Tangos Tutorial – Gadget+SubFind

Initial set up

Make sure you have followed the initial set up instructions. Then download the raw simulation data required for this tutorial. Unpack the tar file 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

Import the simulation

At the unix command line type:

tangos add tutorial_gadget --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 identifies the simulation we’re adding
• --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 subfind’s properties

At the unix command line type:

tangos import-properties --type halo --for tutorial_gadget
tangos import-properties --type group --for tutorial_gadget

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

Generate the merger trees

The merger trees are most simply generated using pynbody’s bridge function. To do this type:

tangos link --for tutorial_gadget

which builds the merger tree for the halos, and then you probably also want to run

tangos link --type group --for tutorial_gadget

to make the merger tree for the groups. If you want to speed up these processes, they can each be MPI parallelised.

The construction of each merger tree should take a couple of minutes, 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

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.