Subhalos which have no descendant

Eunwoo Chung
  • 1
  • 25 Feb '23


I am using the sublink merger tree data to trace the position and velocity of subhalos in the next snapshot.
In main descendant branch, most of them have their descendant, but I found some subhalos have no descendant tree data.
I am not saying the case that their descendant ID is -1, but the case that their descendant doesn’t exist in the main descendant branch at all.
My target objects have total mass between 1e9Msun and 1e11Msun and their DM fractions are bigger than 10%.
I think they have enough DM particle to trace descendant but I wonder how these things can happen.
If there are some possible scenarios, it will be very helpful for my research.

Thank you very much!


Dylan Nelson
  • 25 Feb '23

Hi Eunwoo,

What about picking one, loading all its DM particle IDs, and then going to the next snapshot, and finding which subhalo(s) those same DM particle IDs reside in?

Perhaps they are not in any subhalo (would be strange).

Perhaps the vast majority are in a single subhalo - this would be the natural descendant. If this was the case, perhaps we can understand why the SubLink tree did not pick it.

Perhaps they are distributed among many subhalos, i.e. some sort of fragmentation, stripping, or merging encounter which really disrupted the DM structure.

In any case, I imagine these occurences are very rare? Does this happen 50% of the time, 1% of the time, or 1e-4% of the time?

Eunwoo Chung
  • 25 Feb '23

Hi Dylan,

Thank you for your ideas and I think too that tracing the DM particle IDs can give some solutions.
Then, how can I know that which particle IDs are belonging to specific subhalos?
Maybe I can decide it by their spatial position but I think it will take a very long time because of the snapshot data size.
I wonder if there are any other methods.

Well, I cannot say the rate of these occurrences clearly but I’m sure that these events are very rare and their rates are < 1e-2% roughly.

Dylan Nelson
  • 25 Feb '23

Loading the particles which belong to a specific subhalo is discussed in the data tutorial and documentation.

To find which subhalos the same IDs belong to at a different snapshot, you will need to load all DM IDs in the snapshot (not necessarily all at once). The indices can be mapped back to the parent (sub)halos. An example was discussed in this thread.

Eunwoo Chung
  • 25 Feb '23

Thank you very much and I hope I can share some interesting notes soon :)
Have a great day!

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