Profiles in Pynbody

Radial Density Profile

Making profiles of all kinds of quantities is easy – here’s a simple example that shows how to plot a density profile:

import pynbody
import matplotlib.pylab as plt

# load the snapshot and set to physical units
s = pynbody.load('testdata/')

# load the halos
h = s.halos()

# center on the largest halo and align the disk

# convert all units to something reasonable (kpc, Msol etc)

# create a profile object for the stars (by default this is a 2D profile)
p = pynbody.analysis.profile.Profile(h[1].s,min=.01,max=50)

# make the figure and sub plots
f, axs = plt.subplots(1,2,figsize=(14,6))

# make the plot
axs[0].plot(p['rbins'],p['density'], 'k')
axs[0].set_xlabel('R [kpc]')
axs[0].set_ylabel(r'$\Sigma_{\star}$ [M$_{\odot}$ kpc$^{-2}$]')

# make a 3D density plot of the dark matter (note ndim=3 in the constructor below)
p = pynbody.analysis.profile.Profile(h[1].d,min=.01,max=50,ndim=3)

axs[1].plot(p['rbins'],p['density'], 'k')
axs[1].set_xlabel('R [kpc]')
axs[1].set_ylabel(r'$\rho_{DM}$ [M$_{\odot}$ kpc$^{-3}$]')

(Source code, png, hires.png, pdf)


Below is a more extended description of the profile module.

The Profile Class

The Profile class is meant to be a general-purpose class to satisfy (almost) all simulation profiling needs. Profiles are the most elementary ways to begin analyzing a simulation, so the Profile class is designed to be an extension of the syntax implemented in the SimSnap class and its derivatives.

Creating a Profile instance simply defines the bins etc. Importantly, it also stores lists of particle indices corresponding to each bin, so you can easily identify where the particles belong.

Let’s quickly load a snapshot:

In [1]: import pynbody; from pynbody.analysis import profile; import matplotlib.pylab as plt

In [2]: s = pynbody.load('testdata/'); s.physical_units()

In [3]: h = s.halos()

In [4]: pynbody.analysis.angmom.faceon(h[1])
Out[4]: <pynbody.transformation.GenericRotation at 0x113e0d810>

We’re centered on the main halo (as in the cookbook example above) and we make the Profile instance:

In [5]: p = profile.Profile(h[1].s,min='.01 kpc', max='50 kpc')

With the default parameters, the profile is made in the xy-plane. To make a spherically-symmetric 3D profile, specify ndim=3 when creating the profile.

In [6]: pdm_sph = profile.Profile(s.d,min='.01 kpc', max = '250 kpc')

Even though we use s.d here (i.e. the full snapshot, not just halo 1), the whole snapshot is still centered on halo 1.


You can pass unit strings to min and max and the conversion will be done automatically into whatever the current units of the snapshot are so you don’t have to explicitly do any unit conversions.

Automatically-generated profiles

Many profiling functions are already implemented – see the Profile documentation for a full list with brief descriptions. You can also check the available profiles in your session using derivable_keys() just like you would for a SimSnap:

In [7]: p.derivable_keys()

Additionally, any existing array can be ‘profiled’. For example, if the metallicity [Fe/H] is a derived field stored under ‘feh’, then plotting a metallicity profile is as simple as:

In [8]: plt.plot(p['rbins'].in_units('kpc'),p['feh'],'k')
Out[8]: [<matplotlib.lines.Line2D at 0x113e35b90>]

In [9]: plt.xlabel('$R$ [kpc]'); plt.ylabel('[Fe/H]')
Out[9]: <matplotlib.text.Text at 0x109147510>

If the array doesn’t exist but is deriveable (check with s.derivable_keys()), it is automatically calculated.

In addition, you can define your own profiling functions in your code by using the Profile.profile_property decorator:

def random(self):
   import numpy as np
   return np.random.rand(self.nbins)

Now this will be automatically derivable for any newly-created profile as 'random'.

