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.


Version 1.7 adds many new features, mainly in the galpy.potential and galpy.df modules. The biggest new additions are:

  • A general framework for spherical distribution functions defined using \(f(E,L)\) models. Specifically, general solutions for (a) isotropic \(f(E)\) models, (b) \(f(E,L)\) models with constant anisotropy \(\beta\), and (c) \(f(E,L)\) models with Osipkov-Merritt-type anisotropy are implemented for any potential/density pair (not necessarily self-consistent). These distribution functions can be evaluated, sampled exactly, and any moment of the distribution function can be calculated. Documentation of this is currently available at NEW in v1.7 Spherical distribution functions. Distribution functions with constant anisotropy require the JAX.
  • In addition to the general solution, the distribution function of a few well-known distribution functions was added, including (a) Hernquist distribution functions that are isotropic, have constant anisotropy, or have Osipkov-Merritt type anisotropy; (b) an isotropic Plummer profile; (c) the isotropic NFW profile (either using the approx. from Widrow 2000 or using an improved approximation) and the Osipkov-Merritt NFW profile (new approximate form); (d) the King model (also added as a potential as KingPotential).

Other new additions include:

  • New or improved potentials and potential wrappers:
  • Other changes to Potential classes, methods, and functions:
    • Functions to compute the SCF density/potential expansion coefficients based on an N-body representation of the density (scf_compute_coeffs_spherical_nbody, scf_compute_coeffs_axi_nbody, and scf_compute_coeffs_nbody).
    • An NFWPotential can now be initialized using rmax/vmax, the radius and value of the maximum circular velocity.
    • Potential functions and methods to compute the zero-velocity curve: zvc and zvc_range. The latter computes the range in R over which the zero-velocity curve is defined, the former gives the positive z position on the zero-velocity curve for a given radius in this range.
    • rhalf Potential function/method for computing the half-mass radius.
    • tdyn Potential function/method for computing the dynamical time using the average density.
    • Potential.mass now always returns the mass within a spherical shell if only one argument is given. Implemented faster versions of many mass implementations using Gauss’ theorem (including SCFPotential and DiskSCFPotential).
    • Mixed azimuthal,vertical 2nd derivatives for all non-axisymmetric potentials in function evaluatephizderivs and method phizderiv. Now all second derivatives in cylindrical coordinates are implemented.
    • Function/method plotSurfaceDensities/plotSurfaceDensity for plotting, you’ll never guess, the surface density of a potential.
    • Re-implementation of DoubleExponentialDiskPotential using the double-exponential formula for integrating Bessel functions, resulting in a simpler, more accurate, and more stable implementation. This potential is now accurate to ~machine precision.
    • Potentials are now as much as possible numerically stable at r=0 and r=inf, meaning that they can be evaluated there.

Other additions and changes include:

  • Added the inverse action-angle transformations for the isochrone potential (in actionAngleIsochroneInverse) and for the one-dimensional harmonic oscillator (in actionAngleHarmonicInverse). Also added the action-angle calculation for the harmonic oscilator in actionAngleHarmonic. Why yes, I have been playing around with the TorusMapper a bit!
  • Renamed galpy.util.bovy_coords to galpy.util.coords, galpy.util.bovy_conversion to galpy.util.conversion, and galpy.util.bovy_plot to galpy.util.plot (but old from galpy.util import bovy_X will keep working for now). Also renamed some other internal utility modules in the same way (bovy_symplecticode, bovy_quadpack, and bovy_ars; these are not kept backwards-compatible). Trying to make the code a bit less egotistical!
  • Support for Python 3.9.


