Andromeda is younger than Earth

Simulations based on new data suggest that our Milky Way’s sister galaxy is the result of a merging of two star systems at least 1.8 billion years ago

Matt Williams, Universe Today

Computer simulations based on new observations suggest that Andromeda galaxy formed from a recent galactic collision. Photo: Wikipedia Commons/Adam Evans

Since ancient times, astronomers have looked up at the night sky and seen Andromeda, the closest galaxy to our own. By the 20th century, they that this giant spiral galaxy was the Milky Way’s sister and moving towards us. In 4.5 billion years, it will merge with our own galaxy to form a supergalaxy.

The astronomers were wrong about the Andromeda galaxy in one major respect – its age. According to a recent study by a team of French and Chinese astronomers, this giant spiral galaxy formed from a major merger that occurred less than three billion years ago. This means that Andromeda, as we know it today, is effectively younger than our very own Solar System, which has it beat by about 1.5 billion years.

The study, titled “A 2-3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts,” appeared in the Monthly Notices of the Royal Astronomical Society. Led by Francois Hammer, the principal investigator of the Galaxies, Etoiles, Physique et Instrumentation (GEPI) department at the Paris Observatory, the University of Strasbourg (also in France) the team included members from the Chinese Academy of Sciences.

The researchers relied on surveys that noted the many differences between the Andromeda and Milky Way galaxies. The first of these studies, which took place between 2006 and 2014, demonstrated all Andromeda has a wealth of young blue stars in its disk (less than 2 billion years old) that undergo random motions over large scales. This is in contrast to the stars in the Milky Way’s disk, which are subject only to simple rotation.

In addition, deep observations conducted between 2008 and 2014 with the French-Canadian telescope in Hawaii (CFHT) indicated Andromeda’s vast halo, which is 10 times the size of the galaxy itself, is populated by gigantic currents of stars. The most prominent of these is the “Giant Stream,” a warped disk that has shells and clumps at its very edges.

Using this data, the French-Chinese collaboration then put together a detailed numerical model of Andromeda using the two most powerful computers available in France – the Paris Observatory’s MesoPSL and the National Center for Scientific Research’s (CNRS) IDRIS-GENCI supercomputer. The data suggested the observations could be explained by a recent galactic collision.

The researchers concluded that between 7 and 10 billion years ago, Andromeda consisted of two galaxies that had slowly began orbiting each other and collided 1.8 to three billion years ago. This collision is what gave birth to Andromeda as we know it today. That effectively makes it younger than our solar system, which formed almost 4.6 billion years ago.

The researchers were able to calculate mass distributions for the galaxies that merged to formed Andromeda, which indicated that the larger galaxy was four times the size of the smaller. Most importantly, the team was able to reproduce in detail all the structures that compose Andromeda today – including the bulge, the bar, the huge disk, and the presence of young stars.

The presence of young blue stars in its disk, which has remained unexplained until now, is attributable to a period of intense star formation after the collision. In addition, structures like the “Giant Stream” and the shells of the halo appear to have come from the smaller parent galaxy, whereas the diffuse clumps and the warped nature of the halo were derived from the larger one.



Their study also explains why the features attributed to the smaller galaxy show a dearth of heavy elements compared to the other areas. That is, it was less massive and so formed fewer heavy elements and stars. This study is significant when it comes to galactic formation and evolution, mainly because it is the first numerical simulation that has succeeded in reproducing a galaxy in such detail.

It is also significant given that such a recent impact could have left materials in the Local Group. In other words, this study could have implications far beyond our galactic neighborhood. It is also a good example of how increasingly sophisticated instruments are leading to more detailed observations which, when combined with increasingly sophisticated computers and algorithms, lead to correspondingly detailed models.

Further Reading: Paris Observatory, Monthly Notices of the Royal Astronomical Society

Matt Williams is the curator of Universe Today’s Guide to Space. He is also a freelance writer, a science fiction author and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful British Columbia.

This article first appeared in Universe Today