I'm working with the merger trees of individual subhalos to produce mock observations of mergers. My idea is to make an image with only the two "merging" subhalos, so I'm wondering if there is a way to extract the Subfind ID of the next progenitor from a tree. A NextProgenitorID is given in the trees, but as I understand this is an internal ID that refers to the tree, or am I wrong?
How can I get straightforward the Subfind ID?
The more common (and simple) approach would be to use all the "FoF halo" particles, then you naturally include the central and all satellites, so definitely the two merging galaxies. Also, of course, all other satellites will be included.
If you really want to include only the central, and one satellite, then you're right, you would need to do two "subhalo-scope" loads, and combine these two sets of particles. If you are trying to figure out the subhalo ID of the companion to include, and you know where it is in the merger tree, then you can use SubfindID (in the SubLink merger tree).
Thank you for the clarification, I'll try both approaches and see which one suits best for my aims.