The Andromeda Galaxy, also called M31, is a very close neighbor. It is 2.55 million light-years away and is approaching the Milky Way. In doing so, it crunches on its way the small galaxies it crosses, and an international research team has just found the trace of it in its halo, the region that surrounds the spiral galaxies.
In the universe, the “big ones” eat the “small” ones: The most impressive galaxies grow by merging with the small dwarf galaxies that are in their vicinity. Thus, in the last billion years, Andromeda has absorbed several galaxies, traces of which can still be found by analyzing the spherical clusters left in the halo. These very dense groups of stars once belonged to the galaxies now integrated into Andromeda.
Astronomers, led by Geraint Lewis and Dougal Mackey of the University of Sydney, have studied Andromeda’s halo, which is much larger and more complex than the Milky Way, and reveals a turbulent past. By observing the size of the clusters and the direction in which they move, they tracked their movements, thus reconstructing the history of Andromeda’s “meals”. Their study published in the journal Nature indicates the existence of two different growth periods in which Andromeda engulfed several dozen galaxies.