Tangos Tutorial – Gadget+AHF

Initial set up

This tutorial imports a gadget-run simulation with an AHF halo catalogue and its associated merger tree information.

Make sure you have followed the initial set up instructions.

Then download the raw simulation data required for this tutorial. You need two files:

• the simulations snapshots, tutorial_gadget.tar.gz
• the AHF catalogues and the merger tree files, tutorial_gadget_ahf.tar.gz

required for this tutorial.

Unpack both tar files either in your home folder or the folder that you pointed the TANGOS_SIMULATION_FOLDER environment variable to.

For most Linux or macOS systems, the following typed at your bash command line will download the required data and unpack it in the correct location:

cd $TANGOS_SIMULATION_FOLDER
curl https://zenodo.org/record/5155467/files/tutorial_gadget.tar.gz?download=1 | tar -xz
curl https://zenodo.org/record/5155467/files/tutorial_gadget_ahf.tar.gz?download=1 | tar -xz

Import the simulation

At the unix command line type:

tangos add tutorial_gadget_ahf --min-particles 100

The process should take about a minute on a standard modern computer, during which you’ll see a bunch of log messages scroll up the screen.

Let’s pick this command apart

• tangos is the command-line tool to administrate your tangos database
• add is a subcommand to add a new simulation
• tutorial_gadget_ahf identifies the simulation we’re adding. Note that tangos automatically spots the AHF outputs in this folder and adapts its behaviour accordingly. If you add tutorial_gadget, it’ll instead see the SubFind catalogues (see the alternative tutorial).
• --min-particles 100 imports only halos/groups with at least 100 particles. (The default value is 1000 particles, but this tutorial dataset is fairly low resolution so we’ll keep these small halos.)

Note that all tangos command-line tools provide help. For example tangos --help will show you all subcommands, and tangos add --help will tell you more about the possible options for adding a simulation.

At this point, the database knows about the existence of timesteps and their halos and groups in our simulation, but nothing about the properties of those halos or groups. We need to add more information before the database is useful.

If you want to speed up this process, it can be MPI parallelised since version 1.8.

Import AHF’s properties

At the unix command line type:

tangos import-properties Mvir Rvir Xc Yc Zc --for tutorial_gadget_ahf

The process should take about a minute on a standard modern computer, during which you’ll see a bunch of log messages scroll up the screen.

The example command line lists a few properties, Mvir, Rvir, Xc, Yc and Zc to import from the AHF AHF_halos files. The added directive --for tutorial_gadget_ahf specifies which simulation you want to apply this operation to. It’s not strictly necessary to add this if you only have one simulation in your database.

Import the merger trees

The merger trees can be imported from the AHF AHF_mtree files. Further information on how to create or handle the merger tree files with the codes from the AHF toolbox see AHF’s documentation. To import the merger trees type

tangos import-ahf-trees --for tutorial_gadget_ahf

Note that you can also use the built-in tree builder, as described in other tutorials such as the SubFind example. But compared to the default implementation, ahf trees has the significant runtime advantages.

Importing the merger tree should take a minute or so, and again you’ll see a log scroll up the screen while it happens.

Add some more interesting properties

Let’s finally do some science. We’ll add dark matter density profiles; from your shell type:

tangos write dm_density_profile --with-prerequisites --include-only="NDM()>5000" --type=halo --for tutorial_gadget_ahf

If you want to speed up this process, it can be MPI parallelised.

Here,

• tangos write is the same script you called above to add properties to the database
• dm_density_profile is an array representing the dark matter density profile; to see all available properties you can call tangos list-possible-properties
• --with-prerequisites automatically includes any underlying properties that are required to perform the calculation. In this case, the dm_density_profile calculation actually needs to know an accurate center for the halo (known as shrink_center), so that calculation will be automatically performed and stored
• --include-only allows an arbitrary filter to be applied, specifying which halos the properties should be calculated for. In the present case, we use that to insist that only halos with more than 5000 particles have their density profiles calculated
• --type=halo calculates the properties only for halos (as opposed to groups)

Explore what’s possible

Now that you have a minimal functioning tangos database, proceed to the data exploration tutorial.