radial_distance

halotools.mock_observables.radial_distance(xs, ys, zs, xc, yc, zc, period, return_signed_1d_results=False)

Calculate the radial distance between the positions of a set of satellites and their centrals, accounting for periodic boundary conditions.

If the coordinates xc, yc, zc of each satellite’s central are not known in advance, these can be computed by knowing the halo_hostid of the satellites, as shown in the Examples.

Parameters:

xs, ys, zsndarrays

Arrays of length (ngals, ) storing the xyz-coordinates of the satellites

xc, yc, zcndarrays

Arrays of length (ngals, ) storing the xyz-coordinates of the centrals

periodfloat or 3-element sequence

Length of the periodic box in each Cartesian direction. If a single float is given, the box will be assumed to be a periodic cube. If the points are not in a periodic box, set period to np.inf.

return_signed_1d_resultsbool, optional

If set to True, the signed distances in each dimension will be returned in addition to the radial distance. Default is False, in which case only the radial distance will be returned

Returns:

dradndarray

Array of shape (ngals, ) storing the radial distances

Examples

For demonstration purposes we will use a fake halo catalog and compute the radial distances between all the satellites and their host halos.

>>> from halotools.sim_manager import FakeSim
>>> halocat = FakeSim()
>>> satmask = halocat.halo_table['halo_upid'] != -1
>>> satellites = halocat.halo_table[satmask]

Since Halotools catalogs do not come with the host halo position pre-computed, we will need to calculate this information from knowledge of the halo_hostid of each satellite.

>>> from halotools.utils import crossmatch
>>> idxA, idxB = crossmatch(satellites['halo_hostid'], halocat.halo_table['halo_id'])
>>> satellites['halo_x_host_halo'] = np.nan
>>> satellites['halo_y_host_halo'] = np.nan
>>> satellites['halo_z_host_halo'] = np.nan
>>> satellites['halo_x_host_halo'][idxA] = halocat.halo_table['halo_x'][idxB]
>>> satellites['halo_y_host_halo'][idxA] = halocat.halo_table['halo_y'][idxB]
>>> satellites['halo_z_host_halo'][idxA] = halocat.halo_table['halo_z'][idxB]

>>> xs, ys, zs = satellites['halo_x'], satellites['halo_y'], satellites['halo_z']
>>> xc, yc, zc = satellites['halo_x_host_halo'], satellites['halo_y_host_halo'], satellites['halo_z_host_halo']
>>> satellites['radial_distance'] = radial_distance(xs, ys, zs, xc, yc, zc, halocat.Lbox)