This version mainly consists of changes to the internal functioning of galpy; some of the new outward-facing features are:

  • ChandrasekharDynamicalFrictionForce is now implemented in C, leading to 100x to 1000x speed-ups for orbit integrations using dynamical friction compared to the prior pure-Python version.
  • New potentials:
  • Some notable internal changes:
    • Fixed a bug in how DiskSCFPotential instances are passed to C for orbit integration that in particular affected the McMillan17 Milky-Way potential (any hole in the surface density was effectively ignored in the C code in v1.5).
    • The performance of orbit animations is significantly improved.
    • All main galpy C extensions are now compiled into a single shared-object library libgalpy.
    • Binary wheels are now automatically built for Windows, Mac, and most major Linux distributions upon every push to the master branch and these are automatically uploaded to PyPI upon release. See the Installation Instructions for more info. Binary wheels on Windows are also built for every push on AppVeyor, see the Windows installation instructions.


This version will be the last to support Python 2.7 as this version of Python is reaching end-of-life on January 1 2020.

  • This version’s highlight is a fully re-written implementation of galpy.orbit.Orbit such that it can now contain and manipulate multiple objects at once. galpy.orbit.Orbit can be initialized with an arbitrary shape of input objects in a variety of ways, manipulated in a manner similar to Numpy arrays, and all Orbit methods work efficiently on Orbit instances containing multiple objects. Some methods, such as orbit integration and those for fast orbital characterization are parallelized on multi-core machines. Orbit instances can contain and manipulate millions of objects simultaneously now.
  • Added the galpy.potentials.mwpotentials module with various Milky-Way-like potentials. Currently included are MWPotential2014, McMillan17 for the potential from McMillan (2017), models 1 through 4 from Dehnen & Binney (1998), and the three models from Irrgang et al. (2013). See this section of the API documentation for details.
  • Added a (JSON) list with the phase-space coordinates of known objects (mainly Milky Way globular clusters and dwarf galaxies) for easy Orbit.from_name initialization. For ease of use, Orbit.from_name also supports tab completion for known objects in this list in IPython/Jupyter.
  • Added galpy.potential.to_amuse to create an AMUSE representation of any galpy potential, allowing galpy potentials to be used as external gravitational fields in AMUSE N-body simulations.
  • New or improved potentials and potential wrappers:
    • MovingObjectPotential: Re-wrote potential.MovingObjectPotential to allow general mass distributions for the moving object, implemented now as standard galpy potentials. Also added a C implementation of this potential for fast orbit integration.
    • IsothermalDiskPotential: The one-dimensional potential of an isothermal self-gravitating disk (sech^2 profile).
    • NumericalPotentialDerivativesMixin: a Mixin class to add numerically-computed forces and second derivatives to any Potential class, allowing new potentials to be implmented quickly by only implementing the potential itself and obtaining all forces and second derivatives numerically.
    • DehnenSmoothWrapperPotential: Can now decay rather than grow a potential by setting decay=True.
    • Added support to combine Potential instances or lists thereof through the addition operator. E.g., pot= pot1+pot2+pot3 to create the combined potential of the three component potentials (pot1,pot2,pot3). Each of these components can be a combined potential itself. As before, combined potentials are simply lists of potentials, so this is simply an alternative (and perhaps more intuitive) way to create these lists.
    • Added support to adjust the amplitude of a Potential instance through multiplication of the instance by a number or through division by a numer. E.g., pot= 2.*pot1 returns a Potential instance that is the same as pot1, except that the amplitude is twice larger. Similarly, pot= pot1/2. decreases the amplitude by a factor of two. This is useful, for example, to quickly change the mass of a potential. Only works for Potential instances, not for lists of Potential instances.
  • New or improved galpy.orbit.Orbit functionality and methods:
    • Added support for 1D orbit integration in C.
    • Added support to plot arbitrary combinations of the basic Orbit attributes by giving them as an expresion (e.g., orb.plot(d2='vR*R/r+vz*z/r')); requires the numexpr package.
    • Switched default Sun’s vertical height zo parameter for Orbit initialization to be the value of 20.8 pc from Bennett & Bovy (2019).
    • Add Python and C implementation of Dormand-Prince 8(5,3) integrator.


  • 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.