Halo and Subhalo Questions

Craig Cissel
  • 10 Jun

I am doing a project where I need Positions and velocities of Galaxy Clusters (Halos?) at specific snap shots.
and within each Galaxy Cluster I need each Galaxies (Sub-Halo?) position and velocity.

I wanted to see if my interpretation of the catalog were correct.

For the Cluster:
I believe the GroupCM and GroupPos are both two different ways of expressing the position. GroupVel would be the velocity.

And how would I be able to correctly identify each Clusters identifier so I can find which subgroups belong to it?

For the Galaxy:
The SubHaloCM and SubhaloPos are two different ways of expressing position and the SubhaloVel is the velocity?

Does the SubhaloParent refer to the parent cluster, or the SubhalGrNr? or are they locations of the parent cluster in two separate tables?

Are all these in an absolute xyz dimensions, or are say the Galaxies position in reference to the center of the cluster?

Thanks!

Craig

Dylan Nelson
  • 10 Jun

Hi Craig,

I would use GroupPos and SubhaloPos for the locations of the clusters halos and member galaxies, respectively.

First, you would pick a cluster, e.g. by selecting on Group_M_Crit200 (the halo mass). Then you have its position, and also GroupNsubs and GroupFirstSub, which together give you the list (so to speak) of the member galaxies. The first one is the central galaxy (e.g. BCG) itself, while all the remaining ones are satellites.

You can verify you have the right subhalos (member galaxies) by confirming, as you say, that their SubhaloGrNr is equal to the cluster (index/ID) that you selected.

All the positions are in "box coordinates", so to make them relative, you would want e.g. GalaxyPosRel_x = GroupPos_x - SubhaloPos_x. Careful of periodic boundaries if the cluster is near the edge of the box.

Craig Cissel
  • 10 Jun

Thank you so much!

Craig

Craig Cissel
  • 1
  • 13 Jun

Which data set should I be looking at for this information? Also I have been following the getting started tutorial online and I am running into an error with the Illustris-3 data set. As I downloaded both the Snapshot and the FoF & Subfind[Groupcat] for group 135. However I can't find any fof_subhalo_tab hdf5 files.
I keep running into the error:
" Unable to open file (unable to open file: name = './Illustris_3/output//groups_135/fof_subhalo_tab_135.0.hdf5', errno = 2, error message = 'No such file or directory', flags = 0, o_flags = 0)"

when following the example script. Yet I have all the files from the 135 group. Is there any other supplementary data I need to download for the example files that I am missing? All I see are hdf5 files prefaced with either groups (2 such files) or snap (30 such files). I also was required

Thanks!

Dylan Nelson
  • 13 Jun

Hi Craig,

The group catalog is called groups_*.hdf5 for Illustris and fof_subhalo*.hdf5 for TNG, but these files are essentially the same. You can see where in the python script it searches for both files, so as long as you have these files in the path it is looking for, it should work.

Craig Cissel
  • 13 Jun

Thank you!

I realize now it was just a simple path error. My apologies for the inconvenience.

You've been an amazing help!

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