Hi, sorry in advance if this is a very silly question, but I've been trying to look at individual properties of central galaxies/host halos ("Groups" or "Halos" in the catalogs) and subhalos/satellite galaxies of a halo.
When I load the data for TNG100-1, using:
I get (4371211,) subhalos in total. This shows that the number of halos is larger than the number of subhalos, but each individual halo can host multiple subhalos, so should these numbers be correct? It's very possible I've misunderstood the documentation while loading the data for the given fields, and I want to obtain the correct number of halos/subhalos in the simulation to apply the correct cuts.
Also, when loading from the individual files for mass within a given aperture, from "aperture_masses.hdf5", I only get 203524 quantities (so 203524 subhalos in this file). Are only certain subhalos selected for these individual properties, and if so, how are they selected?
Thank you so much for any help
Dylan Nelson
29 Jan
A halo can have zero, one, or more than one subhalos.
The reason a halo can have zero subhalos is if there is no gravitationally bound structure in the halo with at least 20 particles/cells (the minimum required for a subhalo). This often occurs, only, for very small halos, i.e. halos having ~32 particles/cells in total (the minimum required for a halo). That is, there are a few million very small halos in the catalogs. In almost any analysis of interest, you would ignore these objects (by enforcing a minimum subhalo/halo/galaxy mass of interest).
"Aperture masses" is a supplementary catalog, and all supplementary catalogs are computed for some subset of the data, as described in the documentation. In this case the docs state "Restricted to subhalos with at least one star (for TNG50 and TNG100), or at least 1000 stars (for TNG300)."
Alice Chen
30 Jan
Ok, that makes sense for both the aperture mass files and halo/subhalo numbers (I thought that was the case for the halos, since the majority of them have very low masses, <10^9 solar masses, but wanted to make sure).
Thank you so much!
Hi, sorry in advance if this is a very silly question, but I've been trying to look at individual properties of central galaxies/host halos ("Groups" or "Halos" in the catalogs) and subhalos/satellite galaxies of a halo.
When I load the data for TNG100-1, using:
I get (6291349,) halos. However, when I load the sub halos for the same simulation, using:
I get (4371211,) subhalos in total. This shows that the number of halos is larger than the number of subhalos, but each individual halo can host multiple subhalos, so should these numbers be correct? It's very possible I've misunderstood the documentation while loading the data for the given fields, and I want to obtain the correct number of halos/subhalos in the simulation to apply the correct cuts.
Also, when loading from the individual files for mass within a given aperture, from "aperture_masses.hdf5", I only get 203524 quantities (so 203524 subhalos in this file). Are only certain subhalos selected for these individual properties, and if so, how are they selected?
Thank you so much for any help
A halo can have zero, one, or more than one subhalos.
The reason a halo can have zero subhalos is if there is no gravitationally bound structure in the halo with at least 20 particles/cells (the minimum required for a subhalo). This often occurs, only, for very small halos, i.e. halos having ~32 particles/cells in total (the minimum required for a halo). That is, there are a few million very small halos in the catalogs. In almost any analysis of interest, you would ignore these objects (by enforcing a minimum subhalo/halo/galaxy mass of interest).
"Aperture masses" is a supplementary catalog, and all supplementary catalogs are computed for some subset of the data, as described in the documentation. In this case the docs state "Restricted to subhalos with at least one star (for TNG50 and TNG100), or at least 1000 stars (for TNG300)."
Ok, that makes sense for both the aperture mass files and halo/subhalo numbers (I thought that was the case for the halos, since the majority of them have very low masses, <10^9 solar masses, but wanted to make sure).
Thank you so much!