The purpose of the NBodyLab simulation testbed is to examine the evolution of different types of galaxies, using underlying software and hardware components that can be improved upon by undergraduate researchers.
In the testbed, a galaxy is modeled as a system of particles under mutual gravitational attraction. From Newton's law of gravitation, each particle of mass M attracts another particle of mass m with a F= -(GMm/r^3)*r. The most direct approach is to compute the force exerted on each of the n particles by all of the other n-1 particles at each time step. By Newton's second law of motion, F=ma, the force calculation has yielded the acceleration for each particle. Two first-order differential equations involving acceleration and velocity must be solved, using numerical integration, to estimate new positions and velocities of each particle for the next timestep. N-body models and their applications have been studied from the time of Newton to the present day. The computational power becoming available to produce large-n simulations and compare them to deep-space observations will greatly advance our understanding of galaxy formation.
There are four major components of the simulation testbed:
Several excellent n-body simulation packages are in use by the astrophysics community. We have chosen to work primarily with NEMO, a Unix-based stellar dynamics toolbox maintained by the University of Maryland. The use of NEMO has been enhanced by using a host computer equipped with an experimental "Gravity Pipeline-Molecular Dynamics" (MD-GRAPE) PC card. This special-purpose accelerator card was designed to rapidly calculate all the forces between all of the particles for each time step in the numerical integration. The testbed also provides a server-side encapsulation of NEMO so that a constrained subset can be run over the web.
Most of the galaxy models presented in the testbed are NEMO models, documented at: plummer, baredisk, expdisk, disk, homsph, polytrope, and spiral. The "premade disk-bulge-halo model" option was produced by the package GalatICS, which generates N-body realizations of axisymmetric galaxy models consisting of a disk, bulge and halo. The "premade Sellwood galactic bulge model" and "premade Dubinski Milky Way/Andromeda" options are described here.
Other initial galaxy settings use NEMO utilities to rotate, scale, add velocity, and add spin.
The numerical integration options include a well-proven, highly-refined integrator from NEMO, hackcode1 (which does not call the MD-GRAPE card) and an experimental, simple integrator called "Pomona". The Pomona integrator can run with or without the MD-GRAPE2 card.
NEMO's hackcode1 outperforms the Pomona College integrator with the MD-GRAPE card in many simulations, particularly with small numbers of particles, because hackcode1's tree algorithm is much more sophisticated than the Pomona integrator's direct approach. If you read about n-body methods you'll see are many ways the Pomona integrator (a particle-particle code using the Euler 1st order, Taylor 2nd order, or Runge Kutta 4th order methods) can be improved as a research project.
Other parameters for integration include the number of iterations, delta t (all time steps are equal to dt), and a small "softening factor" used in the denominator of the force calculation to prevent numerical overflows during close encounters.
Visualization of the time evolution is provided by a mosaic of xy-plane snapshots and an animated GIF. An openGL application for Windows 2000 is available here.
If you would like more information about NBodyLab, the MDGRAPE-2 or related consulting services please email Interconnect Technologies. NBodyLab is open-source software.