Some Consequences of Using Gravitational Softening to Model N-body Galaxies
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**Session 51 -- Galaxies**
*Oral presentation, Thursday, June 15, 1995, 10:00am - 11:30am*

## [51.03] Some Consequences of Using Gravitational Softening to Model N-body Galaxies

*Richard A. Gerber (NASA Ames Research Center/NRC)*
``Softened'' gravity is commonly used to model collisionless
galactic systems. Modification of the $1/r^2$ force at short range
diminishes two-body scattering and allows for larger time steps
in computational experiments.
But the effects of using a softened potential must be well
understood when designing experiments and analyzing results of numerical
runs. Some of the most basic results of Newtonian physics are modified
as a consequence of changing the force law. For example,
a spherical shell of matter produces a force in its interior.

This paper examines some of the dynamical
effects of using softened gravity with the
goal of determining under which circumstances the desired physics is
being faithfully modeled. In particular the
calculation of global quantities -- such as force fields,
total energies and virial relationships -- are studied.
It is shown that softening, introduced as a short range
modification of gravity, can have global effects, e.g.,
the familiar form of the scalar virial theorem is no
longer valid.

A simple physical system, a spherical galaxy undergoing
global oscillations, is used to illustrate some of the
effects. The dynamical behavior of the model is
examined with different values of softening using
both Particle Mesh (PM) grid and tree N-body codes.

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