How a smash-up shaped our celestial neighborhood

Paris Observatory

Computer simulations show how the Andromeda Galaxy and Magellanic Clouds formed from the collision of two massive galaxies in the Local Group 6 billion years ago.

The Andromeda Galaxy and the Magellanic Clouds — the Milky Way’s most prominent galactic neighbors — took on their current shape because of a huge collision between two galaxies billions of years ago, according to new numerical simulations.

Our celestial neighborhood, known as the Local Group includes nearly 40 galaxies in all. But it’s dominated by two giant spiral galaxies: Andromeda and our own Milky Way. Researchers have long thought that Andromeda — which is 2.5 million light-years away in the constellation of the same name — formed from a mash-up of two galaxies of smaller mass. To get at the details surrounding the galactic merger and its consequences, a team of astronomers led by the Paris Observatory’s Francois Hammer modeled the galaxy’s structural evolution.

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The team was able to reproduce most of the Andromeda Galaxy’s peculiar properties, such as its large, thin disk and massive central bulge. That lent confidence to an analysis indicating that the two galaxies – one slightly more massive than our Milky Way and the other a third as massive — started to merge about 9 billion years ago. The mash-up was complete about 3.5 billion years later.

The researchers noted in a news release that the collision must have been particularly violent to generate the rotation required to form the Andromeda Galaxy’s giant disk.

The simulations also predict that an amount of mass equivalent to one-third that of the Milky Way could have been expelled during the interaction through the formation of gigantic tidal tails — thin, elongated regions of stars and interstellar gas. The gas within one of these tails may have formed the Magellanic Clouds, which today are satellite galaxies attached to the Milky Way.

The implication is that the Large and Small Magellanic Clouds were actually space invaders from the galaxy next door, coming at us at a velocity of 620,000 mph (1 million kilometers per hour).

“If confirmed, these results may have important consequences in cosmology, by supporting both the hypothesis that most spiral galaxies have been formed by mergers, and the prediction that many dwarf galaxies may originate from tidal tails during such events,” researchers said in the news release.

The team’s results are being reported in two research papers, published by The Astrophysical Journal and Astrophysical Journal Letters.

More information on Andromeda and the Magellanic Clouds:

* Milky Way, Andromeda had similar origins * Andromeda is a cannibal and heading our way * Collision caused rings around Andromeda * New Hubble photos show galaxy collisions * Milky Way clouds are speeding through space * Galactic neighbors are just passing through

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“Does M31 Result From an Ancient Major Merger?” is being published in the Dec. 10 issue of The Astrophysical Journal, with Hammer as well as Y.B. Yang, J.L. Wang, M. Puech, H. Flores and S. Fouquet listed as authors. Yang and Hammer are the authors of “Could the Magellanic Clouds Be Tidal Dwarves Expelled From a Past-Merger Event Occurring in Andromeda?” — which was published Nov. 20 in Astrophysical Journal Letters. Yang and Wang represent the National Astronomical Observatories of the Chinese Academy of Sciences, and the other researchers are from the Paris Observatory.

John Roach is a contributing writer for Connect with the Cosmic Log community by hitting the “like” button on the Cosmic Log Facebook page or following’s science editor, Alan Boyle, on Twitter (@b0yle).


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