Magellanic Clouds: en route from Andromeda?
The origin of the Large and Small Magellanic Clouds (and other nearby satellite galaxies of our Milky Way) is still unclear.
Structurally and compositionally, the two Clouds differ from other Milky Way satellites: they are the only blue, gas-rich irregular galaxies in the immediate outskirts of the Milky Way. Furthermore, recent (2006) accurate measurements of their proper motions show that they are near their escape velocity from our Galaxy. This suggests that the two Clouds are currently having their first "close encounter" with the Milky Way. But where did they come from?
A heavy hitter in the Local Group is the Andromeda Galaxy, M31. In 2008, large-scale structures around M31 were discovered, suggesting a very tumultuous history of major merger events. During this galactic churning and shredding, gas-rich dwarf galaxies could form out of the material expelled during the collisions. And some of this material may have been ejected in the direction of the Milky Way (note that we see M31 almost edge-on, so we lie close to the orbital plane of that galaxy).
Enter Yang & Hammer (Chinese National Astronomical Observatories, and Observatoire de Paris, respectively). Using the updated details of the movements of the Clouds, they investigated possible past trajectories the two galaxies could have followed. Taking into account the possible gravitational effects from the Virgo Cluster and the Great Attractor, the authors demonstrate that it is indeed possible, based on the movements of the galaxies, that the Clouds could have travelled from M31 to the Milky Way.
The travel time to reach us ranges from 4 to 8 billion years, and the main cause of this uncertainty is the sketchy knowledge of the structure of our Galaxy's halo.
Yang & Hammer (2010 Oct 13) Could the Magellanic Clouds be tidal dwarves expelled from a past-merger event occurring in Andromeda? arXiv:1010.2748. (Accepted as a Letter to Astrophysical Journal, 10 October, 2010)
nothing more to see. please move along.