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The Andromeda galaxy, the closest large spiral to the Milky Way, appears calm and tranquil as it wheels through space. But appearances can be deceiving. Astronomers have new evidence that Andromeda was involved in a violent head-on collision with the neighbouring dwarf galaxy Messier 32 (M32) more than 200 million years ago.

Dramatic proof of the galactic smash-up came from images taken by the Infrared Array Camera (IRAC) on NASA's Spitzer Space Telescope. World renowned space pioneer Dr Giovanni Fazio, from Harvard University, designed this camera with which the discovery was made. These images revealed a never-before-seen dust ring deep within the Andromeda galaxy. When combined with a previously observed outer ring, the presence of both dust rings suggests a long-ago disturbance whose effects are still expanding outward through Andromeda.

According to Prof. David Block, Director of Wits University's Anglo American Cosmic Dust Laboratory and lead author of this scientific paper: "These dust rings are like ripples in a pond. Plop a stone into water and you get an expanding series of rings or waves. Let a small galaxy collide nearly head-on with a larger one and you will see waves (or rings) of gas and dust, which propagate outwards, caused as a result of the violent gravitational interaction.

While our Atlantic Ocean was still in the process of formation, Messier 32 ploughed head-long into the disk of gas and stars of our Andromeda Spiral. Only roaming dinosaurs beheld the secrets, until the Spitzer Space Telescope unlocked them."

The other South African astronomer on the NATURE team is Robert Groess, who is currently completing his PhD at Wits University, under the supervision of Block. Groess used sophisticated image processing techniques to extract crucial data from the Spitzer Space Telescope images and also had to electronically remove myriads of foreground stars in our
Milky Way Galaxy.

Research team members Frederic Bournaud and Francoise Combes (Observatoire de Paris) conducted a series of computer simulations to model the collision between Andromeda and M32. They found that M32 plunged through the disk of Andromeda along Andromeda's polar axis approximately 210-million years ago. Since M32 is much less massive than Andromeda, the latter was not substantially disrupted. Nevertheless, the head-on collision created density waves that continue to ripple outward, creating the prominent rings detected by the Spitzer.

Astronomers have predicted that Andromeda and the Milky Way will collide in approximately 5 to 10-billion years. That collision will erase the separate identities of each galaxy, leaving a single elliptical galaxy in their place.

More information:

  For more information call Prof. David Block on 011 717 6108 or or visit his website: www.davidblock.com

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