Question regarding how Halo Catalog chunks are divided

Sophia Nasr
  • 4 May

Hello,

I've read the documentation on the website, and I am left a bit more confused now: on the documentation page and description of the files, it says that snapshot chunks are not divided by spaces, but rather by group membership. Okay this makes sense, but then it says that different particle types of one halo can be stored across different chunks... This is where I'm confused: I was under the impression that the Halo Catalog chunks represent halos with their respective subhalos according to the linking length. But if the above is true about snapshots, then, how are the Halo Catalog file chunks divided up?

For example, when I try to load the 'Subhalo' group for different files, when there is a Subhalo group, and I look at the center of mass field SubhaloCM, is it normal that each file that has a subhalo group has identical centers of mass for all their subhalos? Then, do they all contain identical information? I know they do not because some of the files do not have a Subhalo group, ie they're empty (I originally thought maybe this was a region with few particles, and therefore not much data, but it looks like what I was thinking was incorrect), so my question is how the Halo Catalog chunks of one redshift/snapshot are divided, ie what is the difference between the information one file contains (example, fof_subhalo_tab.069.3.hdf5) from that which another file (example, fof_subhalo_tab.069.50.hdf5) contains?

Thanks a lot. I'm very confused about what I'm seeing here, and so I think the answer to my questions would be to understand this part first.

Dylan Nelson
  • 5 May

Hi Sophia,

File "chunks" are purely numerical and have no physical meaning. The separation of groups or subhalos or particles across file chunks should be ignored. For instance, it doesn't matter if a particular file chunk has a 'Subhalo' group or not. If you use the provided helper scripts, this is basically what they take care of.

Physically, it would be unusual for multiple subhalos to have the same SubhaloCM. If you load this fields for all subhalos of a given snapshot, using e.g. il.groupcat.loadSubhalos(), then I imagine you won't find any duplicates.

Sophia Nasr
  • 6 May

Hi Dylan,

Can you show me a mini script on how I'd do that in Python? The script I got from the repository linked on the website does not use il to import the halo catalog files, so it would be nice to see how I could do it for a folder named "Halo Catalog" (for example), if possible. Could you help with that and show me an example?

Dylan Nelson
  • 6 May

Hello,

if you follow the Example Scripts Walkthrough, the very top starts with an example of loading from the group catalog.

Sophia Nasr

Aha yes I did go through that tutorial I just didn't connect the name change. I'll use that, thank you.

Also edited this since I figured it out. Thanks a bunch! I was initially working with the python file directly (for group catalog analysis), but now I'm working with it as you suggested and it works perfectly.

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