Quick start

The example below builds an NFW halo of concentration 10 with a constant velocity anisotropy of 0.25. Method I (Truncation in phase space) truncates the halo in phase space so that the truncated mass equals the virial mass of the untruncated halo. The model is then given in physical units, with a virial mass of \(10^{12}\,M_\odot\), about the mass of the Milky Way, and a scale radius of 20 kpc.

import pianistpy as pp
import astropy.units as u

# an untruncated NFW density profile with a constant velocity anisotropy of 0.25
halo = pp.Component(pp.DoublePowerLaw.nfw(), pp.Anisotropy(beta_c=0.25))

# the untruncated distribution function
df = pp.compute_df(halo)

# Method I: truncate the distribution function so that the truncated mass equals the virial mass of the
# untruncated NFW halo of concentration 10 (the mass inside 10 scale radii)
model = pp.truncate(df, pp.Truncation(mass_t=1.0, concentration=10))

# physical units: a halo of 1e12 solar masses with R_vir = c * r_0 = 200 kpc
model = model.to("physical", mass_0=1e12 * u.Msun, r_0=20 * u.kpc)

# draw ten million equal-mass particles centered at the origin and write them to a file
particles = pp.Sampler(model).sample(10_000_000, seed=1)
particles.write("NFW_Const0p25_Mt1e12_PARIC.h5")

# the line-of-sight velocity dispersion at three projected radii
sigma = pp.sigma_los(model, [2, 20, 100] * u.kpc)

pp.solve(halo, pp.Truncation(mass_t=1.0, concentration=10)) runs compute_df and truncate in one call.

The Examples page shows self-contained Jupyter tutorial notebooks, and the notebooks that reproduce the result figures of Chiang+(2026d).