Thanks Britton,


That’s exactly what I was looking for. I will see what I can do with that outline in paste.


Dave Semeraro

Visualization Group

Texas Advanced Computing Center

University of Texas Austin


From: Britton Smith []
Sent: Wednesday, March 20, 2019 6:53 PM
To: Discussion of the yt analysis package <>
Subject: [yt-users] Re: halos and particles question


Hi Dave,


Yeah, a halo's basically a gravitationally bound clump of particles.


The easiest way to get the particles in each halo is to assume the halos are spherical. This is reasonably ok unless you know for certain you need the exact particle membership for each halo. If that's the case, see below. If you've managed to make a halo catalog using a yt-endorsed method, then you should have the centers of mass and radii for each halo. You should then be able to create spherical data containers for each center/radius and grab all the particles, something like below:


Note the use of 'particle_position' for both the data_ds and the halos_ds. The halo catalog dataset is treated as particle data, where each particle is a halo. The use of data_ds.arr and .quan is to make sure the center/radius are in the correct unit system.


If you need the exact member particles for each halo, this really isn't implemented at the moment, but could be with moderate work done on the halo finders.


Let me know if you have more questions. Feel free to come onto the slack channel if you want to discuss.




On Wed, Mar 20, 2019 at 1:49 PM Dave Semeraro <> wrote:

Hi there,


I was wondering what is the correct way to obtain a list of particles that lie inside each halo in a catalog. I think I can figure out how to brute force this but I was wondering if there was a YT way to do it. In the end I would like particle positions (x,y,z) for particles grouped by which halo they are in.


I am not an astrophysicist and did not stay in a holiday inn express last night. I don’t even know what a halo is really for. I just need to get disjoint sets of particles that are grouped together for some work I am doing with tessellation.



I was thinking halos calculate a position and center for clumps of particles. I would like separate lists of particle positions for each of those clumps. So far I can generate the halos (cut and paste from the docs). I just don’t know the next step. I am working with the enzo_cosmology_plus dataset as an example.




Dave Semeraro

Visualization Group

Texas Advanced Computing Center

University of Texas Austin

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