Possible galpy extensions

Introduction

The following is a brief list of possible extensions that would be useful to implement. I am not directly working on any of these and am not planning on it, but if you want to add something along these lines to galpy, I would be happy to consult. To be accepted in the main repository through pull request all extensions should at the very least add documentation to the API and tests to the test suite. Please let the maintainer of the code know if you start working on one of these.

Some of these are simple and could be done quickly as part of learning how to work with galpy. Other would require more significant changes to the structure of galpy. The larger extensions are marked by *

Potentials

• Add new potentials (see #232 or #242 for example pull requests for new potentials): e.g.,
  • A Tri-axial logarithmic potential
  • An Einasto density profile
  • A radially exponential disk with a vertical sech profile, or with a general vertical profile
  • A power-law potential with flattening in the density
  • A power-law potential with an exponential cut-off and flattening in the density (bulge model in Binney & Tremaine 2008)
  • TwoPowerSphericalPotentials with flattening in the density (halo model in Binney & Tremaine 2008) (done! in jobovy/twopowertri)
  • Double exponential disk with a central hole (ISM model in Binney & Tremaine 2008)
  • Models 1 and 2 of Binney & Tremaine (2008) [requires the three potentials above)
  • A three-dimensional spiral potential
  • A three-dimensional bar potential: a Ferrers bar
  • A triaxial Staeckel potential in a sensible way
  • Basis function expansions, w/ a way to compute them for a given potential. For example, Hernquist & Ostriker (done! in SeaifanAladdin/SCF)
  • A three-Miyamoto-Nagai disk approximation to an exponential disk from Smith et al. (2015)
• Add adiabatically-contracted NFW potentials or add adiabatic contraction more generally
• Add rotation, tumbling, and movement of center of potential more generally as a wrapper around other potentials

Orbit integration

• *Add support for dynamical friction (note: there is an old branch where this was started that could serve as a starting point; there is also a newer branch)
• Integrate R(Z)Orbits in C without integrating phi [see this issue]
• *Better orbit integration: Add interpolation methods for integrators such that the step size can be larger than the requested output time step (right now, the step size is always smaller than the requested output step size, but for sufficiently smooth potentials, the step size can sometimes be increased with intermediate points found through interpolation; see NR).
• *Implement the integration of the phase-space volume (using Orbit.integrate_dxdv) for 3D orbits. This will require writing the integration routine that uses all of the relevant second derivatives of the potential, both in python and C, and implementing the necessary second derivatives for a large number of potentials (in python and C).
• Add the IAS15 symplectic integrator (Rein & Spiegel 2015); cannot use the rebound version, as that is incompatible GPL'ed code.
• Add the Dormand-Prince 853 integrator.

Action-angle

• Add frequencies and angles for the adiabatic approximation:
  • in Python
  • in C
• Implement spherical actionAngle calculations in C in a similar way as actionAngleAdiabatic or actionAngleStaeckel.
• Implement the transformation from actions and angles to configuration space for the isochrone potential
• Implement the transformation from actions and angles to configuration space for spherical potentials
• Implement the triaxial Staeckel fudge (Sanders & Binney 2015)
• Implement frequencies from Fourier analysis of integrated orbits (see Binney & Spergel, Laskar, or Valluri & Merritt)
• Implement actionAngleIsochroneApprox for triaxial potentials; requires to do the phi fit in actionsFreqsAngles as well (in progress in jobovy/twopowertri).
• *Implement the torus machinery or interact with Paul McMillan's code (this github repository)

Distribution functions

• Implement simple spherical DFs (these will only be merged into the main repository if deemed useful; i.e., if you have used it in a paper).
• Generalize diskdf to be able to use any potential [see this issue]
• Implement a particle-spray model for tidal streams (see, e.g., Fardal et al.)
• Improvements in the DF for a tidal stream:
  • Perform actionAngle approximations as (x,v) -> (J,O,a) rather than (R,vR,...) -> (J,O,a) [see this issue]
  • Add error convolutions to callMarg [see this issue]
  • Look into including the Jacobian (l,b,...) -> (X,V) properly in lb callMarg [see this issue]
  • Use detdOdJp from nearest trackpoint when evaluating the DF, or interpolate between two nearest, as done in streamgapdf to compute the kicks due to a subhalo interaction [see this issue]

Miscellaneous

• *Add support for interacting with N-body codes like NEMO or Gadget. This includes
  • Translate galpy potentials to external potentials that can be used with these codes;
  • Use galpy to sample initial conditions (e.g., for a disk using quasiisothermaldf) and write commands/files to run these through N-body codes (with or without an external potential).