The Cartwheel Galaxy
On the topic of merging galaxies the Hubble Space Telescope produced this famous image of the Cartwheel galaxy. A very strange galaxy indeed, apparently a spiral galaxy which has been obliterated by another galaxy passing straight through it - like a stone hitting water. Well the HST image shows two candidate galaxies that could have been responsible, but which one was it? Some astronomers favoured the top galaxy, because its yellow colour reveals that it is only comprised of old stars as its star creating gas was stripped away during the collision. Other astronomers favoured the bottom galaxy which is blue due to its vast number of young newly created stars, which they explain as having formed due to the impact with the Cartwheel galaxy.