Hello,
I am studying the satellites fraction (Nsat/(Nsat+Ncentral)) as a function of the total satellites mass, at a given redshift.
I am not sure that I am disentangling correclty central subhalo and satellites.

This is my procedure:
At a given redshift, the number of satellites is given by len(SubhaloField) - len(GroupFirstSub). Is it correct?
The total mass of satellites is given by the sum of the SubhaloMass for those galaxies that don't match their ID i with the IDs given by GroupFirstSub.
Am I correctly disentangling satellites and central subhalo?

Thanks for your answer,
F.

Dylan Nelson

25 Aug '22

First, every halo (except for the very small, essentially unresolved halos near the simulation resolution limit and containing just ~32 particles) has one, and exactly one, central. So your fraction as defined is really Nsat/(Nsat+1). Perhaps a different quantity related to satellites would have more information, e.g. the number of satellites above a given mass (or luminosity), which would often be called "richness".

To obtain the number of satellites in a given halo, you can use the GroupNsubs field (subtract one to remove the central).

Hello,

I am studying the satellites fraction (Nsat/(Nsat+Ncentral)) as a function of the total satellites mass, at a given redshift.

I am not sure that I am disentangling correclty central subhalo and satellites.

This is my procedure:

At a given redshift, the number of satellites is given by len(

`SubhaloField`

) - len(`GroupFirstSub`

). Is it correct?The total mass of satellites is given by the sum of the

`SubhaloMass`

for those galaxies that don't match their ID`i`

with the IDs given by`GroupFirstSub`

.Am I correctly disentangling satellites and central subhalo?

Thanks for your answer,

F.

First, every halo (except for the very small, essentially unresolved halos near the simulation resolution limit and containing just ~32 particles) has one, and exactly one, central. So your fraction as defined is really

`Nsat/(Nsat+1)`

. Perhaps a different quantity related to satellites would have more information, e.g. the number of satellites above a given mass (or luminosity), which would often be called "richness".To obtain the number of satellites in a given halo, you can use the

`GroupNsubs`

field (subtract one to remove the central).Please see also the FAQ question How do I find the parent halo of a subhalo? How do I find the subhalos of a halo?