I want to know which galaxies merge in an exact snapshot.
Q1: The NextProgenitorID obtained by using the loadtree function belongs to the Subhalo at the time of the snapshot, or to the FirstProgenitor?
Q2: I found that SubhaloID and SubfindID are different, so what function can I use to read the corresponding details through Subhalo? through the function "loadsubhalo"?
I'm afraid I don't fully understand question (1), perhaps you can rephrase.
For question (2), the tree documentation states that SubfindID is "Index of this subhalo in the Subfind group catalog." i.e. this is the ID you would use with il.groupcat.loadSubhalo().
for question(1), it means whether the nextprogenitor is the nextProgenitor of the firstprogenitor or the subhalo?
And what's more, how I can get subfindID through the nextprogenitorID?
Perhaps this FAQ helps?
Also the red arrow in the schematic diagram of the trees should clarify things.
thanks for your help.
I get the answer to question 1
but for question 2, it means that I get the firstprongenitorID, but I cannot use the function "loadsubhalo" with the firstprogenitorID. So I want to konw how read the detail infromation through firstprogenitorID.
FirstProgenitorID is internal to the tree. If you want to use it, you can use it to locate an entry in the tree.
Once you have located (any) entry in the tree, you can use its SubfindID to find its ID in the group catalogs, and then load further information about it, from the group catalogs, or its member particles/cells from the snapshot.