Re: Galaxy Interaction Simulations

Ben Weiner (
11 Mar 94 05:11:20 GMT

hos@corona.UCSC.EDU (Chris Mihos) writes:

[much deleted]
[some junk I said about spiral arms, disk stability etc]

>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.


I agree that the mass distribution at large radius is fantastically
more important than any issues of disk stability - the disk stability
issue was just something I brought up to illustrate the present state
of comparative ignorance about what galaxies are actually like even
in the "quiescent" (hah!) state - it should have absolutely no effect
on what happens during an interaction. (Although, the question of
how much accreting mass it takes to disrupt a stellar+gaseous disk
is very interesting. But that's not the kind of interaction the
thread was originally about I don't think.)

>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...

yes definitely, I mean it is a very good thing to know that N-body
mergers make something that looks like stuff we see on the sky,
and it's good to try and figure out what merger remnants look like,
especially (IMHO) if we can figure out what old merger remnants look
like. Although many of the questions like triggering star formation
are beyond the ability of present simulations (in any but the vaguest
sense). Comparing models to obervations is always tough because
you have a convolution associated with projecting the velocities
and densities onto the sky and it all can get degenerate ...

>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...

Tell me about it, I have a 3000 line fluid dynamics code in my lap -
and it's only 2-D. Heh heh