# delta_sigma_from_precomputed_pairs¶

halotools.mock_observables.delta_sigma_from_precomputed_pairs(galaxies, mass_enclosed_per_galaxy, rp_bins, period, cosmology=FlatLambdaCDM(name="WMAP5", H0=70.2 km / (Mpc s), Om0=0.277, Tcmb0=2.725 K, Neff=3.04, m_nu=[ 0. 0. 0.] eV, Ob0=0.0459))[source] [edit on github]

Calculate $$\Delta\Sigma(r_p)$$, the galaxy-galaxy lensing signal as a function of projected distance, assuming the mass around each galaxy has been precomputed.

By using the particle_masses argument, the function works equally well with DM-only simulations as with hydro simulations that include particles of variable mass.

Example calls to this function appear in the documentation below.

Parameters: galaxies : array_like Numpy array of shape (num_gal, 3) containing 3-d positions of galaxies. Length units are comoving and assumed to be in Mpc/h, here and throughout Halotools. See the Formatting your xyz coordinates for Mock Observables calculations documentation page for instructions on how to transform your coordinate position arrays into the format accepted by the galaxies and particles arguments. mass_enclosed_per_galaxy : array_like Numpy array of shape (num_gal, num_rp_bins+1) storing the mass enclosed inside each of the cylinders defined by the input rp_bins. The mass_enclosed_per_galaxy argument can be calculated using the total_mass_enclosed_per_cylinder function, as demonstrated in the Examples section below. rp_bins : array_like Numpy array of shape (num_rbins+1, ) of projected radial boundaries defining the bins in which the result is calculated. Length units are comoving and assumed to be in Mpc/h, here and throughout Halotools. period : array_like Length-3 sequence defining the periodic boundary conditions in each dimension. If you instead provide a single scalar, Lbox, period is assumed to be the same in all Cartesian directions. Length units are assumed to be in Mpc/h, here and throughout Halotools. Delta_Sigma : array_like Numpy array of shape (num_rbins-1, ) storing $$\Delta\Sigma(r_p)$$ in comoving units of $$h M_{\odot} / {\rm Mpc}^2$$ assuming h=1.

Examples

For demonstration purposes we will calculate delta_sigma_from_precomputed_pairs using a mock catalog generated with the FakeSim that is generated on-the-fly.

>>> from halotools.sim_manager import FakeSim
>>> halocat = FakeSim()


Now let’s populate this halo catalog with mock galaxies.

>>> from halotools.empirical_models import PrebuiltHodModelFactory
>>> model = PrebuiltHodModelFactory('leauthaud11', threshold = 11.)
>>> model.populate_mock(halocat)


Now we retrieve the positions of our mock galaxies and transform the arrays into the shape of the ndarray expected by the delta_sigma function. We transform our x, y, z points into the array shape used by the pair-counter by taking the transpose of the result of numpy.vstack. This boilerplate transformation is used throughout the mock_observables sub-package:

>>> x = model.mock.galaxy_table['x']
>>> y = model.mock.galaxy_table['y']
>>> z = model.mock.galaxy_table['z']
>>> galaxies = np.vstack((x, y, z)).T


The return_xyz_formatted_array function also performs this same transformation, and can also be used to place mock galaxies into redshift-space for additional observational realism.

Let’s do the same thing for a set of particle data:

>>> px = model.mock.ptcl_table['x']
>>> py = model.mock.ptcl_table['y']
>>> pz = model.mock.ptcl_table['z']
>>> particles = np.vstack((px, py, pz)).T


The default Halotools catalogs come with ~1e6 particles. Using this many particles may be overkill: in many typical use-cases, the delta_sigma function converges at the percent-level using an order of magnitude fewer particles. The code below shows how to (optionally) downsample these particles using a Halotools convenience function.

>>> from halotools.utils import randomly_downsample_data
>>> num_ptcls_to_use = int(1e4)
>>> particles = randomly_downsample_data(particles, num_ptcls_to_use)
>>> particle_masses = np.zeros(num_ptcls_to_use) + halocat.particle_mass


Whether or not you perform additional downsampling, you will need to account for the fact that you are not using the entire snapshot of particles by providing the downsampling_factor argument:

>>> total_num_ptcl_in_snapshot = halocat.num_ptcl_per_dim**3
>>> downsampling_factor = total_num_ptcl_in_snapshot/float(len(particles))

>>> rp_bins = np.logspace(-1, 1, 10)
>>> period = model.mock.Lbox


Now we calculate the mass enclosed by the cylinders around each galaxy:

>>> from halotools.mock_observables import total_mass_enclosed_per_cylinder

>>> mass_encl = total_mass_enclosed_per_cylinder(galaxies, particles, particle_masses, downsampling_factor, rp_bins, period)


At this point, we know the total mass enclosing every galaxy in our sample. Now suppose we are only interested in calculating the lensing signal around some subsample of our galaxies. Then we just build a mask for the sample of galaxies we are interested in and proceed as follows:

>>> cenmask = model.mock.galaxy_table['gal_type'] == 'centrals'

>>> rp, ds = delta_sigma_from_precomputed_pairs(cens, mass_encl_cens, rp_bins, period, cosmology=halocat.cosmology)

Take care with the units. The values for $$\Delta\Sigma$$ returned by the delta_sigma functions are in comoving units of $$h M_{\odot} / {\rm Mpc}^2$$ assuming h=1, whereas the typical units used to plot $$\Delta\Sigma$$ are in physical units of $$M_{\odot} / {\rm pc}^2$$ using the value of little h appropriate for your assumed cosmology.