halo_18.png

Halo 18 is an example of a galaxy that had a head-on collision (see the panel at z=1.2) and had a disc flip (you can see this by comparing the disc orientation at z=1.4 and z=0). This image and Image 2 show how these two exemplary galaxies evolve with time: each panel corresponds to a different time defined by redshift (z). Redshift is a quantity used by astronomers to measure the time in the universe, with z=0 corresponding to the present day, and larger z corresponding to earlier times. Each z has two panels associated with it, which show how the galaxy looks in two planes (like the xy and xz planes in 3D space).