conditional_cylindrical_isolation

halotools.mock_observables.conditional_cylindrical_isolation(sample1, sample2, rp_max, pi_max, marks1=None, marks2=None, cond_func=0, period=None, num_threads=1, approx_cell1_size=None, approx_cell2_size=None)[source] [edit on github]

Determine whether a set of points, sample1, is isolated, i.e. does not have a neighbor in sample2 within an user specified cylindrical volume centered at each point in sample1, where various additional conditions may be applied to judge whether a matching point is considered to be a neighbor.

For example, conditional_cylindrical_isolation can be used to identify galaxies as isolated if no other galaxy with a greater stellar mass lies within a projcted 500 kpc and line-of-sight 3 Mpc. Different additional criteria can be built up from different combinations of input marks1, marks2 and cond_func.

See the Examples section for further details, and also Galaxy Catalog Analysis Example: Identifying isolated galaxies, Part II for a tutorial on usage with a mock galaxy catalog.

Parameters:

sample1 : array_like

Npts1 x 3 numpy array containing 3-D positions of points.

See the Formatting your xyz coordinates for Mock Observables calculations documentation page, or the Examples section below, for instructions on how to transform your coordinate position arrays into the format accepted by the sample1 and sample2 arguments.

Length units are comoving and assumed to be in Mpc/h, here and throughout Halotools.

sample2 : array_like

Npts2 x 3 numpy array containing 3-D positions of points.

rp_max : array_like

radius of the cylinder to search for neighbors around galaxies in sample1. If a single float is given, rp_max is assumed to be the same for each galaxy in sample1. Length units are comoving and assumed to be in Mpc/h, here and throughout Halotools.

pi_max : float

half the length of cylinders to search for neighbors around galaxies in sample1. If a single float is given, pi_max is assumed to be the same for each galaxy in sample1. Length units are comoving and assumed to be in Mpc/h, here and throughout Halotools.

marks1 : array_like, optional

Npts1 x N_marks array of marks. The supplied marks array must have the appropriate shape for the chosen cond_func (see Notes for requirements). If this parameter is not specified, all marks will be set to unity.

marks2 : array_like, optional

Npts2 x N_marks array of marks. The supplied marks array must have the appropriate shape for the chosen cond_func (see Notes for requirements). If this parameter is not specified, all marks will be set to unity.

cond_func : int, optional

Integer ID indicating which function should be used to apply an additional condition on whether a nearby point should be considered as a candidate neighbor. This allows, for example, stellar mass-dependent isolation criteria on a galaxy-by-galaxy basis.

Default is 0 for an unconditioned calculation, in which case points will be considered neighbor candidates regardless of the value of their marks. See Notes for a list of options for the conditional functions.

period : array_like, optional

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 comoving and assumed to be in Mpc/h, here and throughout Halotools.

num_threads : int, optional

Number of threads to use in calculation, where parallelization is performed using the python multiprocessing module. Default is 1 for a purely serial calculation, in which case a multiprocessing Pool object will never be instantiated. A string ‘max’ may be used to indicate that the pair counters should use all available cores on the machine.

approx_cell1_size : array_like, optional

Length-3 array serving as a guess for the optimal manner by how points will be apportioned into subvolumes of the simulation box. The optimum choice unavoidably depends on the specs of your machine. Default choice is to use rp_max/10 in the xy-dimensions and pi_max/10 in the z-dimension, which will return reasonable result performance for most use-cases. Performance can vary sensitively with this parameter, so it is highly recommended that you experiment with this parameter when carrying out performance-critical calculations.

approx_cell2_size : array_like, optional

Analogous to approx_cell1_size, but for sample2. See comments for approx_cell1_size for details.

Returns:

is_isolated : numpy.array

array of booleans indicating if each point in sample1 is isolated.

Notes

The conditional_cylindrical_isolation function only differs from the cylindrical_isolation function in the treatment of the input marks. In order for a point p2 in sample2 with mark \(w_{2}\) to be considered a neighbor of a point p1 in sample1 with mark \(w_{1}\), two following conditions must be met:

  1. p2 must lie within an xy-distance rp_max and a z-distance pi_max of p1, and
  2. the input conditional marking function \(f(w_{1}, w_{2})\) must return True.

