### Difference between the host halo and its central subhalo.

Alexandres Lazar
• 1
• 2 Jul '17

When doing analysis from these quantities, what are the differences between the host halo and its central subhalo in the group and subfind catalogs? Are they the same thing really?

For example, I want to analyze the orbtial energy between a satellite and its host halo. I select a range of host halo and then index out there subhalos:

``````import illustris_python as il
import numpy as np

# Mass limits
Mmin = 8e+11
Mmax = 2.4e+12
h0   = 0.704

# Constructing a Boolean array
( halos['Group_M_TopHat200'] > Mmin*h0/1e+10 ) & ( halos['Group_M_TopHat200'] < Mmax*h0/1e+10  )
).nonzero()[0]

# The central subhalo
firstSub  = firstSub.astype('int32')

# The satelite
secondSub = firstSub+1
``````

If you were to compare masses between `firstSub` and the halos from `mask`, they are not at all one to one.

So in the case of orbits or comparing virial units to the satellite subhalo, what do you really as the halo in comparison, the `firstSub` or the `mask` halo?

Dylan Nelson
• 3 Jul '17

Hi Alexandres,

It depends on the quantity. Specifically, as you mention, for masses, these are different.

The `Group_M_Crit200` or `Group_M_TopHat200` type values are spherical-overdensity calculations, as described in the documentation. In contrast, the central subhalo of that group has a `SubhaloMass` field which is the total gravitationally bound mass, according to the Subfind algorithm.

They mean different things and may be useful in different contexts. If by "virial mass" you want to use e.g. a "mass within r_200crit" as a definition, then the SO quantities would be appropriate.

Alexandres Lazar
• 1
• 3 Jul '17

Thank you for clearing that up!

Alexandres Lazar
• 1
• 3 Jul '17

I have one more question regarding this, do people usually have the primary/central subhalo as the host halo since the central subhalo mass is the host halos mass not bound to any other substructure and interacts gravitationally to the satellites, or is that depend on how papers define their samples? Or is the host halo just the dark matter halo of the central subhalo?

I am asking this because I am looking at the virial mass of the milky ways dark matter halo as well as comparing its virial units to satellites residing reside in the MW dark matter halo, and I just want to end this confusion. It would either be comparing then the virial values of the central galaxy (subhalo) or the virial values of the dark matter halo (which are really big numbers in my analysis ie. normalizing satellite radial velocities with host halos virial velocity).

Dylan Nelson
• 4 Jul '17

Hi Alexandres,

If I understand you correctly, I think there is no right answer. This is simply a definition, and in making an analysis you should just clearly state which definition you use. Or, even better, use both (or all possible definitions), and compare the results, looking for a conclusion which is definition-independent.

If you are comparing to some other theory model, or observational data, then of course you need to carefully match the definition as used in that other work.

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