Public Data Access Overview / Discussion Forum

First, Is it possible to distinguish satellite and field subhalos in Illustris public data? I would like to investigate the environmental effects on dark halos and less massive galaxies embedded their halos in detail.

Second, it is very fundamental question how to use dataset. In Example Scripts tutorial, the relation between 'star formation rate' and 'total mass' was plotted using a subroutine 'loadSubhalos' written in groupcat.py. I would like to replace 'total mass' to total mass of different type using the field of 'SubhaloMassType'. Could you tell me how to extract and plot to total mass of different type?

Hi Kohei,

For the plotting, you can change the example as follows:

type_to_plot = 0 # 0=gas, 1=dm, 4=stars, 5=bhs

fields = ['SubhaloMassType','SubhaloSFRinRad']
subhalos = il.groupcat.loadSubhalos(basePath,135,fields=fields)

import matplotlib.pyplot as plt
mass_msun = subhalos['SubhaloMassType'][:,type_to_plot] * 1e10 / 0.704
plt.plot(mass_msun,subhalos['SubhaloSFRinRad'],'.')

As for the subhalos, there are two aspects:

1. Each subhalo is tagged by the Subfind algorithm as either a "central" or "satellite". If you download the group catalogs, the list of subhalos which are centrals is exactly equal to the GroupFirstSub field. This is because the first subhalo in each group is the central, which will be followed by zero or more satellites.

In the online API, this information is called "primary_flag". For example, Subhalo 0 for Illustris-3 Snap-135 has primary_flag=1 (it is the central subhalo of the most massive group). But, Subhalo 1 has primary_flag=0, because it is the first (most massive) satellite in the group.

1. Instead of using the Subfind results, you may want to define satellites versus field galaxies in a different way. For instance, by requiring that field galaxies have no nearby neighbor more massive than itself within 500 kpc.

It depends what exactly you're trying to do, or compare against.

Hi Dylan,

Thank you so much for explaining me how to use 'SubhaloMassType'!

For satellites and fields, my definition which I'd like to require data sample is that while satellites are within a viral radius (ex. 300kpc for MW halo) of a host halo, field ones are far from a viral radius of a host halo. Thus, for example, satellites are similar to MW satellites such as Fornax, Sextans and Draco dSphs. On the other hand, fields are LeoA and IC10 dIrrs of the Local Group.

As a starting point, I'd suggested you (1) download the group catalog, (2) define a host halo mass range you're interested in, (3) look at the radial distribution (r/rvir) of all the satellites of those hosts. Perhaps a radial or mass cut on these satellites will already be close to what you want.

There is no isolation metrics computed for field subhalos, so if you wanted to insure they are "sufficiently far from a massive neighbor" you would have to calculate this. But, it shouldn't be hard to then come up with a matched sample (e.g. in mass) to your satellite sample.

Thank you so much for letting me know useful suggestions. I am trying to do your first suggestion.

(1) download the group catalog, (2) define a host halo mass range you're interested in, (3) look at the radial distribution (r/rvir) of all the satellites of those hosts.

I am OK that 'field' subhalos are defined strictly.

Kohei

Sorry, I have another question. I would like to draw mass-concentration relation using group catalogs. Halo mass can be used by "SubhaloMass", and scale length of dark halo can be derived from "SubhaloVmaxRad". However, in order to calculate concentration, we need to get viral radius of dark halo.

How do we derive a viral radius of each dark halo using group catalog?

Virial radii (spherical overdensity) measurements are available in the group catalog, e.g. Group_R_Crit200.

But, these exist only for "FoF Groups" and not for "Subfind Subhalos".

Typically, we will attribute this FoF virial radius to the central subhalo of each group. Which means that satellites have no such measurement available. If you would like such a measurement for both centrals and satellites, you would either have to (a) calculate it from the particle data, or (b) use a field available for all subhalos, such as the half mass radii.

There is a plan to release a catalog of halo structural properties, including NFW and Einasto best-fit parameters, etc, but I don't have a firm time-frame for this at the moment.