I was wondering if there was a way to link the black hole information provided in the supplementary data catalogue (the Blackhole details catalogue) to specific subhalos - for example, the black hole IDs and coordinates to the subhalo ID which they belong to (where one applies).
Yes I would suggest to do this by "ID". You can load all PartType5/ParticleIDs from the snapshot, and then search in this array (it is quite small) for the ID of the black hole of interest. Once you've found it, you have the snapshot index of this particle. Then the problem is the same for any particle of any type: given its index in the snapshot, what subhalo contains it? The answer would be the largest entry in offsets.hdf5/Subhalo/SnapByType which is smaller than the index. A similar logic is used in many places in the example scripts for loading purposes.
Thanks for your reply; just to clarify, does this approach work for the original Illustris data as well as for the IllustrisTNG data (where the offset files can be downloaded)?
That's correct, the same offsets exist for original Illustris, they are just included in the group catalog files, instead of as separate files.