There are multiple conditional functions available. In general, each requires a different number of marks per point, N_marks. All conditional functions return a boolean and get passed two arrays per pair, w1 and w2, each of length N_marks. You can pass in more than one piece of information about each point by choosing a the input marks arrays to be multi-dimensional of shape (N_points, N_marks).

The available marking functions, cond_func, and the associated integer ID numbers are:

  1. trivial (N_marks = 1)
    \[f(w_1,w_2) = True\]
  2. greater than (N_marks = 1)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] > w_2[0] \\ False & : w_1[0] \leq w_2[0] \\ \end{array} \right.\end{split}\]
  3. less than (N_marks = 1)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] < w_2[0] \\ False & : w_1[0] \geq w_2[0] \\ \end{array} \right.\end{split}\]
  4. equality (N_marks = 1)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] = w_2[0] \\ False & : w_1[0] \neq w_2[0] \\ \end{array} \right.\end{split}\]
  5. inequality (N_marks = 1)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] \neq w_2[0] \\ False & : w_1[0] = w_2[0] \\ \end{array} \right.\end{split}\]
  6. tolerance greater than (N_marks = 2)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] > (w_2[0]+w_1[1]) \\ False & : w_1[0] \leq (w_2[0]+w_1[1]) \\ \end{array} \right.\end{split}\]
  7. tolerance less than (N_marks = 2)
    \[\begin{split}f(w_1,w_2) = \left \{ \begin{array}{ll} True & : w_1[0] < (w_2[0]+w_1[1]) \\ False & : w_1[0] \geq (w_2[0]+w_1[1]) \\ \end{array} \right.\end{split}\]

Examples

In this first example, we will show how to calculate the following notion of galaxy isolation. A galaxy is isolated if there are zero other more massive galaxies within a projected distance of 750 kpc and a z-distance of 500 km/s.

First we create a random distribution of points inside the box, and also random z-velocities.

>>> Npts = 1000
>>> Lbox = 250.
>>> x = np.random.uniform(0, Lbox, Npts)
>>> y = np.random.uniform(0, Lbox, Npts)
>>> z = np.random.uniform(0, Lbox, Npts)
>>> vz = np.random.normal(loc = 0, scale = 100, size = Npts)

We place our points into redshift-space, formatting the result into the appropriately shaped array used throughout the mock_observables sub-package:

>>> from halotools.mock_observables import return_xyz_formatted_array
>>> sample1 = return_xyz_formatted_array(x, y, z, period = Lbox, velocity = vz, velocity_distortion_dimension='z')

Now we will choose random stellar masses for our galaxies:

>>> stellar_mass = np.random.uniform(1e10, 1e12, Npts)

Since we are interested in whether a point in sample1 is isolated from other points in sample1, we set sample2 to sample1 and both marks1 and marks2 equal to stellar_mass.

>>> sample2 = sample1
>>> marks1 = stellar_mass
>>> marks2 = stellar_mass

All units in Halotools assume h=1, with lengths always in Mpc/h, so we have:

>>> rp_max = 0.75

Since h=1 implies \(H_{0} = 100\) km/s/Mpc, our 500 km/s velocity criteria gets transformed into a z-dimension length criteria as:

>>> H0 = 100.0
>>> pi_max = 500./H0

Referring to the Notes above for the definitions of the conditional marking functions, we see that for this particular isolation criteria the appropriate cond_func is 2. The reason is that this function only evaluates to True for those points in sample2 that are more massive than the sample1 point under consideration. Thus the only relevant points to consider as candidate neighbors are the more massive ones; all other sample2 points will be disregarded irrespective of their distance from the sample1 point under consideration.

>>> cond_func = 2
>>> is_isolated = conditional_cylindrical_isolation(sample1, sample2, rp_max, pi_max, marks1, marks2, cond_func, period=Lbox)