The map was created by using faint background galaxies as light sources, against which gas could be seen by the characteristic absorption features of hydrogen. (keckobservatory.org)
Some of these galaxies may be foreground or background galaxies in the vicinity of the cluster. (atlasoftheuniverse.com)
The team managed to combine the data by using background galaxies which did not change position in the 12 years. (sciencedaily.com)