Re: Barnes-Hut treecode (was Re: Galaxy Interaction Simulations)

Steinn Sigurdsson (
8 Mar 94 14:05:55

In article <> (Wayne Hayes) writes:

amenable to softening, and thus may be a much better than the Barnes
and Hut method for an ``ideal'' N-body simulation where your N is close
to the actual N of the system. (I think the only systems we're even
close to such a simulation is, just maybe, simulating a single small
globular cluster.) I've tried to contact the authors via e-mail but

Just a comment here...

Systems for which "N" approaches the actual number of particles
do exist. Trivially, N=3-10 are instances, (actually 1-10 is a better
range :-) - certainly the solar system people use the actual "N"
by and large.

As for the large "N" systems, well, open clusters are within range
of the current generation of hardware and software, at least to the
single particle level - people are not really modeling them much, as
such, but rather as a waypoint towards modeling globular clusters.

To model globular clusters with unsoftened particles (ie including
2bod relaxation) is not possible, it probably will be in a year or two,
the HARP project is probably the closest. For those interested in
collisionless dynamics, which still allows modeling of some
interesting aspects of globulars, I have a working code that
has done a full realisation of globular clusters, anything from
Pal 1 to Omega Cen for, say, 1000 dynamical times - but it doesn't
include two body relaxation and has limited application.

We're in a race with the cosmology code people for the largest N,
our (non-interacting) galaxy simulations have hit N=10^7 and we'll
get to N=10^8 in a matter of months. That's already enough for
dwarf galaxies and we'll soon be tracing the luminous component of
full size ellipticals particle-for-particle. However, all these
approaches trade something off, and there really is no one code that
is universal - it's something people have thought about a lot, but
is not within current technology, yet. Hence the interest in hybrid
codes and general N-body solvers.

* Steinn Sigurdsson Lick Observatory *
* "standard disclaimer" *
* The laws of gravity are very,very strict *
* And you're just bending them for your own benefit - B.B. 1988*