Accessing Offset Files

Charlotte Wilkinson
  • 24 Oct '16

Using the Subfind Subhalos group catalogue I have selected a number of starburst galaxies, however I need to track them in the merger tree. How do I access the offset files to get the corresponding 'Subhalo_SubfindSubhaloID' parameter so i can match my subhalos to the merger tree. I'm working on disk.

Dylan Nelson
  • 3 Nov '16

Hi Charlotte,

Unfortunately I am not exactly sure what Subhalo_SubfindSubhaloID means here, can you clarify?

In general, if you load the main progenitor branch merger tree of a subhalo index i at snapshot snap0, and want to load the particle data (e.g the position of member DM particles) of the main progenitor of this subhalo at some earlier snapshot snap1, then you could do:

tree = il.sublink.loadTree(basePath,snap0,i,fields=['SubfindID','SnapNum'],onlyMPB=True)

TreeIndex_at_snap1 = np.where(tree['SnapNum'] == snap1)
assert len(TreeIndex_at_snap1[0]) == 1

SubfindID_at_snap1 = tree['SubfindID'][TreeIndex_at_snap1]

particle_data = il.snapshot.loadSubhalo(basePath,snap1,SubfindID_at_snap1,'dm','Coordinates')
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