No, this post is not about gravitational waves. Keep scrolling.

Same galaxy disc simulation as before but now with the dark matter shown in pink because Andres Soolo thinks that would be appropriate for some reason. Also, the gas temperature is more realistic now. Since it's hotter the disc initially expands in the vertical direction because it hasn't been setup to be in equilibrium, but after that it settles now quite nicely. I could probably fix that just by making the disc a bit thicker to begin with.

This temperature is a bit on the low side - 1500 K is about as cold as HI can get before it becomes molecular. At a more realistic 5000 K it still basically works, though the disc starts to evaporate after a few billion years. For the final runs I'll probably try both temperatures.

I'm slowly iterating down to re-creating the observational data. The current sim running has the correct velocity profile. The next step will be to shrink it down to the correct radius. Sometimes funny things can happen when making things smaller, hence I'm being very cautious and proceeding incrementally.

Simulations have to use a vastly smaller number of particles than in reality - in this case 10,000 gas particles and 10,000 dark matter. That means every particle is vastly more massive than in reality, so the gravitational field isn't nearly as uniform as it should be. When particles get very close together they can experience extremely high accelerations and scatter off into the void. That can't happen in reality because the masses are so much smaller. Simulations deal with this by using a "softening length" - when particles get within a certain radius, the gravitational force reaches a maximum instead of increasing to infinity. So it's important to check that scattering isn't important and adjust the softening length if needed.

So far, so good.