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Hi Dylan!
I believe there is an inconsistency in the TNG50-3 sublink merger tree file. Let us look at the parents of the Subhalo 2406 at snapshot 34

From the merger tree visualization in the website, it has its parents as:
(i) SubfindID 2151 (Snapshot 33) (ii) SubfindID 2194 (Snapshot 33) (iii) SubfindID 2184 (Snapshot 32) (iv) SubfindID 2198 (Snapshot 33) (v) SubfindID 2194 (Snapshot 32)

Now, from the merger tree that I am obtaining using the illustris_python module, following are the parents:
(i) SubfindID 2151 (Snapshot 33, SubhaloID: 257735), for which, the NextProgenitorID is 258657, corresponding to which is
(ii) SubfindID 2194 (Snapshot 33, SubhaloID: 258657). Similarly, we have the next progenitors as:
(iii) SubfindID 2184 (Snapshot 32, SubhaloID: 258745)
(iv) SubfindID 2194 (Snapshot 32, SubhaloID: 258777)
Now, the NextProgenitorID corresponding to this is: 258783, for which I could not find a corresponding SubfindID.

From the merger tree visualization provided in the website, the SubhaloID corresponding to SubfindID 2198 (Snapshot 33) has to be 258763 (which is not we see in the NextProgenitorID).

Could you please look into this? I am trying to obtain a list of subhalos which merged into other subhalos in the FoF group 0 (snapshot 99)

Hi Pradyumma,

I looked but it seems ok to me, perhaps something just got mixed up in your check?

In [1]: basePath = 'sims.TNG/TNG50-3/output/'

In [2]: import illustris_python as il

In [3]: tree = il.sublink.loadTree(basePath, 34, 2406)

In [4]: tree['SnapNum'].shape
Out[4]: (1046,)

In [5]: tree['SubfindID'][0]
Out[5]: 2406

In [8]: tree['FirstProgenitorID'][0]
Out[8]: 257735

In [9]: ww = np.where(tree['SubhaloID'] == 257735)

In [10]: ww
Out[10]: (array([1]),)

In [12]: tree['SubfindID'][1]
Out[12]: 2151

In [13]: tree['NextProgenitorID'][1]
Out[13]: 258657

In [14]: ww = np.where(tree['SubhaloID'] == 258657)

In [15]: ww
Out[15]: (array([923]),)

In [16]: tree['NextProgenitorID'][923]
Out[16]: 258745

In [17]: tree['SubfindID'][923]
Out[17]: 2194

In [18]: ww = np.where(tree['SubhaloID'] == 258745)

In [19]: ww
Out[19]: (array([1011]),)

In [20]: tree['SubfindID'][1011]
Out[20]: 2184

In [21]: tree['NextProgenitorID'][1011]
Out[21]: 258763

In [22]: ww = np.where(tree['SubhaloID'] == 258763)

In [23]: ww
Out[23]: (array([1029]),)

In [24]: tree['SubfindID'][1029]
Out[24]: 2198

In [25]: tree['NextProgenitorID'][1029]
Out[25]: 258777

In [26]: ww = np.where(tree['SubhaloID'] == 258777)

In [27]: ww
Out[27]: (array([1043]),)

In [28]: tree['SubfindID'][1043]
Out[28]: 2194

In [29]: tree['NextProgenitorID'][1043]
Out[29]: -1