How to know the host halo's position

HE ZHENTAO
  • 24 Jun

I want to calculate the filament accretion from the coordinates of the center point of the halo(called Host halo) and the coordinates of the subhalo. If I choose a halo in the time range Z=0(eg."http://www.tng-project.org/api/Illustris-3/snapshots/135/") in the illustris3 simulation, how do I know the coordinates(x,y,z) of the center of the halo?

Look forword your reply.
Thank you

Dylan Nelson
  • 24 Jun

You can use the GroupPos (for FoF halos) or SubhaloPos (for subhalos) field of the group catalogs. For central subhalos, these two are the same.

In the API you can also see pos_x, pos_y, and pos_z under each subhalo.

HE ZHENTAO
  • 26 Jun

Hi Dylan

Thank you very much for the selfless help you provided.

I plan to continue my research through the Browsable REST API, I have learned that the positions of the subhalo are represented as pos_x, pos_y, and pos_z. But how do I keep track of the center of their host halo?
I looked at the fields related to "parent_halo", does that mean they are represented as host halo? But I don't find any information about the location of "parent_halo" in it.
Incidentally, all the parameters in the API are expressed in abbreviated form, so how do I find out what each parameter means? Please let me know if you have the documentation for this.

Best wishes for your health!
ZHENTAO HE

Dylan Nelson
  • 28 Jun

In terms of documentation, all fields have the same names as in the group catalog documentation, with e.g. the leading "Subhalo" removed. Multi-dimensional fields are split, as in "SubhaloPos" -> "pos_x".

The parent halo of a subhalo is given by SubhaloGrNr or grnr, which is directly linked as you say under related/parent_halo. This object in the catalog has a field GroupPos, which is equal to the SubhaloPos of the central subhalo. If you would like to access GroupPos and other FoF halo fields, then these are available under meta/info.json e.g.

www.tng-project.org/api/TNG100-1/snapshots/99/halos/0/info.json
HE ZHENTAO
  • 28 Jun

Hi Dylan

Thank you very much for your help, these tips are very useful for me.

HE

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