Calculating Derivatives and Dispersions

You can calculate derivatives of profiles automatically. For instance, you might be interested in d phi / dr if you’re looking at a disk. This is as easy as attaching a d_ to the profile name. For example:

In [10]: p_all = profile.Profile(s,min='.01 kpc', max='250 kpc')

In [11]: p_all['pot'][0:10] # returns the potential profile
SimArray([-1883725.20605628, -1774955.4230129 , -1722218.33725498,
       -1690633.68613091, -1668854.69876004, -1652878.17689281,
       -1640433.23593114, -1630243.68972168, -1621605.13215674,
       -1614220.30221359], 'km**2 s**-2')

In [12]: p_all['d_pot'][0:10] # returns d phi / dr from p["phi"]
SimArray([ 54716.6413401 ,  32302.66586689,  16865.02197728,  10673.15462517,
         7551.40390378,   5684.51994658,   4527.07851737,   3765.77138574,
         3204.80569384,   2767.54060378], 'km**2 kpc**-1 s**-2')

Similarly straightforward is the calculation of dispersions and root-mean-square values. You simply need to attach a _disp or _rms as a suffix to the profile name. To get the stellar velocity dispersion:

In [13]: plt.plot(p['rbins'].in_units('kpc'),p['vr_disp'].in_units('km s^-1'),'k',hold=False)
Out[13]: [<matplotlib.lines.Line2D at 0x114b9d690>]

In [14]: plt.xlabel('$R$ [kpc]'); plt.ylabel('$\sigma_{r}$')
Out[14]: <matplotlib.text.Text at 0x109147510>

In addition to doing this by hand, you can make a QuantileProfile that can return any desired quantile range. By default, this is the mean +/- 1-sigma:

In [15]: p_quant = profile.QuantileProfile(h[1].s, min = '0.1 kpc', max = '50 kpc')

In [16]: plt.plot(p_quant['rbins'], p_quant['feh'][:,1], 'k', hold=False)
Out[16]: [<matplotlib.lines.Line2D at 0x116db5090>]

In [17]: plt.fill_between(p_quant['rbins'], p_quant['feh'][:,0], p_quant['feh'][:,2], color = 'Grey', alpha=0.5)
Out[17]: <matplotlib.collections.PolyCollection at 0x114b9d4d0>

In [18]: plt.xlabel('$R$ [kpc]'); plt.ylabel('[Fe/H]')
Out[18]: <matplotlib.text.Text at 0x109147510>

Making a profile using a different quantity

Radial profiles are nice, but sometimes we want a “profile” using a different quantity. We might want to know, for example, how the mean metallicity varies as a function of age in the stars. Profile calls the function _calculate_x() by default and this simply returns the 3D or xy-plane radial distance, depending on the value of ndim. We can specify a different function using the calc_x keyword. Often these are simple so a lambda function can be used (e.g. if we just want to return an array) or can also be more complicated functions. For example, to make the profile of stars in halo 1 according to their age:

In [19]: s.s['age'].convert_units('Gyr')

In [20]: p_age = profile.Profile(h[1].s, calc_x = lambda x: x.s['age'], max = '10 Gyr')

In [21]: plt.plot(p_age['rbins'], p_age['feh'], 'k', label = 'mean [Fe/H]',hold=False)
Out[21]: [<matplotlib.lines.Line2D at 0x113e15410>]

In [22]: plt.plot(p_age['rbins'], p_age['feh_disp'], 'k--', label = 'dispersion')
Out[22]: [<matplotlib.lines.Line2D at 0x1196afe50>]

In [23]: plt.xlabel('Age [Gyr]'); plt.ylabel('[Fe/H]')
Out[23]: <matplotlib.text.Text at 0x109147510>

In [24]: plt.legend()
Out[24]: <matplotlib.legend.Legend at 0x10a09fc90>

Vertical Profiles and Inclined Profiles

For analyzing disk structure, it is frequently useful to have a profile in the z-direction. This is done with the VerticalProfile which behaves in the same way as the Profile. Unlike in the basic class, you must specify the radial range and maximum z to be used:

In [25]: p_vert = profile.VerticalProfile(h[1].s, '3 kpc', '5 kpc', '5 kpc')

In [26]: plt.plot(p_vert['rbins'].in_units('pc'), p_vert['density'].in_units('Msol pc^-3'),'k', hold=False)
Out[26]: [<matplotlib.lines.Line2D at 0x10bb97b50>]

In [27]: plt.xlabel('$z$ [pc]'); plt.ylabel(r'$\rho_{\star}$ [M$_{\odot}$ pc$^{-3}$]')
Out[27]: <matplotlib.text.Text at 0x109147510>

Similarly, one can make inclined profiles using the VerticalProfile, but the snapshot needs to be rotated first:

In [28]: s.rotate_x(60) # rotate the snapshot by 60-degrees
Out[28]: <pynbody.transformation.GenericRotation at 0x116c9e1d0>

In [29]: p_inc = profile.InclinedProfile(h[1].s, 60, min = '0.1 kpc', max = '50 kpc')