The presence of many dwarf galaxies around larger galaxies, such as Andromeda or our own Milky Way Galaxy, has long been known : it would be remnants of larger galaxies slowly eaten by their bulky neighbour, and which astronomers thought independent from each other. This study reveals that in fact, around the Andromeda Galaxy, the majority of them are arranged in a gigantic flattened structure, more than a million light years long, rotating on itself.

This discovery was carried out thanks to the PAndAS (Pan-Andromeda Archaeological Survey1) survey by an international team between 2008 and 2011, with the telescope Canada-France-Hawaii (CFHT) but also the American Keck telescope. This survey allowed them to discover and characterize a very large number of new dwarf galaxies around Andromeda. To do this, the researchers used a new technique that is to study simultaneously the brightness and the position of the stars in the dwarf galaxies, as well as models of distribution of star luminosity in these galaxies.
Previous research had already indicated the possibility of the structure, but the new study shows its existence with more statistical weight, and highlights the rotation of these satellites. Such a structure was also discovered around the Milky Way these past ten years.

The discovery allows to constrain the different theories of galaxy formation, as some theories expect that galaxies grow by accumulation of dark matter from dwarf galaxies accreted from random directions.

Reference

A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy
Rodrigo A. Ibata, Geraint F. Lewis, Anthony R. Conn, Michael J. Irwin, Alan W. McConnachie, Scott C. Chapman, Michelle L. Collins, Mark Fardal, Annette M. N. Ferguson, Neil G. Ibata, A. Dougal Mackey, Nicolas F. Martin, Julio Navarro, R. Michael Rich, David Valls-Gabaud, Lawrence M. Widrow
Nature, 3 January 2013