I want to claculate the pericentre distance of subhalo that is accreted into a halo. I'm going into the merger trees of a given subhalo to estimate when this sub was accreted by its FirstSub at z=0 by considering when SubhaloID of the subhalo differs from the SubhaloID of the central one. So, how could I get the minimum distance between the central sub and the satellite and how could I calculate how many times the subhalo has passes through the pericentre distance?
Thanks in advance,
Yes I would suggest to derive a new field SubhaloRelPos[SnapNum,3] for each subhalo of interest. It should be the position of that subhalo, relative to its z=0 host halo (which also moves in time). With the merger tree you should follow both backwards in time, and subtract their positions. After you have this, you can easily calculate quantities such as the minimum distance, or when this occurred. If you need precise answers, you may want to interpolate (or integrate) between the snapshot spacings, since the pericentre could happen very fast.
Alright, that makes sense. Thanks a lot!
Just one last thing. When I'm trying to estimate the positions of both the FirstSub and the satellite backward in time I get many entries of the positions for given SnapNum, how is this happening? Shouldn't I have just one single value for each snapshot?
You should use only the main progenitor branch, not the entire tree.
Alright, thanks very much for your help.