of the metallicity distribution of the stream stars with these two dwarf satellites together with their alignment, physical proximity, and distorted morphological appearance, point to a common origin. The well-known disparity in properties between the Milky Way and M31 stellar haloes would be understandable if the majority of M31's stellar halo arose as relatively recent tidal debris from prolonged bouts of aggressive tidal interaction with its two nearest neighbour satellites. Together with recent observations of tidal debris in the Milky Way halo, these results clearly demonstrate that the epoch of galaxy building still continues, and that substructure in the form of huge, recently-deposited tidal streams, could be a generic feature of large galaxy haloes.

Surface density of RGB stars over the southeastern halo of M31. The over-density of stars is seen as a stream extending out of M31 close to, but distinct from, the minor axis. [ GIF | TIFF ]

Some references: 

• CCI Annual Report 2001, 14.
• R. Ibata, M. Irwin, G.  Lewis, A. M. N. Ferguson and N. Tanvir, 2001, "A Giant Stream of Metal-Rich Stars in the Halo of the Galaxy M31", Nature, 412, 49.
• M. Irwin et al., 2001, "The Andromeda Stream: A Giant Trail of Tidal Stellar Debris in the Halo of M31", ING Newsl., 5, 3.
• Irwin, M, Lewis, G, Ibata, R, 2003, "Galactic Cannibals", Frontiers, No 17 Autum 2003.
  
• "Our Galaxy's "Sister" is a Cannibal", AAO Press Release, 4 July 2001.