What’s new?

This page gives some of the key improvements in each galpy version. See the HISTORY.txt file in the galpy source for full details on what is new and different in each version.


  • Added dynamical friction as the ChandrasekharDynamicalFrictionForce class, an implementation of dynamical friction based on the classical Chandrasekhar formula (with recent tweaks from the literature to better represent the results from N-body simulations).
  • A general EllipsoidalPotential superclass for implementing potentials with densities that are constant on ellipsoids (functions of \(m^2 = x^2 + y^2/b^2 + z^2/c^2\)). Also implemented in C. Implementing new types of ellipsoidal potentials now only requires three simple functions to be defined: the density as a function of m, its derivative with respect to m, and its integral with respect to m^2. Makes implementing any ellipsoidal potential a breeze. See examples in the new-potentials section below.
  • New or improved potentials and potential wrappers:
  • New Potential methods and functions:
    • Use nested lists of Potential instances wherever lists of Potential instances can be used. Allows easy adding of components (e.g., a bar) to previously defined potentials (which may be lists themselves): new_pot= [pot,bar_pot].
    • rtide and ttensor: compute the tidal radius of an object and the full tidal tensor.
    • surfdens method and evaluateSurfaceDensities function to evaluate the surface density up to a given z.
    • r2deriv and evaluater2derivs: 2nd derivative wrt spherical radius.
    • evaluatephi2derivs: second derivative wrt phi.
    • evaluateRphiderivs: mixed (R,phi) derivative.
  • New or improved galpy.orbit.Orbit functionality and methods:
    • Orbit.from_name to initialize an Orbit instance from an object’s name. E.g., orb= Orbit.from_name('LMC').
    • Orbit initialization without arguments is now the orbit of the Sun.
    • Orbits can be initialized with a SkyCoord.
    • Default solarmotion= parameter is now ‘schoenrich’ for the Solar motion of Schoenrich et al. (2010).
    • rguiding: Guiding-center radius.
    • Lz: vertical component of the angular momentum.
    • If astropy version > 3, Orbit.SkyCoord method returns a SkyCoord object that includes the velocity information and the Galactocentric frame used by the Orbit instance.
  • galpy.df.jeans module with tools for Jeans modeling. Currently only contains the functions sigmar and sigmalos to calculate the velocity dispersion in the radial or line-of-sight direction using the spherical Jeans equation in a given potential, density profile, and anisotropy profile (anisotropy can be radially varying).
  • Support for compilation on Windows with MSVC.


  • A fast and precise method for approximating an orbit’s eccentricity, peri- and apocenter radii, and maximum height above the midplane using the Staeckel approximation (see Mackereth & Bovy 2018). Can determine these parameters to better than a few percent accuracy in as little as 10 \(\mu\mathrm{s}\) per object, more than 1,000 times faster than through direct orbit integration. See this section of the documentation for more info.
  • A general method for modifying Potential classes through potential wrappers—simple classes that wrap existing potentials to modify their behavior. See this section of the documentation for examples and this section for information on how to easily define new wrappers. Example wrappers include SolidBodyRotationWrapperPotential to allow any potential to rotate as a solid body and DehnenSmoothWrapperPotential to smoothly grow any potential. See this section of the galpy.potential API page for an up-to-date list of wrappers.
  • New or improved potentials:
  • New or improved galpy.orbit.Orbit methods:
    • Method to display an animation of an integrated orbit in jupyter notebooks: Orbit.animate. See this section of the documentation.
    • Improved default method for fast calculation of eccentricity, zmax, rperi, rap, actions, frequencies, and angles by switching to the Staeckel approximation with automatically-estimated approximation parameters.
    • Improved plotting functions: plotting of spherical radius and of arbitrary user-supplied functions of time in Orbit.plot, Orbit.plot3d, and Orbit.animate.
  • actionAngleStaeckel upgrades:
    • actionAngleStaeckel methods now allow for different focal lengths delta for different phase-space points and for the order of the Gauss-Legendre integration to be specified (default: 10, which is good enough when using actionAngleStaeckel to compute approximate actions etc. for an axisymmetric potential).
    • Added an option to the estimateDeltaStaeckel function to facilitate the return of an estimated delta parameter at every phase space point passed, rather than returning a median of the estimate at each point.
  • galpy.df.schwarzschilddf:the simple Schwarzschild distribution function for a razor-thin disk (useful for teaching).


  • Full support for providing inputs to all initializations, methods, and functions as astropy Quantity with units and for providing outputs as astropy Quantities.
  • galpy.potential.TwoPowerTriaxialPotential, a set of triaxial potentials with iso-density contours that are arbitrary, similar, coaxial ellipsoids whose ‘radial’ density is a (different) power-law at small and large radii: 1/m^alpha/(1+m)^beta-alpha (the triaxial generalization of TwoPowerSphericalPotential, with flattening in the density rather than in the potential; includes triaxial Hernquist and NFW potentials.
  • galpy.potential.SCFPotential, a class that implements general density/potential pairs through the basis expansion approach to solving the Poisson equation of Hernquist & Ostriker (1992). Also implemented functions to compute the coefficients for a given density function. See more explanation here.
  • galpy.actionAngle.actionAngleTorus: an experimental interface to Binney & McMillan’s TorusMapper code for computing positions and velocities for given actions and angles. See the installation instructions for how to properly install this. See this section and the galpy.actionAngle API page for documentation.
  • galpy.actionAngle.actionAngleIsochroneApprox (Bovy 2014) now implemented for the general case of a time-independent potential.
  • galpy.df.streamgapdf, a module for modeling the effect of a dark-matter subhalo on a tidal stream. See Sanders et al. (2016). Also includes the fast methods for computing the density along the stream and the stream track for a perturbed stream from Bovy et al. (2016).
  • Orbit.flip can now flip the velocities of an orbit in-place by specifying inplace=True. This allows correct velocities to be easily obtained for backwards-integrated orbits.
  • galpy.potential.PseudoIsothermalPotential, a standard pseudo-isothermal-sphere potential. galpy.potential.KuzminDiskPotential, a razor-thin disk potential.
  • Internal transformations between equatorial and Galactic coordinates are now performed by default using astropy’s coordinates module. Transformation of (ra,dec) to Galactic coordinates for general epochs.


  • Full support for Python 3.
  • galpy.potential.SnapshotRZPotential, a potential class that can be used to get a frozen snapshot of the potential of an N-body simulation.
  • Various other potentials: PlummerPotential, a standard Plummer potential; MN3ExponentialDiskPotential, an approximation to an exponential disk using three Miyamoto-Nagai potentials (Smith et al. 2015); KuzminKutuzovStaeckelPotential, a Staeckel potential that can be used to approximate the potential of a disk galaxy (Batsleer & Dejonghe 1994).
  • Support for converting potential parameters to NEMO format and units.
  • Orbit fitting in custom sky coordinates.