Aperture Quantities

CAESAR comes with a stand-alone function to sum quantities within a user-specified aperture around galaxies, called get_aperture_masses(). This operates directly on galaxies and halos from a CAESAR catalog, so must be used after the catalog has been generated.

get_aperture_masses() can compute masses for any particle type, or HI/H2/SFR. The aperture size can be specified as a constant for all galaxies, or an array of length the number of galaxies. This can be done in 3-D, or in 2-D projected along any principal axis. It is fully cython parallel.

Note that get_aperture_masses() only sums over particles within a galaxy’s halo. For a large aperture, or for satellites near the edge of the halo, this may not give fully accurate answers. Also, this means 2-D projections are done through the entire halo.

Usage

This example computes the quantities listed in myquants in a 2-D aperture projected in z, within an aperture given by twice the stellar half mass radius for each galaxy, on 8 cores:

In [1]: import caesar
In [2]: from caesar.hydrogen_mass_calc import get_aperture_masses
In [3]: sim = caesar.load(CAESARFILE)
In [4]: myquants = ['gas','star','dm','sfr','HI','H2']
In [5]: rhalf = np.array([i.radii['stellar_half_mass'] for i in sim.galaxies])
In [6]: m_apert = get_aperture_masses(SNAPFILE,sim.galaxies,sim.halos,quantities=myquants,aperture=2*rhalf,projection='z',nproc=8)

get_aperture_masses() returns a 2-D array of size (Nquants,Ngal), with the aperture-summed quantities for each galaxy, in the order specified in the quantities option. CAESARFILE and SNAPFILE are the filenames of the CAESAR catalog and particle snapshot, respectively.

Options

get_aperture_masses() requires the original simulation snapshot, as well as the galaxies and halos objects from the corresponding CAESAR catalog. It operates only on galaxies, and cannot operate on a subset of objects.

The following options can be passed to get_aperture_masses():

  • quantities: Can be any particle type (e.g. 'gas'), or else 'HI', 'H2' or 'sfr'. Default: ['gas','star','dm']

  • aperture: The aperture size. Can be a constant, which is assumed to be in comoving kpc, or else an array of length Ngalaxies. Default: 30

  • projection: None gives the 3-D aperture values. Specifying 'x', 'y', 'z' gives the 2-D aperture projected along that axis. Default: None

  • exclude: None includes all particles in halo. satellites or central excludes particles in satellite or central galaxies, respectively. galaxies/both excludes all particles in galaxies. Default: None

  • nproc: Number of OpenMP cores (using joblib, passed as n_jobs). Default: 1

The routine returns a single 2-D array of length (Nquants,Ngal), where Nquants=len(quantities) and Ngal=len(sim.galaxies).