particles within the snapshot files are sorted according to their group/subgroup memberships, according to the FoF or Subfind algorithms. Within each particle type, the sort order is: GroupNumber, SubgroupNumber, BindingEnergy, where particles belonging to the group but not to any of its subgroups ("fuzz") are included after the last subgroup.
And we also know, the ParticleID of particles are:
Constant for the duration of the simulation.
So, as my understanding, in the last z=0 snapshot, the particleID inside a specific subhalo should have been totally shuffled to a chaotic status. However, when I try to find the subhaloID based on the ParticleID referring to this thread, I found the code means: The particles of specific type is indexed according to the subhaloID (all the particleID in a continuous range are inside a specific subhalo). I also checked Subhalo/SnapByType in the offset hdf5 file, I found the first particleID of a subhalo always increases with the subhaloID.
That is odd: how can the particleID be constant during simulation and indexed according to its subhaloID in a snapshot happen at the same time?
Dylan Nelson
24 Sep '22
Perhaps you are mixing up IDs and indices, they are different.
Each particle has an ID, and this is constant in time.
The ordering of these particles in a snapshot, i.e. the indices that each particle has, can be arbitrary.
We chose to order them according to "group/subgroup membership", which changes in time.
The index of a given particle (i.e. its location in the snapshot) changes in time.
As we know from the Data Specification:
And we also know, the ParticleID of particles are:
So, as my understanding, in the last
z=0snapshot, the particleID inside a specific subhalo should have been totally shuffled to a chaotic status. However, when I try to find the subhaloID based on the ParticleID referring to this thread, I found the code means: The particles of specific type is indexed according to the subhaloID (all the particleID in a continuous range are inside a specific subhalo). I also checkedSubhalo/SnapByTypein the offset hdf5 file, I found the first particleID of a subhalo always increases with the subhaloID.That is odd: how can the particleID be constant during simulation and indexed according to its subhaloID in a snapshot happen at the same time?
Perhaps you are mixing up IDs and indices, they are different.
Each particle has an ID, and this is constant in time.
The ordering of these particles in a snapshot, i.e. the indices that each particle has, can be arbitrary.
We chose to order them according to "group/subgroup membership", which changes in time.
The index of a given particle (i.e. its location in the snapshot) changes in time.