galpy: A python LIBRARY FOR GALACTIC DYNAMICS

Abstract

I describe the design, implementation, and usage of galpy, a python package for galactic-dynamics calculations. At its core, galpy consists of a general framework for representing galactic potentials both in python and in C (for accelerated computations); galpy functions, objects, and methods can generally take arbitrary combinations of these as arguments. Numerical orbit integration is supported with a variety of Runge-Kutta-type and symplectic integrators. For planar orbits, integration of the phase-space volume is also possible. galpy supports the calculation of action-angle coordinates and orbital frequencies for a given phase-space point for general spherical potentials, using state-of-the-art numerical approximations for axisymmetric potentials, and making use of a recent general approximation for any static potential. A number of different distribution functions (DFs) are also included in the current release; currently these consist of two-dimensional axisymmetric and non-axisymmetric disk DFs, a three-dimensional disk DF, and a DF framework for tidal streams. I provide several examples to illustrate the use of the code. I present a simple model for the Milky Way's gravitational potential consistent with the latest observations. I also numerically calculate the Oort functions for different tracer populations of stars and compare it to a new analytical approximation. Additionally, I characterize the response of a kinematically-warm disk to an elliptical m = 2 perturbation in detail. Overall, galpy consists of about 54,000 lines, including 23,000 lines of code in the module, 11,000 lines of test code, and about 20,000 lines of documentation. The test suite covers 99.6 % of the code. galpy is available at

The Phoenix Stream: A Cold Stream in the Southern Hemisphere

“…EriPhe and NGC 1261 share a similar heliocentric distance as the Phoenix stream. Using GALPY (Bovy 2015) and literature proper motions for NGC 1261 (Dambis 2006), we integrate the cluster orbit and find that it roughly aligns with the Phoenix stream and that its motion is retrograde with respect to the solar motion. The close proximity of NGC 1261 and the orbit alignment with the stream may suggest that they could share a common origin with the EriPhe OD.…”

Galactic rotation in Gaia DR1

“…The different color bins display excellent agreement with each other, except for the very bluest stars, which are likely affect by residual streaming motions from birth. That A and B do not strongly depend on color (and, thus, on velocity dispersion) is expected in the limit of well-mixed, solar-neighborhood populations in an axisymmetric Galaxy, for which A and B should be constant to within 1 km s −1 kpc −1 (Bovy 2015). I combine the four bins blueward of B − V = 1 (except for the bluest bin) which contain the majority of the stars in the sample (304,267 out of 315,946) to obtain a single value of each Oort constant.…”

Spiral- and bar-driven peculiar velocities in Milky Way-sized galaxy simulations

“…This bar acts as a perturbation to a logarithmic axisymmetric potential with a circular velocity of 220 km s −1 . The bar's effect on the kinematics of a population of stars with a radial velocity dispersion of 31.4 km s −1 is computed using galpy 1 (Bovy 2015).…”