Friends-of-Friends (FoF) is a popular halo finder which is used in most N-body simulations. It works by grouping dark matter particles that are close together into groups called halos. This is good in the sense that it is simple, requires little memory, and runs fast.

But on the flip side, particles would also often get clumped together in a halo although they are headed in different directions and belong to different coherently moving structures.

This is shown in the images on the left. The top images shows just the particles in the simulation before any halo finder is run on them. The middle image shows how the particles are correctly assigned into 3 separate halos which appear to be self-bound and in equilibrium. The last image shows an example of a problematic FoF group.

There are 3 distinct halos that get identified as one big halo because arbitrary particles between them have artificially decreased the 3 distances. This leads to highly non-spherical halo shapes and results in incorrect determination of various halo properties such as mass, velocity, etc. Due to some of these inconsistencies the FoF halo finder can lead to significant errors in the conclusions astronomers draw from the data sets that use it.