Random info.

Orphans: When a subhalo can no longer be found in the N-body merger trees, a ‘subresolution merging time’ is computed for the subhalo (based on its last known orbital properties and the algorithm of Boylan-Kolchin et al. 2008). The associated galaxy is then an orphan, which continues to evolve as normal (although we have no detailed knowledge of its position within its host halo) until the subresolution merging time has passed, at which point it is assumed to merge with the central galaxy of its host halo. (Knebe et al. 2018)

How galaxies are identified in Illustris: DM haloes are identified using the standard friends-of-friends (FoF) approach (Davis et al. 1985) with a linking length equal to 0.2 times the mean interparticle separation. The algorithm is applied to the DM particles, keeping only haloes with at least 32 DM particles. After this step, baryonic resolution elements are assigned to the same FoF group as their nearest DM particle. Substructure within the FoF groups is identified using an extension of the SUBFIND algorithm (Springel et al. 2001; Dolag et al. 2009), which can be applied to hydrodynamic simulations. (Rodriguez-Gomez et al. 2015)