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Hello!

I have been trying to get the dark matter fraction of some satellite subhalos, but I am not sure that I am doing it correctly due to some doubts I have regarding how to get the subhalos IDs in order to work with them. I am using JupyterLab, so I have been using the load functions available over there.

I understand that the subhalos IDs are the same as their index in an array, but I want to be sure if I am getting this right. In order to work with the subhalos at z=0 using the TNG100-1 simulation, firstly I formed an array with the loadSubhalos function that contains the mass of all their particles within twice the stellar half mass radius (subhalos_mass = il.groupcat.loadSubhalos(basePath, 99, fields=['SubhaloMassInRad'])). Once I found out which where the IDs of the central halos (using the "GroupFirstSub" field), I selected the most massive central halo's satellite subhalos as it follows: subhalos_satelites = subhalos_mass[1:17185]. The indexes of this new array called "subhalos_satelites" shoud be the satellite subhalos' IDs, am I right?

Secondly, I tried to study their dark matter fraction, but it is here where I am having trouble: I tried doing this for the first 10 satellite subhalos and I am not sure I am reading the information correctly when I use the load functions. I am attaching an image where you can see what I did. In my opinion, the variables called "subhalos_satelites[i]" and "total_mass" should have the same value but, when I printed them out to check this, I found out that this was not the case. Maybe I am not using the load functions correctly or I am mixing up the satellite subhalos' information.

If my explanation was not clear enough, let me know and I will try to explain it in a different way.

Many thanks in advance,
Elisa.

Hello Elisa,

Your subhalos_satelites contains masses (floating point numbers), not IDs (which are integers).

If you want to get the IDs of the satellite subhalos of a given halo, you don't need to load any arrays of subhalos. Rather, you just need the GroupFirstSub and GroupNsubs, then:

haloID = 1234

halo = il.groupcat.loadSingle(basePath, snapNum, haloID=haloID)

central_subhalo_ID = halo['GroupFirstSub']
all_subhalo_IDs = np.arange(halo['GroupFirstSub'], halo['GroupFirstSub']+halo['GroupNsubs'])
satellite_subhalo_IDs = np.arange(halo['GroupFirstSub']+1, halo['GroupFirstSub']+halo['GroupNsubs'])

Hello Dylan,

I think I understand now, but I have two questions regarding those variables you defined. Firstly, how do you know which halo ID do you initially have to ask for? You wrote haloID = 1234 as an example, but where can I see which halo IDs are there in order for me to set that variable? Secondly, why are you adding halo['GroupFirstSub']+halo['GroupNsubs'] when you use them as inputs?

Hi Elisa,

I'm not entirely sure what you mean by "how do you know which halo... to ask for"? You can pick any halo(s) you want - perhaps you can clarify.

Second, this is to set the stop parameter for np.arange().

Hi,

What I wanted to ask is how do I know that the halo that I'm asking for exists. I know I can ask for any halo I want, but that request must be made from a "list" of available halos, must not it? So how can I be sure that the halo that I am choosing exists?
I am sorry if this sounds confusing, maybe I am making it more complicated than it actually is and there is no restriction to the halo IDs.

Hi Elisa,

The halos which exist are 0, 1, 2, ..., Nhalos-1 where "Nhalos" can be found in the Header of the group catalog files. Any of those numbers is a valid halo.

(Similarly for subhalos).

Oh I see, I did not know that! Thank you very much for all your help, Dylan!