Getting Started

swiftgalaxy facilitates analyses of individual galaxies from cosmological hydrodynamical simulations executed with the SWIFT code. The core SWIFTGalaxy class makes selecting particles belonging to a galaxy, transforming their coordinates, masking its particle arrays, working with spherical or cylindrical coordinates, accessing its integrated properties computed by a halo finder, and more, easy. It is built upon the SWIFTDataset from swiftsimio and takes advantage of that module’s lazy-loading and unit-awareness features. Users already familiar with usage of swiftsimio will find that any syntax to interact with a SWIFTDataset also works with a SWIFTGalaxy.

Requirements

python3.8 or higher is required (>=3.9 is preferred).

Python packages

Required:

swiftsimio, required to provide the SWIFTDataset class and related functionality (note that swiftsimio has additional required dependencies).
numpy, required to support various array operations.
unyt, required for unit calculations.

Optional:

scipy, required to specify rotations via the Rotation class.
velociraptor, required to enable support for Velociraptor halo finder outputs.

Installing

swiftgalaxy can be installed using the python packaging manager, pip, or any other packaging manager that you wish to use:

pip install swiftgalaxy

Note

swiftgalaxy is not hosted on PyPI yet, so pip install swiftgalaxy will fail. Use the instructions below instead.

Note that this will also install required dependencies.

To set up the code for development, first clone the latest master from github:

git clone https://github.com/SWIFTSIM/swiftgalaxy.git

and install with pip using the -e flag,

pip install -e swiftgalaxy/

Quick start

Assuming we have a snapshot file snap.hdf5, and a halo catalogue provided by Velociraptor halos.properties, halos.catalog_groups, etc., with the default names for the arrays of coordinates, velocities and particle_ids, we can initialise a SWIFTGalaxy for the first row (indexed from 0) in the halo catalogue very easily:

from swiftgalaxy import SWIFTGalaxy, Velociraptor
sg = SWIFTGalaxy(
    'snap.hdf5',
    Velociraptor(
        'halos',
        halo_index=0
    )
)

Like a SWIFTDataset, the particle datasets are accessed as below, and all data are loaded ‘lazily’, on demand.

sg.gas.particle_ids
sg.dark_matter.coordinates

However, information from the halo finder is used to select only the particles identified as bound to this galaxy. The coordinate system is centred in both position and velocity on the centre and peculiar velocity of the galaxy, as determined by the halo finder. The coordinate system can be further manipulated, and all particle arrays will stay in a consistent reference frame at all times.

Again like for a SWIFTDataset, the units and metadata are available:

sg.units
sg.metadata

The halo finder interface is accessible as shown below. What this interface looks like depends on the halo finder being used, but will provide values for the individual galaxy of interest.

sg.halo_finder

In this case with Velociraptor, we can get the virial mass like this:

sg.halo_finder.masses.mvir

The further features of a SWIFTGalaxy are detailed in the next sections.