some questions about TNG50-1, connections between fof halo and central subhalo

Yaofei He
  • 13 Aug '23

I have some questions about TNG50-1 data.

1.There are two kinds of data in group catalogs, one called as FoF Halos, and the other called as Subfind Subhalos. There are many subhalos in one halo, so the total number of subhalos should be greater than the halos’ number. But on the contrary, the halos are less than subhalos in TNG50-1 z=0 simulation. Why?

2.And what's their connection? How do one fof halo corresponds to subhalo(Or how can we connect a subhalo of id 0 with its host fof halo of one certain id? How could I find it?)?

3.In my mind, if one fof halo contains many subhalos, the center subhalo would extend to the edge of its host halo, and other satellites would each occupies a piece of the host halo so that the center subhalo looks fragmentary. Is this statement right?

4.Would a sphere with a radius of Group_R_Crit200 (a parameter of group catalog) contain all the dark matter particles in corresponding fof halo? And would it contain all dm particles in corresponding center subhalo?

Thank you for your help!

Dylan Nelson
  • 14 Aug '23

(1) Not all halos have multiple subhalos. The smallest halos have, in fact, zero subhalos. If you focus only on the more massive (well resolved) halos, your intuition will be correct.

(2) The fields GroupFirstSub and SubhaloGrNr map between the two types of objects.

(3) That is correct, you will find a schematic illustration in Springel+ 2001 (Figure 3).

(4) Roughly yes, but this is never exactly guaranteed. A FoF halo can extend past R200c, and similarly, non-FoF particles can be within R200c.

Yaofei He
  • 1
  • 23 Aug '23

So, a halo won't be in the subfind subhalos catalog data even when it could be regarded as the center subhalo of itself? Or every subhalo of subfind catalog belongs to a halo with at least two subhalos?

Dylan Nelson
  • 23 Aug '23

Every subhalo of the subfind catalog belongs to a halo with at least one subhalo.

A halo has exactly one subhalo if it has only a central, and no satellites.

(For very low-mass halos, which you likely do not care about, they can be so small that the halo contains no subhalo at all).

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