Galaxy spiral movement bug

[Sirius18933]

when i running the galaxy collision of milky way and andromeda, something strange happened with the galaxy spiral movement.
the galaxy spiral shape broken. i restarting my computer and it's not affect it.

[unl0cker]

You are stepping the time up too much.

[Sirius18933]

no. i running the system with 50000 year per step. just like actually the system andromeda and milky way collision simulation time speed.

this is how i illustrating what happen :
                                                                                   /                                            \
                                                                                   \-o-\                                      /-o-/
Before, the galaxy shape of andromeda is like this          / then turns to like this         \

That's how i illustrating what happen with the spiral arm.

[Darvince]

That's due to the Milky Way's gravity shifting the stars.

[unl0cker]

weird... this does not happen in real sims of this particular encounter.

Following the logic it                            \
should be like this then:  (mw) <---->  /-o-/
                                                              _ /


----


can you post the file?

[Sirius18933]

Here's the file. the spiral looks stranger. 

[unl0cker]

I've never played with these galaxies in U.S.

But you are right, the sim requires a bit of tweaking to work.

What you think of this one? I think the approach is better, but after the "collision" mine
has unwanted results. It does not aggregate back again.


* Don't forget to set RK4 calcs. It's always more precise.

[unl0cker]

And here is the mother's milk alone...

I had to dirty fix it to make the arm spirals last longer, as I suppose they will do, I say suppose because I really don't know as I've never studied galactic evolution.

But by 1B yrs they are gone, and what we get is a flat disk.

The problem probably lays in the fact that dust has no gravity in the sim, so if you make the milky way body's gravity enough to have some effect on the outer arms will cause the center to be "demolished", and setting a smaller gravity for it, as I did, will have almost no impact on the outer arms, rendering the whole thing flat after some spins, because the outer arms will not spin as fast as the center.

The really is different I think because each blob of stars have some pull on it's neighbors. So the center black hole has no pull on the outer arm's stats, but it sets a gravity chain reaction that in the end moves the outer stars as well thought proxy. Well, this is my wild guess. I never studied none of this. On a second reading, they have no clue too... thus, dark matter dirty fix.