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.
Galactic cannibalism
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.
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.
Galactic cannibalism
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.
History of the Andromeda Galaxy. Credit: University of Sydney.
The first period of growth goes back almost ten billion years. At the time, the Andromeda Galaxy did not look at all like it does today, it was in full construction, and the galaxies it absorbed at the time certainly contributed to its evolution. The second period is newer, a billion years or a little more, and it left more interpretable traces. What amazed the scientists is that the stars coming from the two different periods do not rotate in the same direction at all. The first population of clusters develops perpendicular to the second. A peculiarity that specialists do not currently explain.
Milky Way and Andromeda are racing against each other and will clash
Collisions between galaxies are not uncommon in the universe. They were even frequent at the beginning of its history, when the universe was smaller and the galaxies were de facto closer to each other. Today, most galaxies show signs of merging or close to collisions, and the Milky Way, which is a spiral galaxy like Andromeda, is no exception to this rule. It has previously swallowed several small satellite galaxies, and it is in the process of swallowing Sagittarius’ galaxy. However, it is more difficult to determine its history as it is observed from within. It is therefore the work of Andromeda that is similar “cosmic archeology”in the words of the authors, is important: it provides an external reading of the evolution of galaxies and contributes to the understanding of the history of the Milky Way.
This story also ends in four billion years in a giant fusion. The Andromeda Galaxy is actually on a collision course with the Milky Way. And the two galaxies are heading towards each other and will collide head-on. From this encounter, hundreds of millions of new stars and a huge egg-shaped galaxy will be born as a result of the merger. The distances between the stars are so great in the galaxies that there is not really a risk of collision between the stars coming from the two galaxies. The sun should thus survive this event, which will last for millions of years.