Re: Galaxy Interaction Simulations

Chris Mihos (hos@corona.UCSC.EDU)
7 Mar 1994 16:35:52 GMT

we basically agree here, but i wanted to make some qualifications...


In article <>,
Ben Weiner <> wrote:
> (Ross Chandler) writes:
>>You don't need huge numbers of particles to get realistic looking results.
> ^^^^^^^
>"looking" - that's the problem. I don't want to criticize your work,
>maybe just add a little cold water from the astronomer's perspective.
>There are many nice pictures of simulations that look like interacting
>galaxies (dating back to Toomre & Toomre 1972). But an observer can
>remain suspicious - there are enough inputs into any interaction
>simulation that you can tweak to eventually make a nice picture,

"an observer"? come on, ben, theory people are allowed to be skeptical,
too. in fact, i often find them a harder sell than the observers!

>inefficient, and the biggest problem is the sqrt(N) particle noise.
>This can make it real hard to resolve perturbations on a background
>density, such as spiral arms. N-body simulations are basically still
>not sufficiently fine to settle questions of spiral arm formation ...
>which ties into things like disk stability, disk heating etc.
Absolutely. Nbody simulations of isolated disks are very difficult --
effects such as spiral arm formation and long term evolution are
not feasible as of yet.

>Then one takes two "disk galaxies" (neither of which quite behaves like
>a real disk galaxy) and whacks them together - particles everywhere;
>the problem isn't the simulations, it's interpreting them.
>I guess my opinion boils down to this: we don't fully understand the
>dynamics of isolated galaxies yet, interacting galaxies are yet
>another can of worms. Worth studying but not yet clear what we're

Well, remember that the perturbation due to the interaction is usually
much stronger than that due to internal disk processes like spiral arms.
The interaction timescale is short (~ 1 Gyr), so root N noise wont
degrade the simulation too badly (given LARGE N) over the timescale
involved. So I think in terms of the overall dynamical evolution of
an interaction, we have a decent feeling for what is going on. In terms
of the finely sampled distribution function of the stellar orbits,
agreed -- things are a little dicier. But inroads are being made.

Two caveats:
1) The biggest problem in this game is that we dont know what the
mass distribution of galaxies looks like at large radius, ie out
past a few disk scale lengths. This has a big effect on the interaction
dynamics, and worries me MUCH more than disk stability issues.

2) Because of this, uniqueness of "orbital solutions" for interacting
systems is in question (including my own work in this arena, so I'm
not merely casting dispersion on others' work here). I do think
individual solutions can tell us something about the interaction
geometry and, crudely speaking, the time scale of the interaction,
but as far as a unique solution to the orbit, doubtful. But dont
throw the baby out with the bathwater -- there is good information
in the simulations, but, as you say, caution in the interpretation
is warranted...

After this long discussion, let me make the statement that all of
this refers to purely Nbody dynamics; adding gas dynamics and
star formation makes things MUCH more complicated. But that's
enough for today...