Head-on collision explains Andromeda's strange shape

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A two-decade-long riddle about the bizarre shape of the Milky Way's nearest spiral-shaped galaxy, Andromeda, has been solved, suggests a new study.
Instead of having the flat plane and outflung arms that are the hallmarks of a mature spiral galaxy, Andromeda has a warped plane and several rather chaotic, overlapping outer rings.

The reason, according to an international team of astronomers, is that Andromeda suffered a head-on collision with a smaller galaxy some 210 million years ago.
The evidence comes from infrared images of Andromeda taken by NASA's orbiting Spitzer Space Telescope. They show a previously hidden, tilted second ring that protrudes from the heart of the galaxy. This ring is likely to be the shockwave of gas and dust from a colossal collision.

The theory has been put to the test in a computer simulation. It suggests a dwarf galaxy called M32 probably drove straight into the heart of Andromeda, which is also known as M31.
"While head-on collisions may have been common in the early universe, only a handful are known nearby," says the paper, whose lead author is David Block of the University of Witwatersrand in South Africa. "The discovery of one in our near-neighbour M31 affords the unique opportunity of studying such a collision at unprecedented spatial resolution."
Andromeda, first spotted as "a little cloud" by the Persian astronomer Abd-al-Rahman Al Sufi in 964, is itself headed for a collision with the Milky Way. However, the two galaxies are more than 2 million light years apart and the collision will probably take place between 3 billion and 6 billion years from now.



The dwarf galaxy M32 probably crashed into
the heart of Andromeda 210 million years ago,
setting off shock waves that created two dusty
rings, marked in blue and green.
(Image: NASA/JPL/P. Barmby/CfA)

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