conditional_spherical_isolation

halotools.mock_observables.conditional_spherical_isolation(sample1, sample2, r_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 spherical 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_spherical_isolation can be used to identify galaxies as isolated if no other galaxy with a greater stellar mass lies within 500 kpc. 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.

r_max : array_like

radius of spheres to search for neighbors around galaxies in sample1. If a single float is given, r_max is assumed to be the same for each galaxy in sample1. You may optionally pass in an array of length Npts1, in which case each point in sample1 will have its own individual neighbor-search radius.

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 r_max/10 in each 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_spherical_isolation function only differs from the spherical_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 a distance r_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 5 Mpc.

First we create a random distribution of points inside the box:

>>> 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)

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:

>>> sample1 = np.vstack((x,y,z)).T

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

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.

>>> r_max = 5.0
>>> cond_func = 2
>>> is_isolated = conditional_spherical_isolation(sample1, sample2, r_max, marks1, marks2, cond_func, period=Lbox)