Halos and sub-halos detection
HaloMaker (version 2) is the first step of the GalICS pipeline.
From the snapshots of an N-body simulation (Gadget, RAMSES, Enzo formats), it identifies halos and sub-halos, depending on which one of the two halo-finder available for this version is used: a friend of friend algorithm or Adaptahop, written by Stéphane Colombi (IAP). Note that only Adaptahop detects sub-halos. The code also measures the instantaneous physical properties of these halos, such as their virial masses, radius, potential and kinetic energy, spin and
profile. If the friend of friend algorithm has been used, the center of each halo is defined by the position of the particle closest to the center of mass. If Adaptahop has been chosen as halo-finder, the center is defined as the position of its particle with the highest SPH density. ( The density of each particle was computed by Adaptahop ).
A linked list of all the particles contained in these halos and sub-halos
which is key for the construction of their merging history tree is also stored in the binary outputs called tree_brick_xxx where xxx is the number of the snapshot.